Infrasound detection is important in natural disasters monitoring, military defense, underwater acoustic detection, and other domains. Fiber-optic Fabry–Perot (FP) acoustic sensors have the advantages of small structure size, long-distance detection, immunity to electromagnetic interference, and so on. The size of an FP sensor depends on the transducer diaphragm size and the back cavity volume. However, a small transducer diaphragm size means a low sensitivity. Moreover, a small back cavity volume will increase the low cut-off frequency of the sensor. Hence, it is difficult for fiber-optic FP infrasound sensors to simultaneously achieve miniaturization, high sensitivity, and extremely low detectable frequency. In this work, we proposed and demonstrated a miniaturized and highly sensitive fiber-optic FP sensor for mHz infrasound detection by exploiting a Cr-Ag-Au composite acoustic-optic transducer diaphragm and a MEMS technique-based spiral micro-flow hole. The use of the spiral micro-flow hole as the connecting hole greatly reduced the volume of the sensor and decreased the low-frequency limit, while the back cavity volume was not increased. Combined with the Cr-Ag-Au composite diaphragm, a detection sensitivity of -123.19 dB re 1 rad/μPa at 5 Hz and a minimum detectable pressure (MDP) of 1.2 mPa/Hz1/2 at 5 Hz were achieved. The low detectable frequency can reach 0.01 Hz and the flat response range was 0.01–2500 Hz with a sensitivity fluctuation of ±1.5 dB. Moreover, the size of the designed sensor was only 12 mm×Φ12.7 mm. These excellent characteristics make the sensor have great practical application prospects.

Controlling the dispersion characteristic of metasurfaces (or metalenses) along a broad bandwidth is of great importance to develop high-performance broadband metadevices. Different from traditional lenses that rely on the material refractive index along the light trajectory, metasurfaces or metalenses provide a new regime of dispersion control via a sub-wavelength metastructure, which is known as negative chromatic dispersion. However, broadband metalenses design with high-performance focusing especially with a reduced device dimension is a significant challenge in society. Here, we design, fabricate, and demonstrate a broadband high-performance diffractive-type plasmonic metalens based on a circular split-ring resonator metasurface with a relative working bandwidth of 28.6%. The metalens thickness is only 0.09λ0 (λ0 is at the central wavelength), which is much thinner than previous broadband all-dielectric metalenses. The full-wave simulation results show that both high transmissive efficiency above 80% (the maximum is even above 90%) and high average focusing efficiency above 45% (the maximum is 56%) are achieved within the entire working bandwidth of 9–12 GHz. Moreover, an average high numerical aperture of 0.7 (NA=0.7) of high-efficiency microwave metalens is obtained in the simulations. The broadband high-performance metalens is also fabricated and experimental measurements verify its much higher average focusing efficiency of 55% (the maximum is above 65% within the broad bandwidth) and a moderate high NA of 0.6. The proposed plasmonic metalens can facilitate the development of wavelength-dependent broadband diffractive devices and is also meaningful to further studies on arbitrary dispersion control in diffractive optics based on plasmonic metasurfaces.

In recent studies, visible light communication (VLC) has been predicted to be a prospective technique in the future 6G communication systems. To suit the trend of exponentially growing connectivity, researchers have intensively studied techniques that enable multiple access (MA) in VLC systems, such as the MIMO system based on LED devices to support potential applications in the Internet of Things (IoT) or edge computing in the next-generation access network. However, their transmission rate is limited due to the intrinsic bandwidth of LED. Unfortunately, the majority of visible light laser communication (VLLC) research with beyond 10 Gb/s data rates concentrates on point-to-point links, or using discrete photodetector (PD) devices instead of an integrated array PD. In this paper, we demonstrated an integrated PD array device fabricated with a Si-substrated GaN/InGaN multiple-quantum-well (MQW) structure, which has a 4×4 array of 50 μm×50 μm micro-PD units with a common cathode and anode. This single-integrated array successfully provides access for two different transmitters simultaneously in the experiment, implementing a 2×2 MIMO-VLLC link at 405 nm. The highest data rate achieved is 13.2 Gb/s, and the corresponding net data rate (NDR) achieved is 12.27 Gb/s after deducing the FEC overhead, using 2.2 GHz bandwidth and superposed PAM signals. Furthermore, we assess the Huffman-coded coding scheme, which brings a fine-grain adjustment in access capacity and enhances the overall data throughput when the user signal power varies drastically due to distance, weather, or other challenges in the channel condition. As far as we know, this is the first demonstration of multiple visible light laser source access based on a single integrated GaN/InGaN receiver module.

Low-power, flexible, and integrated photodetectors have attracted increasing attention due to their potential applications of photosensing, astronomy, communications, wearable electronics, etc. Herein, the samples of ZnO microwires having p-type (Sb-doped ZnO, ZnO:Sb) and n-type (Ga-doped ZnO, ZnO:Ga) conduction properties were synthesized individually. Sequentially, a p-n homojunction vertical structure photodiode involving a single ZnO:Sb microwire crossed with a ZnO:Ga microwire, which can detect ultraviolet light signals, was constructed. When exposed under 360 nm light illumination at -0.1 V, the proposed photodiode reveals pronounced photodetection features, including a largest on/off ratio of 105, responsivity of 2.3 A/W, specific detectivity of ∼6.5×1013 Jones, noise equivalent power of 4.8×10-15 W Hz-1/2, and superior photoelectron conversion efficiency of ∼7.8%. The photodiode also exhibits a fast response/recovery time of 0.48 ms/9.41 ms. Further, we propose a facile and scalable construction scheme to integrate a p-ZnO:Sb⊗n-ZnO:Ga microwires homojunction component into a flexible, array-type detector, which manifests significant flexibility and electrical stability with insignificant degradation. Moreover, the as-constructed array unit can be integrated into a practical photoimaging system, which demonstrates remarkable high-resolution single-pixel imaging capability. The results represented in this work may supply a workable approach for developing low-dimensional ZnO-based homojunction optoelectronic devices with low-consumption, flexible, and integrated characteristics.

Polarization is one of the basic characteristics of electromagnetic (EM) waves, and its flexible control is very important in many practical applications. At present, most of the multifunction polarization metasurfaces are electrically tunable based on PIN and varactor diodes, which are easy to operate and have strong real-time performance. However, there are still some problems in them, such as few degrees of freedom of planar structure control, complex circuit, bulky sample, and high cost. In view of these shortcomings, this paper proposes a Miura origami based reconfigurable polarization conversion metasurface for multifunctional control of EM waves. The interaction between the electric dipoles is changed by adjusting the folding angle θ, thereby tuning the operating frequency of the polarization conversion and the polarization state of the reflected wave. This mechanical control method brings more degrees of freedom to manipulate EM waves. And the processed sample is with lightweight and low cost. To verify the performance of the proposed origami polarization converter, a Miura origami structure loaded with metal split rings is designed and fabricated. The operating frequency of the structure can be tuned in different folding states. In addition, by controlling the folding angle θ, linear-to-linear and linear-to-circular polarization converters can be realized at different folding states. The proposed Miura origami polarization conversion metasurface provides a new idea for reconfigurable linear polarization conversion and multifunctional devices.

We experimentally demonstrate and numerically analyze large arrays of whispering gallery resonators. Using fluorescent mapping, we measure the spatial distribution of the cavity ensemble’s resonances, revealing that light reaches distant resonators in various ways, including while passing through dark gaps, resonator groups, or resonator lines. Energy spatially decays exponentially in the cavities. Our practically infinite periodic array of resonators, with a quality factor (Q) exceeding 107, might impact a new type of photonic ensembles for nonlinear optics and lasers using our cavity continuum that is distributed, while having high-Q resonators as unit cells.

In this paper, a patch-antenna-array enhanced quantum cascade detector with freely switchable operating modes among mid-wave, long-wave, and dual-color was proposed and discussed. The dual-color absorption occurs in a single active region through an optimized coupled miniband diagonal-transition subbands arrangement, and a successful separation of the operation regimes was realized by two nested antenna arrays with different patch sizes up to room temperature. At 77 K, the 5.7-μm channel achieved a peak responsivity of 34.6 mA/W and exhibited a detectivity of 2.0×1010 Jones, while the 10.0-μm channel achieved a peak responsivity of 87.5 mA/W, giving a detectivity of 5.0×1010 Jones. Under a polarization modulation of the incident light, the minimum cross talk of the mid-wave and the long-wave operating modes was 1:22.5 and 1:7.6, respectively. This demonstration opens a new prospect for multicolor infrared imaging chip integration technology.

Metasurfaces provide an effective technology platform for manipulating electromagnetic waves, and the existing design methods all highlight the importance of creating a gradient in the output phase across light scattering units. However, in the emerging research subfield of meta-waveguides where a metasurface is driven by guided modes, this phase gradient-oriented approach can only provide a very limited emission aperture, significantly affecting the application potential of such meta-waveguides. In this work, we propose a new design approach that exploits the difference between meta-atoms in their light scattering amplitude. By balancing this amplitude gradient in the meta-atoms against the intensity decay in the energy-feeding waveguide, a large effective aperture can be obtained. Based on this new design approach, three different wavefront shaping functionalities are numerically demonstrated here on multiple devices in the terahertz regime. They include beam expanders that radiate a plane wave, where the beam width can increase by more than 900 times as compared to the guided wave. They also include a metalens that generates a Bessel-beam focus with a width 0.59 times the wavelength, and vortex beam generators that emit light with a tunable topological charge that can reach -30. This amplitude gradient design approach could benefit a variety of off-chip light shaping applications such as remote sensing and 6G wireless communications.

Temperature sensing is essential for human health monitoring. High-sensitivity (>1 nm/°C) fiber sensors always require long interference paths and temperature-sensitive materials, leading to a long sensor and thus slow response (6–14 s). To date, it is still challenging for a fiber optic temperature sensor to have an ultrafast (∼ms) response simultaneously with high sensitivity. Here, a side-polished single-mode/hollow/single-mode fiber (SP-SHSF) structure is proposed to meet the challenge by using the length-independent sensitivity of an anti-resonant reflecting optical waveguide mechanism. With a polydimethylsiloxane filled sub-nanoliter volume cavity in the SP-SHSF, the SP-SHSF exhibits a high temperature sensitivity of 4.223 nm/°C with a compact length of 1.6 mm, allowing an ultrafast response (16 ms) and fast recovery time (176 ms). The figure of merit (FOM), defined as the absolute ratio of sensitivity to response time, is proposed to assess the comprehensive performance of the sensor. The FOM of the proposed sensor reaches up to 263.94 (nm/°C)/s, which is more than two to three orders of magnitude higher than those of other temperature fiber optic sensors reported previously. Additionally, a three-month cycle test shows that the sensor is highly robust, with excellent reversibility and accuracy, allowing it to be incorporated with a wearable face mask for detecting temperature changes during human breathing. The high FOM and high stability of the proposed sensing fiber structure provide an excellent opportunity to develop both ultrafast and highly sensitive fiber optic sensors for wearable respiratory monitoring and contactless in vitro detection.

Transparent absorbers, with a functional integration of broadband electromagnetic shielding, microwave camouflage, and optical transparency, have attracted increasing attention in the past decades. Metal mesh, an artificial, optically transparent, conducting material composed of periodic metallic gratings, is the optimal choice for the microwave shielding layer of transparent absorbers because of its excellent compatibility between high transparency and low resistance. However, the micrometer-level periodicity of metallic grating concentrates the diffraction of light, which degrades the imaging quality of cameras and sensors in common. In this study, we report on a generalized Thiessen-polygon-randomization method that prevents the concentration of the diffraction of light in periodic metallic grating and demonstrate an ultrawide-band optically transparent diffraction-immune metamaterial absorber. The absorber is constructed with a multilayer indium-tin-oxide-based metasurface and a Thiessen-polygon-randomized metal-mesh reflector. The lossy metasurface provides multimode absorption, whereas the Thiessen-polygon randomization prevents the concentration of the diffraction of light. The practical sample achieves a 10 dB absorptivity and shielding effectiveness over a range of 8–26.5 GHz, and the optical transparency is also preserved over the entire visible and near-infrared regions. The point spread function and field of view are both improved by using the antidiffraction absorber. Our study paves the way for the application of optically transparent electromagnetic devices, display, and optoelectronic integration in a more practical stage.

Optical physical unclonable functions (PUFs) have emerged as a promising strategy for effective and unbreakable anti-counterfeiting. However, the unpredictable spatial distribution and broadband spectra of most optical PUFs complicate efficient and accurate verification in practical anti-counterfeiting applications. Here, we propose an optical PUF-based anti-counterfeiting label from perovskite microlaser arrays, where randomness is introduced through vapor-induced microcavity deformation. The initial perovskite microdisk laser arrays with regular positions and uniform sizes are fabricated by femtosecond laser direct ablation. By introducing vapor fumigation to induce random deformations in each microlaser cavity, a laser array with completely uneven excitation thresholds and narrow-linewidth lasing signals is obtained. As a proof of concept, we demonstrated that the post-treated laser array can provide fixed-point and random lasing signals to facilitate information encoding. Furthermore, different emission states of the lasing signal can be achieved by altering the pump energy density to reflect higher capacity information. A threefold PUF (excited under three pump power densities) with a resolution of 5×5 pixels exhibits a high encoding capacity (1.43×1045), making it a promising candidate to achieve efficient authentication and high security with anti-counterfeiting labels.

The control of light–matter coupling at the single electron level is currently a subject of growing interest for the development of novel quantum devices and for studies and applications of quantum electrodynamics. In the terahertz (THz) spectral range, this raises the particular and difficult challenge of building electromagnetic resonators that can conciliate low mode volume and high quality factor. Here, we report on hybrid THz cavities based on ultrastrong coupling between a Tamm cavity and an LC circuit metamaterial and show that they can combine high quality factors of up to Q=37 with a deep-subwavelength mode volume of V=3.2×10-4λ3. Our theoretical and experimental analysis of the coupled mode properties reveals that, in general, the ultrastrong coupling between a metamaterial and a Fabry–Perot cavity is an effective tool to almost completely suppress radiative losses and, thus, ultimately limit the total losses to the losses in the metallic layer. These Tamm cavity-LC metamaterial coupled resonators open a route toward the development of single photon THz emitters and detectors and to the exploration of ultrastrong THz light–matter coupling with a high degree of coherence in the few to single electron limit.

Whispering gallery mode (WGM) microcavities have been widely used for high-sensitivity ultrasound detection, owing to their optical and mechanical dual-resonance enhanced sensitivity. The ultrasound sensitivity of the cavity optomechanical system is fundamentally limited by thermal noise. In this work, we theoretically and experimentally investigate the thermal-noise-limited sensitivity of a WGM microdisk ultrasound sensor and optimize the sensitivity by varying the radius and a thickness of the microdisk, as well as using a trench structure around the disk. Utilizing a microdisk with a radius of 300 μm and thickness of 2 μm, we achieve a peak sensitivity of 1.18 μPa Hz-1/2 at 82.6 kHz. To the best of our knowledge, this represents the record sensitivity among cavity optomechanical ultrasound sensors. Such high sensitivity has the potential to improve the detection range of air-coupled ultrasound sensing technology.

Integrating novel materials is critical for the ultrasensitive, multi-dimensional detection of biomolecules in the terahertz (THz) range. Few studies on THz biosensors have used semiconductive active layers with tunable energy band structures. In this study, we demonstrate three THz biosensors for detecting casein molecules based on the hybridization of the metasurface with graphitic carbon nitride, graphene, and heterojunction. We achieved low-concentration detection of casein molecules with a 3.54 ng/mL limit and multi-dimensional sensing by observing three degrees of variations (frequency shift, transmission difference, and phase difference). The favorable effect of casein on the conductivity of the semiconductive active layer can be used to explain the internal sensing mechanism. The incorporation of protein molecules changes the carrier concentration on the surface of the semiconductor active layer via the electrostatic doping effect as the concentration of positively charged casein grows, which alters the energy band structure and the conductivity of the active layer. The measured results indicate that any casein concentration can be distinguished directly by observing variations in resonance frequency, transmission value, and phase difference. With the heterojunction, the biosensor showed the highest response to the protein among the three biosensors. The Silvaco Atlas package was used to simulate the three samples’ energy band structure and carrier transport to demonstrate the benefits of the heterojunction for the sensor. The simulation results validated our proposed theoretical mechanism model. Our proposed biosensors could provide a novel approach for THz metasurface-based ultrasensitive biosensing technologies.

Optical fiber microresonators have attracted considerable interest for acoustic detection because of their compact size and high optical quality. Here, we have proposed, designed, and fabricated a spring-based Fabry–Pérot cavity microresonator for highly sensitive acoustic detection. We observed two resonator vibration modes: one relating to the spring vibration state and the other determined by the point-clamped circular plate vibration mode. We found that the vibration modes can be coupled and optimized by changing the structure size. The proposed resonator is directly 3D printed on an optical fiber tip through two-photon polymerization and is used for acoustic detection and imaging. The experiments show that the device exhibits a high sensitivity and low noise equivalent acoustic signal level of 2.39 mPa/Hz1/2 at 75 kHz that can detect weak acoustic waves, which can be used for underwater object imaging. The results demonstrate that the proposed work has great potential in acoustic detection and biomedical imaging applications.

In this study, we theoretically proposed a method to achieve an electromagnetically induced transparency (EIT)-like effect in a whispering gallery mode resonator (WGMR) and experimentally validated the method in a lithium niobate (LN) device. Benefitting from the electro-optic and inverse piezoelectric effects of the LN material, two modes of the LN WGMR that are close in frequency can be tuned at different tuning rates, resulting in EIT-like resonance lineshapes. By varying the electric field applied to the LN WGMR, the full dynamic of the EIT-like phenomenon can be precisely controlled. The experimental results agreed well with the calculations based on the coupled mode theory. Moreover, we observed a hysteresis resulting from the photorefractive effect of LN. We believe our proposed method and demonstrated devices offer a way to control an EIT-like effect, which could have potential applications in light storage, quantum information processing, and enhanced sensing techniques.

Highly accurate biosensors for few or single molecule detection play a central role in numerous key fields, such as healthcare and environmental monitoring. In the last decade, laser biosensors have been investigated as proofs of concept, and several technologies have been proposed. We here propose a demonstration of polymeric whispering gallery microlasers as biosensors for detecting small amounts of proteins, down to 400 pg. They have the advantage of working in free space without any need for waveguiding for input excitation or output signal detection. The photonic microsensors can be easily patterned on microscope slides and operate in air and solution. We estimate the limit of detection up to 148 nm/RIU for three different protein dispersions. In addition, the sensing ability of passive spherical resonators in the presence of dielectric nanoparticles that mimic proteins is described by massive ab initio numerical simulations.

Valley Hall topological photonic crystals, inspired by topological insulators in condensed matter physics, have provided a promising solution to control the flow of light. Recently, the dynamic manipulation property of topological photonic crystals has been widely studied. Here, we propose a novel solution for programmable valley photonic crystals, called field programmable topological edge array (FPTEA), based on the field reorientation property of nematic liquid crystals and robust valley-protected edge modes. FPTEA is composed of an array of graphene-like lattices with C3 symmetry, in which the birefringence of liquid crystal is larger than 0.5105. Due to the dielectric anisotropy of liquid crystals being sensitive to external fields such as light, heat, electric, and magnetic fields, each lattice is tunable, and the topological propagation routes and even the lattice parameters can be dynamically changed while changing the distribution of external fields. We numerically demonstrate three methods of composing an FPTEA device to design arbitrary passive optical devices by electric driving, thermal inducing, or UV writing. These results show the great application potential of liquid crystals in topological photonic crystals, and enrich the design of programmable integrated topological devices with broad working bandwidth ranging from microwave to visible light.

Dynamical control of perfect absorption plays an indispensable role in optical switch and modulators. However, it always suffers from the limited modulation range, small depth, and susceptible absorption efficiencies. Here, we propose a new strategy based on Friedrich–Wintgen bound states in the continuum (F–W BICs) to realize a tunable perfect absorber with large dynamic modulation range. For proof of concept, we demonstrate a pentaband ultrahigh absorption system consisting of graphene gratings and graphene sheets through elaborately tuning F–W BIC. The nature of the F–W BIC arises from the destructive interference between Fabry–Perot resonance and guided mode resonance modes in the coherent phase-matching condition. The radiation channels are avoided from crossing. The BIC can be dynamically modulated by engineering the Fermi level of graphene gratings, which breaks the traditional modulation methods with an incidence angle. Remarkably, the perfect absorber with this F–W BIC approach achieves the largest modulation range of up to 3.5 THz. We believe that this work provides a new way to dynamically engineer perfect absorption and stimulates the development of multiband ultracompact devices.

Lithium niobate’s substantial nonlinear optical and electro-optic coefficients have recently thrust it into the limelight. This study presents a thorough review of bound states in the continuum (BICs) in lithium niobate metasurfaces, also suggesting their potential for sensing applications. We propose an all-dielectric tunable metasurface that offers high Q factor resonances in the terahertz range, triggered by symmetry-protected BICs. With exceptional sensitivity to changes in the refractive index of the surrounding medium, the metasurface can reach a sensitivity as high as 947 GHz/RIU. This paves the way for ultrasensitive tunable terahertz sensors, offering an exciting path for further research.

The study of pixelated metamaterials that integrate both the functions of linear and circular polarization filters is rapidly growing due to the need for full-Stokes polarization imaging. However, there is a lack of large-area, ultracompact pixelated full-Stokes metamaterials with excellent performance, especially circular polarization filters with a high extinction ratio, a broad operating bandwidth, and a low-cost, high-quality, efficient manufacturing process, which limits the practical applications of pixelated full-Stokes metamaterials. In this study, we propose a universal design and fabrication scheme for large-area, ultracompact pixelated aluminum wire-grid-based metamaterials used in Vis-NIR full-Stokes polarization imaging. The aluminum wire-grid was designed as a linear polarization filter with an average linear polarization extinction ratio of 36,000 and a circular polarization filter with an average circular polarization extinction ratio of 110 in Vis-NIR. A large-area, ultracompact 320×320 pixelated aluminum wire-grid-based full-Stokes metamaterial was fabricated using nanoimprint lithography and nano transfer printing with the advantages of low cost and high efficiency. This metamaterial was used to achieve full-Stokes polarization imaging with errors within 8.77%, 12.58%, 14.04%, and 25.96% for Stokes parameters S0, S1, S2, and S3, respectively. The inversion errors of the compensated Stokes parameters can be reduced to 0.21%, 0.21%, 0.42%, and 1.96%, respectively.

Chiral metasurfaces integrated with active materials can dynamically control the chirality of electromagnetic waves, making them highly significant in physics, chemistry, and biology. Herein, we theoretically proposed a general and feasible design scheme to develop a chiral metadevice based on a bilayer anisotropic metasurface and a monolayer liquid crystal (LC), which can construct and flexibly manipulate arbitrary terahertz (THz) chirality. When the twist angle between the anisotropic axes of two metasurfaces θ is not 0°, the spatial mirror symmetry of the chiral metadevice is broken, resulting in a strong THz chiral response. In addition, the introduction of anisotropic LCs not only enhances the chiral response of the metadevice but also induces the flipping modulation and frequency tunability of the chirality. More importantly, by optimizing the θ, we can flexibly design the arbitrary chiral response and the operating frequency of chirality, thereby promoting the emergence of various chiral manipulation devices. The experimental results show that the maximum circular dichroism can reach -33 dB at 0.94 THz and flip to 28 dB at 0.69 THz by rotating the LC optical axis from the x to y axis, with the maximum operating frequency tunable range of ∼120 GHz. We expect this design strategy can create new possibilities for the advancement of active THz chiral devices and their applications, including chiral spectroscopy, molecular recognition, biosensing, and fingerprint detection.

Exciton-polaritons offer the potential to achieve electrically pumped perovskite polariton lasers with much lower current thresholds than conventional photonic lasers. While optically pumped exciton-polaritons have been widely studied in halide perovskites, electrically-pumped polaritons remain limited. In this study, we demonstrate the use of a solution-processing strategy to develop halide perovskite polariton light-emitting diodes (LEDs) that operate at room temperature. The strong coupling of excitons and cavity photons is confirmed through the dispersion relation from angle-resolved reflectivity, with a Rabi splitting energy of 64 meV. Our devices exhibit angle-resolved electroluminescence following the low polariton branch and achieve external quantum efficiencies of 1.7%, 3.85%, and 3.7% for detunings of 1.1, -77, and -128 meV, respectively. We also explore devices with higher efficiency of 5.37% and a narrower spectral bandwidth of 6.5 nm through the optimization of a top emitting electrode. Our work demonstrates, to our knowledge, the first room-temperature perovskite polariton LED with a typical vertical geometry and represents a significant step towards realizing electrically pumped perovskite polariton lasers.

Optical metasurfaces are two-dimensional ultrathin devices based on single-layer or multilayer arrays of subwavelength nanostructures. They can achieve precise control of phase, amplitude, and polarization on the subwavelength scale. In this paper, a substrate-free all-silicon coded grating is designed, which can realize the phase control of the outgoing beam after the y-polarized plane wave is vertically incident on the metasurface at 0.1 THz. Through a single-layer silicon nanoarray structure, a low-reflection anomalous transmission metasurface is realized, and a variety of different beam deflectors are designed based on these encoded gratings. We propose a coded grating addition principle, which adds and subtracts two traditional coded grating sequences to obtain a new coded grating sequence. The encoded supergrating can flexibly control the scattering angle, and the designed substrate-free all-silicon encoded grating can achieve a deflection angle of 48.59°. In order to verify the principle of coded grating addition, we experimented with cascade operation of two coded sequence gratings to obtain the flexible control of the terahertz beam of the composite supergrating. The principle of grating addition provides a new degree of freedom for the flexible regulation of the terahertz wavefront. At the same time, this method can be extended to the optical band or microwave band, opening up new ways for electromagnetic wave manipulation and beam scanning.

Graphene-based photodetectors have attracted much attention due to their unique properties, such as high-speed and wide-band detection capability. However, they suffer from very low external quantum efficiency in the infrared (IR) region and lack spectral selectivity. Here, we construct a plasmon-enhanced macro-assembled graphene nanofilm (nMAG) based dual-band infrared silicon photodetector. The Au plasmonic nanostructures improve the absorption of long-wavelength photons with energy levels below the Schottky barrier (between metal and Si) and enhance the interface transport of electrons. Combined with the strong photo-thermionic emission (PTI) effect of nMAG, the nMAG–Au–Si heterojunctions show strong dual-band detection capability with responsivities of 52.9 mA/W at 1342 nm and 10.72 mA/W at 1850 nm, outperforming IR detectors without plasmonic nanostructures by 58–4562 times. The synergy between plasmon–exciton resonance enhancement and the PTI effect opens a new avenue for invisible light detection.

Dynamic beam steering with unlimited angular range and fast speed remains a challenge in the terahertz gap, which is urgently needed for next-generation target tracking, wireless communications, and imaging applications. Different from metasurface phased arrays with element-level phase control, here we steer the beam by globally engineering the diffraction of two cascaded metagratings during in-plane rotation. Benefiting from large-angle diffraction and flexible on/off control of the diffraction channels, a pair of metagratings with optimized supercells and proper orientation successfully directs the incoming beam towards any arbitrary direction over the transmission half space, with the steering speed improved more than twice that of the small-angle diffractive designs. Single-beam and dual-beam steering within the solid angle of 1.56π and elevation angle of ±77° has been demonstrated with average throughput efficiency of 41.4% at 0.14 THz, which can be generalized to multiple-beam cases. The dual diffraction engineering scheme offers a clear physical picture for beamforming and greatly simplifies the device structure, with additional merits of large aperture and low power consumption.

High-order Bessel beams are of great interest for most stable long-range optical quantum communications due to their unique nondiffraction, self-healing, and orbital angular-momentum-carrying capabilities. Until now, metasurfaces based on Bessel beam generators are mostly static and focused on generating zero-order Bessel beams. A moiré meta-device made of two cascaded metasurfaces is a simple, effective strategy to dynamically manipulate the wavefront of electromagnetic (EM) waves by mutual rotation between the two metasurfaces. Here, an all-dielectric moiré meta-device integrated with the functions of an axicon and a spiral phase plate to generate terahertz Bessel beams is designed. Not only the order, but also the nondiffraction length of the generated Bessel beam can be continuously tuned. As a proof of concept of the feasibility of the platform, the case of tuning order is experimentally demonstrated. The experimental results are in good agreement with the theoretical expectations. In addition, we also numerically proved that the nondiffraction length of the Bessel beam can be adjusted with the same approach. The moiré meta-device platform is powerful in dynamically manipulating the wavefront of EM waves and provides an effective strategy for continuously controlling the properties of the Bessel beam, which may find applications in optical communications, particle manipulation, and super-resolution imaging.

High sensitivity, high solar rejection ratio, and fast response are essential characteristics for most practical applications of solar-blind ultraviolet (UV) detectors. These features, however, usually require a complex device structure, complicated process, and high operating voltage. Herein, a simply structured n-AlGaN/AlN phototransistor with a self-depleted full channel is reported. The self-depletion of the highly conductive n-AlGaN channel is achieved by exploiting the strong polarization-induced electric field therein to act as a virtual photogate. The resulting two-terminal detectors with interdigital Ohmic electrodes exhibit an ultrahigh gain of 1.3×105, an ultrafast response speed with rise/decay times of 537.5 ps/3.1 μs, and an ultrahigh Johnson and shot noise (flicker noise) limited specific detectivity of 1.5×1018 (4.7×1016) Jones at 20-V bias. Also, a very low dark current of the order of ∼pA and a photo-to-dark current ratio of above 108 are obtained, due to the complete depletion of the n-Al0.5Ga0.5N channel layer and the high optical gain. The proposed planar phototransistor combines fabrication simplicity and performance advantages, and thus is promising in a variety of UV detection applications.

The development of high-performance InP-based quantum dot light-emitting diodes (QLEDs) has become the current trend in ecofriendly display and lighting technology. However, compared with Cd-based QLEDs that have already been devoted to industry, the efficiency and stability of InP-based QLEDs still face great challenges. In this work, colloidal NiOx and Mg-doped NiOx nanocrystals were used to prepare a bilayered hole injection layer (HIL) to replace the classical polystyrene sulfonate (PEDOT:PSS) HIL to construct high-performance InP-based QLEDs. Compared with QLEDs with a single HIL of PEDOT:PSS, the bilayered HIL enables the external quantum efficiencies of the QLEDs to increase from 7.6% to 11.2%, and the T95 lifetime (time that the device brightness decreases to 95% of its initial value) under a high brightness of 1000 cd m-2 to prolong about 7 times. The improved performance of QLEDs is attributed to the bilayered HIL reducing the mismatched potential barrier of hole injection, narrows the potential barrier difference of indium tin oxide (ITO)/hole transport layer interface to promote carrier balance injection, and realizes high-efficiency radiative recombination. The experimental results indicate that the use of bilayered HILs with p-type NiOx might be an efficient method for fabricating high-performance InP-based QLEDs.

Multiplexing and demultiplexing of optical orbital angular momentum (OAM) are critical operations in mode-division multiplexing communications. Traditional Dammann gratings, spiral phase planes, and optical geometric transformations are regarded as convenient methods for OAM mode (de)multiplexing. However, crosstalk between the different modes and the difficulty of mode multiplexing greatly limit their application to mode-division multiplexing communications. Here, using a set of inversely-designed phase planes, we demonstrate an OAM (de)multiplexer based on multiphase plane light conversion that can enable perfect OAM multiplexing communication. The sorted patterns are Gaussian-like and can be coupled easily into single-mode fiber arrays. Inputs from the fiber array are turned into coaxial OAM modes after the phase planes. OAM mode crosstalk generated by the multiplexer is less than -20 dB, with insertion loss of less than 2.6 dB. OAM modes are sorted by the demultiplexer with mode crosstalk below -10 dB, and the sorting results are coupled to the fiber array. OAM modes carrying 10 Gbit/s on–off keying signals were transmitted in a 5 km few-mode fiber. The measured bit-error-rate curves have power penalties of less than 10 dB. The proposed configuration is highly efficient and convenient and will be beneficial for potential applications in quantum information, information processing, and optical communications.

Superoscillation metalenses have demonstrated promising prospects in breaking the theoretical diffraction limitations on the resolution of optical devices and systems. However, most reported superoscillation metalenses have a very small field of view of several tenths of a degree, which greatly limits their applications in imaging and microscopy. Therefore, it is of critical importance to achieve absolute high resolution by increasing the numerical apertures (NAs) of optical devices and systems. Unfortunately, similar to the case in traditional optics, it is challenging to realize a large field of view at high NA, especially in the superoscillation regime. To date, no attempt has been made to achieve flat-field focusing in the superoscillation regime, to our knowledge. Here, we demonstrate a high-NA superoscillation metalens with an entrance aperture stop, which is optimized for superoscillation performance with a comparatively large field of view. The proposed flat-field superoscillation metalens has an effective NA as large as 0.89 and achieves superoscillation focusing within a field of view of 9°. Such a superoscillation metalens may offer a promising way toward superoscillation imaging and fast-scanning label-free far-field superoscillation microscopy.

Metasurfaces consisting of artificial subwavelength structure arrays have shown unprecedented ability to manipulate the phase, amplitude, and polarization of light. Separate and complete control over different spin states, namely the orthogonal circular polarizations, has proven more challenging as compared to the control over orthogonal linear polarizations. Here, we present and experimentally demonstrate several spin-dependent wavefront control metasurfaces in the terahertz regime using all-silicon dielectric structures. Such spin-dependent all-silicon metasurfaces are easy to fabricate and have potential applications in spin-involved ultracompact and miniaturized terahertz optical systems as well as terahertz communication systems.

Metasurfaces composed of spatially arranged ultrathin subwavelength elements are promising photonic devices for manipulating optical wavefronts, with potential applications in holography, metalens, and multiplexing communications. Finding microstructures that meet light modulation requirements is always a challenge in designing metasurfaces, where parameter sweep, gradient-based inverse design, and topology optimization are the most commonly used design methods in which the massive electromagnetic iterations require the design computational cost and are sometimes prohibitive. Herein, we propose a fast inverse design method that combines a physics-based neural network surrogate model (NNSM) with an optimization algorithm. The NNSM, which can generate an accurate electromagnetic response from the geometric topologies of the meta-atoms, is constructed for electromagnetic iterations, and the optimization algorithm is used to search for the on-demand meta-atoms from the phase library established by the NNSM to realize an inverse design. This method addresses two important problems in metasurface design: fast and accurate electromagnetic wave phase prediction and inverse design through a single phase-shift value. As a proof-of-concept, we designed an orbital angular momentum (de)multiplexer based on a phase-type metasurface, and 200 Gbit/s quadrature-phase shift-keying signals were successfully transmitted with a bit error rate approaching 1.67×10-6. Because the design is mainly based on an optimization algorithm, it can address the “one-to-many” inverse problem in other micro/nano devices such as integrated photonic circuits, waveguides, and nano-antennas.

Optical biosensing systems are commonly developed assembling a source, a light–sample interaction area, and a detector as distinct stand-alone elements. We present a compact, inexpensive, and easy-to-use glass chip that monolithically integrates both the interaction and detection elements in a 1 cm2 overall surface. The device working principle is based on evanescent wavelight interaction with the complex refractive index of a liquid mixture, being the index influenced by the mixture’s physical and chemical features. We describe the manufacture of a prototype able to perform investigations on food quality and subsequent tests on the detection of fat content in milk. Theoretical investigations are reported as well as measurements performed on samples in the green spectrum. A sensitivity of about 139 fA/(g/dL) and a limit of detection of 14 ppm have been achieved, better than those of current commercial devices.

Understanding inter-site mutual mode interaction in coupled physical systems is essential to comprehend large compound systems, as this local interaction determines the successive multiple inter-site energy transfer efficiencies. In the present study, we demonstrate that only the non-Hermitian coupling can correctly account for the light transfer between two coupled optical cavities. We also reveal that the non-Hermitian coupling effect becomes crucial as the system dimension decreases. Our results provide important insight for handling general-coupled devices in the subwavelength regime.

Single longitudinal mode continuous-wave operation of GaN-based distributed Bragg reflector (DBR) laser diodes with 10th-order surface gratings is demonstrated. The DBR consists of periodic V-shaped grooves on a 2 μm wide ridge waveguide fabricated by using electron-beam lithography and plasma etching. The effect of different lengths of the DBR section and the gain section on the device performance has been studied. Periodic mode hops to the adjacent longitudinal Fabry–Perot resonator mode at shorter wavelength have been observed when increasing the operation current. Between the mode hops, single longitudinal mode emission at around 405 nm is achieved with a full width at half-maximum of 0.03 nm. A linear redshift of the emission wavelength with increasing temperature of 0.019 nm/K was derived.

To enhance and actively control terahertz (THz) anisotropy and chirality, we have designed and fabricated a THz composite device with a liquid crystal (LC) layer and Si anisotropic metasurface. By initial anchoring and electrically rotating the spatial orientation of the LC optical axis, the different symmetry relationships are obtained in this hybrid device. When the optical axis of LC is parallel or perpendicular to the optical axis of the Si metasurface, the anisotropy of the device will be enhanced or offset, which leads to a tunable phase-shift range of more than 180°. When there is an angle between the two optical axes, due to the destruction of the mirror symmetry in the LC-Si anisotropic medium, the highest circular dichroism of the device reaches 30 dB in the middle orientation state of the LC optical axis, and the active modulation can be realized by changing the bias electric field on the LC layer. This composite device demonstrates rich characteristics for the feasible manipulation of THz polarization conversion and chiral transmission, which can be applied in THz polarization imaging and chiral spectroscopy.

All-optical modulators with ultrahigh speed are in high demand due to the rapid development of optical interconnection and computation. However, due to weak photon–photon interaction, the advancement of all-optical modulators is consequently hampered by the large footprint and high power consumption. In this work, the enhanced sensitivity around an exceptional point (EP) from parity-time (PT) symmetry theory is initiatively introduced into a nonlinear all-optical modulator design. Further, a non-Hermitian all-optical modulator based on PT symmetry is proposed, which utilizes the large Kerr nonlinearity from indium tin oxide (ITO) in its epsilon-near-zero (ENZ) region. The whole system is expected to operate around EP, giving rise to the advantages of nanoscale integration and large modulation depth. This presented modulator with high efficiency and high-speed all-optical control can be commendably extended to the design methodology of various nanostructures and further prompt the development of all-optical signal processing.

The powerful wavefront manipulation capability of metasurfaces originates from their subwavelength or deep subwavelength elements with designable optical responses, especially phase responses. However, they usually suffer from performance degradation as the spatial phase gradient is large. To solve this issue, we propose an accurate and efficient nonlocal diffraction engineering mechanism to tailor an arbitrary large-gradient wavefront utilizing superwavelength-scale elements. The fast-varying phase profile is cut into segments according to 2π zones rather than subwavelength discretization. Each phase segment is accurately implemented by precisely tailoring the diffraction pattern of the element, where diffraction angles, efficiencies, and phases are controlled simultaneously. As proof of the concept, high numerical aperture cylindrical metalenses are designed using this method and experimentally validated at the terahertz band. The cylindrical metalens is further extended to a full-space metalens, which enables high-quality subwavelength imaging with resolved details of 0.65λ. The proposed mechanism offers an efficient way to capture the fast-varying wavefront using relatively coarse geometries with new physical insights.

Chromatic optical lenses have promising applications in three-dimensional imaging, which allows fast spectral tomography without mechanical moving parts. The scanning range of current chromatic optical lenses is limited by their dispersion ability. The recent development in metasurfaces provides ideal blocks for optical wavefront manipulation and dispersion engineering of artificial materials at sub-wavelength scales. Hyper-dispersive metalenses can be realized by utilizing dispersive meta-atoms, which have enhanced dispersion compared to regular diffractive lenses. This is critical for increasing the imaging depth of fast spectral tomography. In this work, a hyper-dispersive metalens is realized with a chromatic dispersion 1.76 times greater than that of a regular diffractive metalens in the THz frequency range of 2.40–2.61 THz by simultaneously controlling the frequency-dependent phase, group delay (GD), and GD dispersion of the metalens. This approach can also be extended to other optical spectra and improve the performance of spectral tomography.

Extended depth of focus (EDOF) optics can enable lower complexity optical imaging systems when compared to active focusing solutions. With existing EDOF optics, however, it is difficult to achieve high resolution and high collection efficiency simultaneously. The subwavelength spacing of scatterers in a meta-optic enables the engineering of very steep phase gradients; thus, meta-optics can achieve both a large physical aperture and a high numerical aperture. Here, we demonstrate a fast (f/1.75) EDOF meta-optic operating at visible wavelengths, with an aperture of 2 mm and focal range from 3.5 mm to 14.5 mm (286 diopters to 69 diopters), which is a 250× elongation of the depth of focus relative to a standard lens. Depth-independent performance is shown by imaging at a range of finite conjugates, with a minimum spatial resolution of ∼9.84 μm (50.8 cycles/mm). We also demonstrate operation of a directly integrated EDOF meta-optic camera module to evaluate imaging at multiple object distances, a functionality which would otherwise require a varifocal lens.

Coherent conversion of microwave and optical photons can significantly expand the capabilities of information processing and communications systems. Here, we experimentally demonstrate the microwave-to-optical frequency conversion in a magneto-optical whispering gallery mode microcavity. By applying a magnetic field parallel to the microsphere equator, the intracavity optical field will be modulated when the magnon is excited by the microwave drive, leading to a microwave-to-optical conversion via the magnetic Stokes and anti-Stokes scattering processes. The observed single-sideband conversion phenomenon indicates a nontrivial optical photon–magnon interaction mechanism derived from the magnon that induced both the frequency shift and modulated coupling rate of optical modes. In addition, we demonstrate the single-sideband frequency conversion with an ultrawide tuning range up to 2.5 GHz, showing its great potential in microwave-to-optical conversion.

Optofluidic resonators are capable of characterizing various fluidic media. Here, we propose an optofluidic microbottle resonator (OFMBR) that is applied to generate pendant droplets, whose maximum mass is related to the liquid surface tension. Mass and type of droplets forming along the OFMBR stem can be monitored in real time by spectrum variation. As a pendant droplet grows, increased droplet gravity introduces a decreased coupling gap and compressive force between the tapered fiber and OFMBR, leading to a resonance wavelength shift. The operation mechanism of the proposed sensors is validated by theoretical simulation and experimental results. From the experimental spectra, a liquid mass sensor with maximum sensitivity of -3.34 pm/mg is obtained, and distilled water and alcohol can be identified. This scheme provides a new thread for droplet generation as well as fluidic properties characterization.

All-optical silicon-photonics-based LiDAR systems allow for desirable features in scanning resolution and speed, as well as leverage other advantages such as size, weight, and cost. Implementing optical circulators in silicon photonics enables bidirectional use of the light path for both transmitters and receivers, which simplifies the system configuration and thereby promises low system cost. In this work, to the best of our knowledge, we present the first experimental verification of all-passive silicon photonics conditional circulators for monostatic LiDAR systems using a nonlinear switch. The proposed silicon nonlinear interferometer is realized by controlling signal power distribution with power-splitting circuits, allowing the LiDAR transmitter and receiver to share the same optical path. Unlike the traditional concept requiring a permanent magnet, the present device is implemented by using common silicon photonic waveguides and a standard foundry-compatible fabrication process. With several additional phase shifters, the demonstrated device exhibits considerable flexibility using a single chip, which can be more attractive for integration with photodetector arrays in LiDAR systems.

We report a light waveguide liquid crystal display (LCD) based on the flexoelectric effect. The display consists of two parallel flat substrates with a layer of flexoelectric liquid crystal sandwiched between them. A light-emitting diode (LED) is installed on the edge of the display and the produced light is coupled into the display. When no voltage is applied, the liquid crystal is uniformly aligned and is transparent. The incident light propagates through the display by total internal reflection at the interface between the substrate and air, and no light comes out of the viewing side of the display. The display appears transparent. When a voltage is applied, the liquid crystal is switched to a micrometer-sized polydomain state due to flexoelectric interaction and becomes scattering. The incident light is deflected from the waveguide mode and comes out of the viewing side of the display. We achieved thin-film-transistor active matrix compatible driving voltage by doping liquid crystal dimers with large flexoelectric coefficients. The light waveguide LCD does not use polarizers as in conventional LCDs. It has an ultrahigh transmittance near 90% in the voltage-off state. It is very suitable for transparent display, which can be used for head-up display and augmented reality display.

We systematically investigate the field distribution of the transverse electric modes in X-cut lithium niobate disks as an example of circular microcavities with anisotropic refractive index. A conserved quantity is discovered, which indicates the absence of chaos that generally exists in deformed microcavities and leads to a nontrivial directional emission. The emission directionality was theoretically investigated and experimentally verified by exciting high-order modes of an X-cut lithium niobate microresonator assisted with second harmonics. The field distribution analysis can enrich the knowledge in designing photonic devices that need precise control of field distribution, such as phase matching in nonlinear processes. Furthermore, the discovered emission phenomenon is momentous in enhancing and controlling communications between on-chip photonic devices.

This paper proposes a micro-LED backlight module with a distributed Bragg reflector (DBR) structure to achieve excellent micro-LED backlight module quality and uses deep reinforcement learning (DRL) architecture for optical design. In the DRL architecture, to solve the computing environment problems of the two extreme structures of micro-scale and macro-scale, this paper proposes an environment control agent and virtual-realistic workflow to ensure that the design environment parameters are highly correlated with experimental results. This paper successfully designed a micro-LED backlight module with a DBR structure by the abovementioned methods. The micro-LED backlight module with a DBR structure improves the uniformity performance by 32% compared with the micro-LED backlight module without DBR, and the design calculation time required by the DRL method is only 17.9% of the traditional optical simulation.

Tunable metamaterial absorbers play an important role in terahertz imaging and detection. We propose a multifunctional metamaterial absorber based on doped silicon. By introducing resonance and impedance matching into the absorber, a broadband absorption greater than 90% in the range of 0.8–10 THz is achieved. At the same time, the light regulation characteristics of the doped semiconductor are introduced into the absorber, and the precise amplitude control can be achieved in the range of 0.1–1.2 THz by changing the pump luminous flux. In addition, based on the principle of light-regulating the concentration of doped silicon carriers, the medium-doped silicon material is replaced by a highly doped silicon material, and a sensor with a sensitivity of up to 500 GHz/RIU is realized by combining the wave absorber with the microfluidic control. Finally, the broadband absorption characteristics and sensing performance of alcohol and water on the prepared device are verified by experiments, indicating that the absorber may have great potential in the field of sensor detection.

Soliton microcombs have shown great potential in a variety of applications ranging from chip-scale frequency metrology to optical communications and photonic data center, in which light couplings among cavity transverse modes, termed as intermode interactions, are long-existing and usually give rise to localized impacts on the soliton state. Of particular interest are whispering gallery mode-based crystalline resonators, which with dense mode families, potentially feature interactions of all kinds. While effects of narrowband interactions such as spectral power spikes have been well recognized in crystalline resonators, those of broadband interactions remain unexplored. Here, we demonstrate microcombs with broadband and dispersive intermode interactions, in home-developed magnesium fluoride microresonators with an intrinsic Q-factor approaching 10 billion. In addition to conventional soliton comb generation in the single-mode pumping scheme, comb states with broadband spectral tailoring effect have been observed, via an intermode pumping scheme. Remarkably, footprints of both constructive and destructive interference on the comb spectrum have been observed, which as confirmed by simulations, are connected to the dispersive effects of the coupled mode family. Our results would not only contribute to the understanding of dissipative soliton dynamics in multi-mode or coupled resonator systems, but also extend the access to stable soliton combs in crystalline microresonators where mode control and dispersion engineering are usually challenging.

Modern machine-learning applications require huge artificial networks demanding computational power and memory. Light-based platforms promise ultrafast and energy-efficient hardware, which may help realize next-generation data processing devices. However, current photonic networks are limited by the number of input-output nodes that can be processed in a single shot. This restricted network capacity prevents their application to relevant large-scale problems such as natural language processing. Here, we realize a photonic processor for supervised learning with a capacity exceeding 1.5×1010 optical nodes, more than one order of magnitude larger than any previous implementation, which enables photonic large-scale text encoding and classification. By exploiting the full three-dimensional structure of the optical field propagating in free space, we overcome the interpolation threshold and reach the over-parameterized region of machine learning, a condition that allows high-performance sentiment analysis with a minimal fraction of training points. Our results provide a novel solution to scale up light-driven computing and open the route to photonic natural language processing.

Terahertz (THz) molecular fingerprint sensing provides a powerful label-free tool for the detection of trace-amount samples. Due to the weak light–matter interaction, various metallic or dielectric metasurfaces have been adopted to enhance fingerprint absorbance signals. However, they suffer from strong background damping or complicated sample coating on patterned surfaces. Here, we propose an inverted dielectric metagrating and enhance the broadband THz fingerprint detection of trace analytes on a planar sensing surface. Enhancement of the broadband signal originates from the effects of evanescent waves at the planar interface, which are excited by multiplexed quasi-bound states in the continuum (quasi-BICs). One can evenly boost the near-field intensities within the analytes by tuning the asymmetry parameter of quasi-BIC modes. The multiplexing mechanism of broadband detection is demonstrated by manipulating the incident angle of excitation waves and thickness of the waveguide layer. Compared to the conventional approach, the THz fingerprint peak value is dramatically elevated, and the largest peak enhancement time is 330. Our work gives a promising way to facilitate the metasensing of the THz fingerprint on a planar surface and will inspire universal THz spectral analysis for trace analytes with different physical states or morphologies.

Optical frequency combs are fundamentally important in precision measurement physics, bringing unprecedented capabilities of measurements for time keeping, metrology, and spectroscopy. In this work, we investigate theoretically the formation of a form of frequency combs in cavity optomagnonics, in which a ferrimagnetic insulator sphere supports optical whispering gallery modes for both light photons and magnons. Numerical simulations of the optomagnonic dynamics show that a robust frequency comb can be obtained at low power under the bichromatic pumping drive, and the comb spacing is adjustable. Furthermore, the optomagnonic frequency comb structure has abundant non-perturbative features, suggesting that the magnon-induced Brillouin light scattering process in cavity optomagnonics may also exhibit phenomena similar to those in atomic–molecular systems. In addition to providing insight into optomagnonic nonlinearity, optomagnonic frequency combs may also provide the feasibility of implementing frequency combs based on spintronic platforms and may find applications for precision metrology based on magnonic devices.

Perovskite-enabled optical devices have drawn intensive interest and have been considered promising candidates for integrated optoelectronic systems. As one of the important photonic functions, optical phase modulation previously was demonstrated with perovskite substrate and complex refractive index engineering with laser scribing. Here we report on the new scheme of achieving efficient phase modulation by combining detour phase design with 40 nm ultrathin perovskite films composed of nanosized crystalline particles. Phase modulation was realized by binary amplitude patterning, which significantly simplifies the fabrication process. Perovskite nanocrystal films exhibit significantly weak ion migration effects under femtosecond laser writing, resulting in smooth edges along the laser ablated area and high diffractive optical quality. Fabrication of a detour-phased perovskite ultrathin planar lens with a diameter of 150 μm using femtosecond laser scribing was experimentally demonstrated. A high-performance 3D focus was observed, and the fabrication showed a high tolerance with different laser writing powers. Furthermore, the high-quality imaging capability of perovskite ultrathin planar lenses with a suppressed background was also demonstrated.

Active terahertz (THz) beam manipulation is urgently needed for applications in wireless communication, radar detection, and remote sensing. In this work, we demonstrate a liquid crystal (LC) integrated Pancharatnam–Berry (PB) metadevice for active THz beam manipulation. Through theoretical analysis and simulation design, the geometric phase of the PB metasurface is engineered to match the tunable anisotropic phase shift of LCs under an external magnetic field, and dynamic beam deflection accompanied by spin conversion is obtained. The experimental results show that the device realizes a dynamic modulation depth of >94% and maximum efficiency of over 50% for the different spin states. Moreover, due to the broadband operating characteristics of devices at 0.7–1.3 THz, the deflection angles are frequency dependent with a scanning range of over ±20° to ±32.5°. Moreover, the two conjugate spin states are always spatially separated in different deflection directions with an isolation degree of over 10 dB. Therefore, this metadevice provides a scheme of active THz beam deflection and spin state conversion, and it also achieves both controllable wavelength division multiplexing and spin division multiplexing, which have important potential in large-capacity THz wireless communication.

An N×N iterative photonic processor is proposed for the first time, we believe, for fast computation of complex-valued matrix inversion, a fundamental but computationally expensive linear algebra operation. Compared to traditional digital electronic processing, optical signal processing has a few unparalleled features that could enable higher representational efficiency and faster computing speed. The proposed processor is based on photonic integration platforms–the inclusion of III-V gain blocks offers net neutral loss in the phase-sensitive loops. This is essential for the Richardson iteration method that is adopted in this paper for complex linear systems. Wavelength multiplexing can be used to significantly improve the processing efficiency, allowing the computation of multiple columns of the inverse matrix using a single processor core. Performances of the key building blocks are modeled and simulated, followed by a system-level analysis, which serves as a guideline for designing an N×N Richardson iteration processor. An inversion accuracy of >98% can be predicted for a 64×64 photonic processor with a >80 times faster inversion rate than electronic processors. Including the power consumed by both active components and electronic circuits, the power efficiency of the proposed processor is estimated to be over an order of magnitude more energy-efficient than electronic processors. The proposed iterative photonic integrated processor provides a promising solution for future optical signal processing systems.

Whispering gallery mode (WGM) microcavities provide increasing opportunities for precision measurement due to their ultrahigh sensitivity, compact size, and fast response. However, the conventional WGM sensors rely on monitoring the changes of a single mode, and the abundant sensing information in WGM transmission spectra has not been fully utilized. Here, empowered by machine learning (ML), we propose and demonstrate an ergodic spectra sensing method in an optofluidic microcavity for high-precision pressure measurement. The developed ML method realizes the analysis of the full features of optical spectra. The prediction accuracy of 99.97% is obtained with the average error as low as 0.32 kPa in the pressure range of 100 kPa via the training and testing stages. We further achieve the real-time readout of arbitrary unknown pressure within the range of measurement, and a prediction accuracy of 99.51% is obtained. Moreover, we demonstrate that the ergodic spectra sensing accuracy is ∼11.5% higher than that of simply extracting resonating modes’ wavelength. With the high sensitivity and prediction accuracy, this work opens up a new avenue for integrated intelligent optical sensing.

Random scattering of light in disordered media can be used for highly sensitive speckle-based wavemeters and spectrometers. However, the multiple scattering events that fold long optical paths within a compact space also make such devices exceedingly sensitive to vibrations and small disturbances to the disordered media. Here, we show how scattering can be engineered so that it can be used for a compact computational spectrometer that is largely insensitive to environmental factors. We designed and fabricated a three-dimensional pseudo-random nano-void pattern with 62% scattering efficiency. The controlled amount of multiple scattering ensured a sufficiently long optical path for the target resolution of 100 pm, with optimal long-term stability. The 200-μm-thick scattering silica substrate was integrated in a compact assembly with a low-cost camera sensor. The target resolution was achieved for full spectrum measurements while single wavelengths could be determined with 50 pm resolution. Such tailored scattering systems can improve the trade-off among cost, size, stability, and spectral resolution in computational spectrometers.

Quantum dots (QDs) offer an interesting alternative for traditional phosphors in on-chip light-emitting diode (LED) configurations. Earlier studies showed that the spectral efficiency of white LEDs with high color rendering index (CRI) values could be considerably improved by replacing red-emitting nitride phosphors with narrowband QDs. However, the red QDs in these studies were cadmium-based, which is a restricted element in the EU and certain other countries. The use of InP-based QDs, the most promising Cd-free alternative, is often presented as an inferior solution because of the broader linewidth of these QDs. However, while narrow emission lines are the key to display applications that require a large color gamut, the spectral efficiency penalty of this broader emission is limited for lighting applications. Here, we report efficient, high-CRI white LEDs with an on-chip color converter coating based on red InP/ZnSe QDs and traditional green/yellow powder phosphors. Using InP/ZnSe QDs with a quantum yield of nearly 80% and a full width at half-maximum of 45 nm, we demonstrate high spectral efficiency for white LEDs with very high CRI values. One of the best experimental results in terms of both luminous efficacy and color rendering performance is a white LED with an efficacy of 132 lm/W, and color rendering indices of Ra≈90, R9≈50 for CCT ≈ 4000 K. These experimental results are critically compared with theoretical benchmark values for white LEDs with on-chip downconversion from both phosphors and red Cd-based QDs. The various loss mechanisms in the investigated white LEDs are quantified with an accurate simulation model, and the main impediments to an even higher efficacy are identified as the blue LED wall-plug efficiency and light recycling in the LED package.

High detectivity is essential for solar-blind deep-ultraviolet (DUV) light detection because the DUV signal is extremely weak in most applications. In this work, we report ultrahigh-detectivity AlGaN-based solar-blind heterojunction-field-effect phototransistors fabricated utilizing dual-float-photogating effect. The p+-Al0.4GaN layer and Al0.4GaN absorber layer deposited on the Al0.6GaN barrier serve as top pin-junction photogate, while the thin Al0.4GaN channel layer with a strong polarization field inside acts as virtual back photogate. Due to the effective depletion of the two-dimensional electron gas at the Al0.6Ga0.4N/Al0.4Ga0.6N heterointerface by the top photogate, the dark current was suppressed below 2 pA in the bias range of 0 to 10 V. A high photo-to-dark current ratio over 108 and an optical gain of 7.5×104 were demonstrated at a bias of 5 V. Theoretical analysis indicates that the optical gain can be attributed to the joint action of the floating top and back photogates on the channel current. As a result, a record high flicker noise (Johnson and shot noise) limited specific detectivity of 2.84×1015(2.91×1017) cm Hz0.5 W-1 was obtained. Furthermore, high response speed at the microsecond level was also shown in the devices. This work provides a promising and feasible approach for high-sensitivity DUV detection.

An integrated polymer microtoroid resonator system fabricated using single-step, two-photon polymerization laser lithography is reported. The integrated microtoroid-waveguide system includes an integrated coupling waveguide for ease of optical coupling and has a quality factor of 16,000. We further demonstrate a nested double-spiral waveguide that allows higher spatial density of data, akin to a macroscopic form of space-division multiplexing. Experimental characterization reveals good transmission properties in the double-spiral waveguide device. In addition, the waveguides are demonstrated to support 30 Gb/s nonreturn-to-zero and 28 Gb/s pulse amplitude modulation 4 high-speed data.

We demonstrated 10×10 arrays of InGaN 17 μm×17 μm micro-light-emitting diodes (μLEDs) with a peak wavelength from 662 to 630 nm at 10–50 A/cm2. The on-wafer external quantum efficiency reached 0.18% at 50 A/cm2. The output power density of the red μLEDs was obtained as 1.76 mW/mm2, which was estimated to be higher than that of 20 μm×20 μm AlInGaP red μLEDs (∼630 nm). Finally, we demonstrate that InGaN red/green/blue μLEDs could exhibit a wide color gamut covering 81.3% and 79.1% of the Rec. 2020 color space in CIE 1931 and 1976 diagrams, respectively.

It is still very challenging to determine a freeform lens for converting a given input beam into a prescribed output beam where not only the irradiance distribution but also the phase distribution hardly can be expressed analytically. Difficulties arise because the ray mapping from the input beam to the output beam is not only intertwined with the required double freeform surfaces but also intertwined with the output phase distribution, whose gradient represents the directions of the output rays. Direct determination of such a problem is very difficult. Here, we develop a special iterative wavefront tailoring (IWT) method to tackle this problem. In a certain iteration, the current calculation data of the double freeform surfaces and the output phase gradient are used to update the coefficients of a Monge–Ampère equation describing an intermediate wavefront next to the entrance freeform surface. The solution to the wavefront equation could lead to an improved ray mapping to be used to update the corresponding phase gradient data and reconstruct the double freeform surfaces. In a demonstrative example that deviates much from the paraxial or small-angle approximation, the new IWT method can generate a freeform lens that performs much better than that designed by a conventional ray mapping method for producing two irradiance distributions in the forms of numerals “1” and “2” on two successive targets, respectively.

An optical funnel, which performs as a passive electromagnetic compressor, can guide electromagnetic waves from a wide inlet to a narrow outlet without reflectance/scattering and squeeze electromagnetic fields uniformly to an air neck. In this study, an optical funnel is designed by precisely filling subwavelength ceramic blocks with a gradient refractive index inside a tapered waveguide. The gradient refractive index is designed by transformation optics, which is isotropic and all above unit, thus exhibiting a broadband feature. Due to the mechanism of impedance matching over the whole funnel, extremely low reflectance/scattering and stable enhancement of fields can be achieved. The field enhancement factor in different regions of the funnel (e.g., in the air neck) can be flexibly designed just by modifying the funnel-width ratios.

Metasurfaces composed of meta-atoms provide promising platforms for manipulating amplitude, phase, and polarization of light. However, the traditional design methods of metasurfaces are time consuming and laborious. Here, we propose a bidirectional cascaded deep neural network with a pretrained autoencoder for rapid design of dielectric metasurfaces in the range of 450 nm to 850 nm. The forward model realizes a prediction of amplitude and phase responses with a mean absolute error of 0.03. Meanwhile, the backward model can retrieve patterns of meta-atoms in an inverse-design manner. The availability of this model is demonstrated by database establishment, model evaluation, and generalization testing. Furthermore, we try to reveal the mechanism behind the model in a visualization way. The proposed approach is beneficial to reduce the cost of computation burden and improve nanophotonic design efficiency for solving electromagnetic on-demand design issues automatically.

In this work, we propose a novel approach to produce three-dimensional (3D) optical trapping with sub-wavelength size through an engineered microsphere, under linear polarization states of an incident light. The engineered microsphere is designed to contain the segmented regions of diffractive patterns and made by focused ion beam fabrication. We simulate and experimentally characterize the focus performance of the engineered microsphere. The emitted light field from the exit surface of the engineered microsphere forms a pair of axially arranged focused beams, and they are connected with a continuous optical field embracing a 3D optical null at the center, forming the so-called optical bottle beam. Experimental results and numerical simulation are in good agreement. Such micro-optics can be used for precise and localized optical trapping.

The design of large-scale, high-numerical-aperture, and broadband achromatism is a big challenge in metalens research. In fact, many colorful imaging systems have RGB color filters, which means the achromatism only for RGB lights would be sufficient. Avoiding broadband achromatism is expected to greatly improve the working efficiency of metalenses. Nevertheless, a proper bandpass filter is necessary under a white light illumination in the metalens integrated imaging system. Here we propose a bandpass-filter-integrated multiwavelength achromatic metalens (NA=0.2), which is designed using a searching optimization algorithm to achieve the achromatism of RGB lights with high efficiencies. The bandpass filter is implemented by composite DBRs and defect layers, by which three desired wavelengths are selected out. The simulations and experiments on the filter-integrated metalens definitely show a good RGB achromatism. Further imaging experiments demonstrate a higher signal-to-noise ratio and resolution compared with the one without the filter. Our approach provides not only an RGB achromatic meta-imaging device but also a new route to access a highly efficient spectrum tailoring metasystem by incorporating bandpass filter designs.

Optical tweezers (OTs) and optical spanners (OSs) are powerful tools of optical manipulation, which are responsible for particle trapping and rotation, respectively. Conventionally, the OT and OS are built using bulky three-dimensional devices, such as microscope objectives and spatial light modulators. Recently, metasurfaces are proposed for setting up them on a microscale platform, which greatly miniaturizes the systems. However, the realization of both OT and OS with one identical metasurface is posing a challenge. Here, we offer a metasurface-based solution to integrate the OT and OS. Using the prevailing approach based on geometric and dynamic phases, we show that it is possible to construct an output field, which promises a high-numerical-aperture focal spot, accompanied with a coaxial vortex. Optical trapping and rotation are numerically demonstrated by estimating the mechanical effects on a particle probe. Moreover, we demonstrate an on-demand control of the OT-to-OS distance and the topological charge possessed by the OS. By revealing the OT–OS metasurfaces, our results may empower advanced applications in on-chip particle manipulation.

With the advent and rapid development of the transformation optics and metamaterials, invisibility cloaks have captivated much attention in recent years. While most cloaking schemes suffer from limited bandwidth, the carpet cloak, which can hide an object on a reflecting plane, can operate over a broadband frequency range. However, the carpet cloaks experimentally realized thus far still have several limitations. For example, the quasi-conformal mapping carpet cloak leads to a lateral shift of the reflected light ray, while the birefringent carpet cloak only works for a specific polarization. In this work, we propose a conformal transformation scheme to tackle these two problems simultaneously. As an example, we design a mid-infrared carpet cloak in a silicon platform and demonstrate its polarization-insensitive property as well as the minimized lateral shift over a broad frequency band from 24 to 28.3 THz.

In this work, a self-powered GaN-based metal-semiconductor-metal photodetector (MSM PD) with high responsivity has been proposed and fabricated. The proposed MSM PD forms an asymmetric feature by using the polarization effect under one electrode, such that we adopt an AlGaN/GaN heterojunction to produce the electric field, and by doing so, an asymmetric energy band between the two electrodes can be obtained even when the device is unbiased. The asymmetric feature is proven by generating the asymmetric current-voltage characteristics both in the dark and the illumination conditions. Our results show that the asymmetric energy band enables the self-powered PD, and the peak responsivity wavelength is 240 nm with the responsivity of 0.005 A/W. Moreover, a high responsivity of 13.56 A/W at the applied bias of 3 V is also achieved. Thanks to the very strong electric field in the charge transport region, when compared to the symmetric MSM PD, the proposed MSM PD can reach an increased photocurrent of 100 times larger than that for the conventional PD, even if the illumination intensity for the light source becomes increased.

Spatiospectral structure of wave phase matching in birefringent crystals has a strong dependence on the geometry of the acousto-optic interaction and incident light spectrum. This dependence defines details of light beam profile transformation. It is especially important for imaging applications related to a large angular aperture and a wide spectral bandwidth of the incident light. In this paper, we demonstrate accurate three-dimensional plotting of a light transmission pattern without small birefringence approximation. The rather complicated shape of the phase-matching locus in the spatiospectral domain inevitably leads to residual spatially nonuniform chromatic aberrations in the spectral image. Theoretical consideration and computational modeling are confirmed by the experiments on Bragg diffraction in paratellurite crystal. The results are especially important for the development of acousto-optical imaging devices and laser beam shaping technologies.

Pressure-temperature cross-sensitivity and its accompanying temperature-related stability is a nerve-wracking obstruction for pressure sensor performance in a wide temperature range. To solve this problem, we propose a novel (to the best of our knowledge) all-silicon dual-cavity optical Fabry–Perot interferometer (FPI) pressure sensor. The all-silicon structure has high intrinsic reflectivity and is able to eliminate the influence of thermal-expansion-mismatch-induced stress and chemical-reaction-induced gas generation, and therefore, in essence, enhances measurement accuracy. From the experiment results, the pressure-temperature cross-sensitivity is reduced to be ～5.96 Pa/°C, which presents the lowest pressure-temperature cross-sensitivity among the FPI pressure sensors with the capability of surviving high temperatures up to 700°C thereby opening the way for high-precision pressure monitoring in various harsh and remote environments.

Multimode and random directionalities are major issues restricting the application of whispering gallery mode microcavity lasers. We demonstrated a 40 μm diameter microring with an off-centered embedded hole and warped geometry from strained III-nitride quantum well multilayers. Single-mode directional whispering gallery mode lasing was achieved by the warped structure and high-order mode suppression induced by the off-centered hole. In addition, the introduction of the off-centered hole reduced the lasing threshold from 3.24 to 2.79 MW/cm2 compared with the warped microdisk without an embedded hole while maintaining a high-quality factor of more than 4000. Directional light emission in 3D was achieved and attributed to the warped structure, which provides a vertical component of the light emission, making it promising for building multifunctional coherent light sources in optoelectronic integration.

Amorphous materials are attractive candidates for fabricating the superconducting nanowire single-photon detectors (SNSPDs) due to their superior tolerance and scalability over crystalline niobium nitride. However, the reduced superconducting transition temperature degenerates both operating temperature and saturation efficiency. Herein, the SNSPD (6.5 nm thickness and 50 nm width) based on the amorphous Mo0.8Si0.2 film with a high optical absorption coefficient demonstrates close-to-unity intrinsic detection efficiency for 1550 nm photons from 75 mK to 2.2 K. Further, a high-performance array SNSPD with optimized 90 nm-width wires is also demonstrated. As-fabricated uniform 4-pixel SNSPD exhibits a saturation plateau for the photon counts at 2.2 K, which overcomes the limitation of operation at low temperature (1 K) for traditional amorphous SNSPDs. Coupled with superior intrinsic quantum efficiency, highly efficient photon counts, and low dark count ratio, this detector paves a way for achieving high efficiency and superior yield for large array systems.

There is still a lack of high-performance terahertz (THz) modulators with wide operation bandwidth and large modulation depth due to the underlying physics limitation behind existing approaches. Meanwhile, for many applications, simple compact THz modulators working straightforward in the transmission mode are also highly desired. Here, we demonstrate a THz modulator with a maximal transmission-amplitude modulation depth of 99.9% (switching ratio of 1000) based on a commonly used silica-on-silicon structure. Different from those reported graphene or metamaterials enhanced proposals, the device we proposed works within a reversible avalanche breakdown region of silicon that has not been studied yet and has the potential to modulate/switch THz waves efficiently. Further, we proved that the modulation depth exceeds 97% in the frequency range from 0.2 to 1 THz in the experiment. The simplicity and generality of this new type of near-perfect THz modulator will undoubtedly attract lots of attention of researchers in the near future due to its potential to be engineered into integrated devices.

Chaotic dynamics in optical microcavities, governed dominantly by manifolds, is of great importance for both fundamental studies and photonic applications. Here, we report the experimental observation of a stable manifold characterized by energy and momentum evolution in the nearly chaotic phase space of an asymmetric optical microcavity. By controlling the radius of a fiber coupler and the coupling azimuth of the cavity, corresponding to the momentum and position of the input light, the injected light can in principle excite the system from a desired position in phase space. It is found that once the input light approaches the stable manifold, the angular momentum of the light experiences a rapid increase, and the energy is confined in the cavity for a long time. Consequently, the distribution of the stable manifold is visualized by the output power and the coupling depth to high-Q modes extracted from the transmission spectra, which is consistent with theoretical predictions by the ray model. This work opens a new path to understand the chaotic dynamics and reconstruct the complex structure in phase space, providing a new paradigm of manipulating photons in wave chaos.

We report a broadband two-dimensional (2D) Kerr and Raman–Kerr frequency comb generation in a silica bottle resonator accounting for azimuthal and axial degrees of freedom and pioneer a method that allows for controlled and reversible switching between a four wave mixing (FWM) state and a stimulated Raman scattering state. The repetition rate of the Raman–Kerr comb is not an integer number of the free spectral range, which spans more than 242 nm with hundreds of teeth. We show that, experimentally and numerically, multiple 2D comb regimes can be selectively accessed via dispersion engineering by exciting different orders of axial modes or modifying the curvature of the axial profile, involving cascaded FWM, Raman lasing, and Raman-assisted FWM. The effect of axial curvature on dispersion is associated with the axial mode number in bottle resonators. Our approach enables dispersion and spectral engineering flexibility in any resonator with localized axial modes.

SnO2 has attracted considerable attention due to its wide bandgap, large exciton binding energy, and outstanding electrical and optoelectronic features. Owing to the lack of reliable and reproducible p-type SnO2, many challenges on developing SnO2-based optoelectronic devices and their practical applications still remain. Herein, single-crystal SnO2 microwires (MWs) are acquired via the self-catalyzed approach. As a strategic alternative, n-SnO2 MW/p-GaN heterojunction was constructed, which exhibited selectable dual-functionalities of light-emitting and photodetection when operated by applying an appropriate voltage. The device illustrated a distinct near-ultraviolet light-emission peaking at ∼395.0 nm and a linewidth ∼50 nm. Significantly, the device characteristics, in terms of the main peak positions and linewidth, are nearly invariant as functions of various injection current, suggesting that quantum-confined Stark effect is essentially absent. Meanwhile, the identical n-SnO2 MW/p-GaN heterojunction can also achieve photovoltaic-type light detection. The device can steadily feature ultraviolet photodetecting ability, including the ultraviolet/visible rejection ratio (R360 nm/R400 nm) ∼1.5×103, high photodark current ratio of 105, fast response speed of 9.2/51 ms, maximum responsivity of 1.5 A/W, and detectivity of 1.3×1013 Jones under 360 nm light at -3 V bias. Therefore, the bifunctional device not only displays distinct near-ultraviolet light emission, but also has the ability of high-sensitive ultraviolet photodetection. The novel design of n-SnO2 MW/p-GaN heterojunction bifunctional systems is expected to open doors to practical application of SnO2 microstructures/nanostructures for large-scale device miniaturization, integration and multifunction in next-generation high-performance photoelectronic devices.

By using a high-contrast grating (HCG, high transmittance >90%) to control the phase shift of incident light, we theoretically designed a novel-structured HCG-integrated superconducting nanowire single-photon detector (HCG-SNSPD) with a high-efficiency and large light-receiving area. Without enlarging the typical single-pixel SNSPD nanowire area (10 μm×10 μm), the effective detection area is expanded to 115 μm, while the absorption efficiency of the nanowire reaches 84.9% at a wavelength of 1550 nm. The effective detection area of HCG-SNSPD is increased by 11.5 times compared to that of conventional single-pixel SNSPDs. Moreover, the absorption efficiencies of HCG-SNSPD exceed 70% at wavelengths ranging from 1460 nm to 1650 nm, indicating high-efficiency broadband detection. This study promotes new possibilities for the application of SNSPDs.

Surface-modified semiconductors show enormous potential for opto-terahertz (THz) spatial modulation due to their enhanced modulation depth (MD) along with their inherent broad bandwidth. Taking full advantage of the surface modification, a performance-enhanced, all-optical, fast switchable THz modulator was achieved here based on the surface-passivated GaAs wafer. With a decreased surface recombination rate and prolonged carrier lifetime induced by passivation, S-passivated GaAs was demonstrated as a viable candidate to enhance THz modulation performance in MD, especially at low photodoping levels. Despite a degraded modulation rate owing to the longer carrier lifetime, this passivated GaAs modulator simultaneously realizes a fast modulation at a 69-MHz speed and as high an MD as ∼94% in a spectral wideband of 0.2–1.2 THz. The results demonstrated a new strategy to alleviate the tradeoff between high MD and speed in contrast to bare surfaces or heterogeneous films/unusual geometry on semiconductors including Si, Ge, and GaAs.

Compared with traditional optical elements, metasurfaces have shown unique advantages in multifunctionality encoded in different frequencies, polarization states, and orbital angular momentums. However, the study of metasurfaces with well-controlled functions under different incident angles is still in its infancy. Here we propose a general method to tailor the angular dispersion over the simplest binary dielectric grating in the transmission mode. We demonstrate that the angular response is strongly related to the number of waveguide modes inside the grating, so one can intentionally reduce or enhance the angular dispersion by controlling the number of waveguide modes. Independent phase manipulation over incident angles is experimentally demonstrated by a metalens with angle-dependent focus. The angular dispersion in orthogonal polarization states is further utilized to demonstrate angle-insensitive and angle-multiplexed wave plates. These devices with simple configuration and clear physics offer a general platform to expand the scope of beam manipulation over metasurfaces.

Recently, multilevel diffractive lenses (MDLs) have attracted considerable attention, mainly due to their superior wave-focusing performance; however, efforts to reduce chromatic aberration are still ongoing. Here, we present a numerical design and experimentally demonstrate a high-numerical aperture (∼0.99), diffraction-limited achromatic multilevel diffractive lens (AMDL), operating in the microwave range of 10–14 GHz. A multi-objective differential evolution (MO-DE) algorithm was incorporated with the three-dimensional (3D) finite-difference time-domain method to optimize the heights and widths of each concentric ring (zone) of the AMDL structure. To the best of our knowledge for the first time, in this study, the desired focal distance was also treated as an optimization parameter in addition to the structural parameters of the zones. Thus, MO-DE diminishes the necessity of predetermined focal distance and center wavelength by also providing an alternative method for phase profile tailoring. The proposed AMDL can be considered an ultra-compact and flat lens since it has the radius of 3.7λc and a thickness of ∼λc, where λc is the center wavelength of 24.98 mm (i.e., 12 GHz). The numerically calculated full width at half maximum values are 0.554λ and focusing efficiency values are varying between 28% and 45.5%. To experimentally demonstrate the functionality of the optimized lens, the AMDL composed of polylactic acid material polymer is fabricated via 3D-printing technology. The numerical and experimental results are compared, discussed in detail, and observed to be in good agreement. Moreover, the verified AMDL in the microwave regime is scaled down to the visible wavelengths to observe achromatic and diffraction-limited focusing behavior between 380 and 620 nm wavelengths.

The achromatic subdiffraction lens with large numerical aperture (NA) is of significant importance in optical imaging, photolithography, spectroscopy, and nanophotonics. However, most of the previous research on subdiffraction lenses has been restricted by limited bandwidth and efficiency as well as severe chromatic aberrations. In this paper, a semicircular gradient index lens (sGRIN) with a modified refractive index profile originated from a Maxwell fish-eye lens is put forward to achieve highly efficient (above 81%) achromatic (4–20 GHz) subdiffraction focusing at the focusing line (around 0.28λ) with large NA of 1.3 and broadband diffraction-limited far-field radiation (4–16 GHz) theoretically, which overcomes the drawbacks of previous works. The presented lens is designed by gradient dielectric metamaterials. Evanescent waves ignited at the lens/air interface and transformation of electromagnetic (EM) waves with high spatial frequency in sGRIN to EM waves with low spatial frequency in air are responsible for subdiffraction focusing and diffraction-limited far-field radiation, respectively. Experimental results demonstrate the excellent performance of achromatic subdiffraction focusing and diffraction-limited far-field radiation. The presented lens has great potential to be applied in subdiffraction imaging systems.

The importance of tunable subwavelength optical devices in modern electromagnetic and photonic systems is indisputable. Herein, a lithography-free, wide-angle, and reconfigurable subwavelength optical device with high tunability operating in the near-infrared regions is proposed and experimentally demonstrated, based on a reversible nanochemistry approach. The reconfigurable subwavelength optical device basically comprises an ultrathin copper oxide (CuO) thin film on an optical thick gold substrate by utilizing the reversible chemical conversion of CuO to sulfides upon exposure to hydrogen sulfide gas. Proof-of-concept experimental results show that the maximal modulation depth of reflectance can be as high as 90% at the wavelength of 1.79 μm with the initial thickness of CuO taken as 150 nm. Partially reflected wave calculations combined with the transfer matrix method are employed to analytically investigate the optical properties of the structure, which show good agreement with experimental results. We believe that the proposed versatile approaches can be implemented for dynamic control management, allowing applications in tunable photonics, active displays, optical encryption, and gas sensing.

A high-birefringence spiral Sagnac waveguide (SSW) device fabricated via direct laser writing (DLW) using a two-photon polymerization (2PP) technique is proposed, designed, and experimentally demonstrated as an ultrahigh magnetic field sensor. The sensor comprises a Y-style tapered waveguide and an SSW containing two microfluidic channels. The SSW has a total length of ∼2.4 mm and a spiral radius of ∼200 μm. Due to the asymmetric structure, the SSW has a high birefringence of 0.016, which can be designed as a magnetic field sensor, as a magnetic fluid can be filled into the microfluidic channel changing the guiding mode and the birefringence and consequently leading to a change in phase of the interferometer when the applied magnetic field changes. The experimental results show that the proposed photonic device has a sensitivity to magnetic fields as high as 0.48 nm/Oe within a range from 10 to 100 Oe. The proposed device is very stable and easy to fabricate, and it can therefore be used for weak magnetic field detection.

Storing a very high frequency (VHF) band (30–300 MHz) electromagnetic wave has many potential applications, such as phase modulation, buffering, and radio frequency memory. It can be effectively achieved by applying coupled resonator-based electromagnetically induced transparency (EIT) due to its slow light effect. However, the wavelength in the VHF band is too long to design resonators, and the group delay is limited by the high resistive loss of metal. The practical application of EIT in the VHF band is still a big challenge. In this work, we propose and experimentally demonstrate EIT response in a high-temperature superconducting (HTS) microwave circuit with coupled-resonator-induced transparency. The chip size of the HTS circuit is only 34 mm×20 mm with a very low transparency frequency of 198.55 MHz. In addition, we implement very large group delay higher than 12.3 μs and 16.2 μs with working temperatures of 65 K and 50 K separately, which is much longer than the previous reported works on slow wave. The fabricated circuit is planar with working temperature about 65 K, and thus can be easily integrated into other microwave devices under the cryogenic conditions provided by a commercial portable Stirling cryocooler. Our proposed method paves a way for studying EIT in the microwave region due to the high quality factor of the HTS resonator, which has great potential use for radio-frequency memory in the future.

Soliton microcombs (SMCs) are spontaneously formed in a coherently pumped high-quality microresonator, which provides a new tool for use as an on-chip frequency comb for applications of high-precision metrology and spectroscopy. However, generation of SMCs seriously relies on advanced experimental techniques from professional scientists. Here, we experimentally demonstrate a program-controlled single SMC source where the intracavity thermal effect is timely balanced using an auxiliary laser during single SMC generation. The microcomb power is adopted as the criteria for microcomb states discrimination and a forward and backward thermal tuning technique is employed for the deterministic single SMC generation. Further, based on a closed-loop control system, the repetition rate stability of the SMC source improved more than 20 times and the pump frequency can be continuously tuned by simply changing the operation temperature. The reliability of the SMC source is verified by consecutive 200 generation trials and maintaining over 10 h. We believe the proposed SMC source will have significant promising influences in future SMC-based application development.

Dissipative Kerr solitons offer broadband coherent and low-noise frequency combs and stable temporal pulse trains, having shown great potential applications in spectroscopy, communications, and metrology. Breathing solitons are a particular kind of dissipative Kerr soliton in which the pulse duration and peak intensity show periodic oscillation. Here we have investigated the breathing dissipative Kerr solitons in silicon nitride (Si3N4) microrings, while the breathing period shows uncertainties of around megahertz (MHz) order in both simulation and experiments. This instability is the main obstacle for future applications. By applying a modulated signal to the pump laser, the breathing frequency can be injection locked to the modulation frequency and tuned over tens of MHz with frequency noise significantly suppressed. Our demonstration offers an alternative knob for the control of soliton dynamics in microresonators and paves a new avenue towards practical applications of breathing solitons.

We study light transmission and reflection from an integrated microresonator device, formed by a circular microresonator coupled to a bus waveguide, with an embedded S-shaped additional crossover waveguide element that selectively couples counter-propagating modes in a propagation-direction-dependent way. The overall shape of the device resembles a “taiji” symbol, hence its name. While Lorentz reciprocity is preserved in transmission, the peculiar geometry allows us to exploit the non-Hermitian nature of the system to obtain high-contrast unidirectional reflection with negligible reflection for light incident in one direction and a significant reflection in the opposite direction.

Magnetostrictive optomechanical cavities provide a new optical readout approach to room-temperature magnetometry. Here we report ultrasensitive and ultrahigh bandwidth cavity optomechanical magnetometers constructed by embedding a grain of the magnetostrictive material Terfenol-D within a high quality (Q) optical microcavity on a silicon chip. By engineering their physical structure, we achieve a peak sensitivity of 26 pT/Hz comparable to the best cryogenic microscale magnetometers, along with a 3 dB bandwidth as high as 11.3 MHz. Two classes of magnetic response are observed, which we postulate arise from the crystallinity of the Terfenol-D. This allows single crystalline and polycrystalline grains to be distinguished at the level of a single particle. Our results may enable applications such as lab-on-chip nuclear magnetic spectroscopy and magnetic navigation.

Manipulating Airy beams to symmetric Airy beams (SABs) with abruptly autofocusing and self-accelerating properties has attracted much attention. With such a particular propagation dynamic, SABs may provide great potential in dynamic signal imaging. On the other hand, the generation of SABs by spatial light modulators suffers from the limitations of phase gradient accuracy, low optical efficiency (40%), and a bulky footprint. Therefore, exploring imaging applications and optimal generation methods of these Airy-type beams deserves further research. Here, based on the coordinate transformation of SAB, an asymmetric Airy beam (AAB) is realized. Symmetric/asymmetric cubic phase microplates (SCPPs/ACPPs) are designed and fabricated for generating SAB/AAB. The SCPP/ACPP demonstrates superior performance: compact construction (60 μm×60 μm×1.1 μm), continuous variation of phase, high efficiency (～81% at 532 nm), and broadband operation from 405 to 780 nm. Dynamic imaging under monochromatic and polychromatic lights is realized by the SAB/AAB, indicating various results at different propagation distances with a certain initial signal. Further investigation reveals that the SCPP on a soft substrate maintains its physical dimensions and optical properties unchanged during stretching. Our work enables wide potential applications in integrated optics, beam manipulation, and imaging.

A novel type of mid-IR microresonator, the chalcogenide glass (ChG) microfiber knot resonator (MKR), is demonstrated, showing easy fabrication, fiber-compatible features, resonance tunability, and high robustness. ChG microfibers with typical diameters around 3 μm are taper-drawn from As2S3 glass fibers and assembled into MKRs in liquid without surface damage. The measured Q factor of a typical 824 μm diameter ChG MKR is about 2.84×104 at the wavelength of 4469.14 nm. The free spectral range (FSR) of the MKR can be tuned from 2.0 nm (28.4 GHz) to 9.6 nm (135.9 GHz) by tightening the knot structure in liquid. Benefitting from the high thermal expansion coefficient of As2S3 glass, the MKR exhibits a thermal tuning rate of 110 pm·°C?1 at the resonance peak. When embedded in polymethyl methacrylate (PMMA) film, a 551 μm diameter MKR retains a Q factor of 1.1×104. The ChG MKRs demonstrated here are highly promising for resonator-based optical technologies and applications in the mid-IR spectral range.

Metamaterial absorbers (MAs) serve as important electromagnetic wave-absorbing devices that have captured the attention of researchers for a long term. Functioning as sensitive detectors to determine perturbations in an ambient environment is another significant subsidiary function. Here, we theoretically propose an optimized fabrication method to implement terahertz MAs with fewer steps and also evaluate both absorption and sensing performances of such MAs realized by the new method. Simulation findings demonstrate that such MAs can basically maintain the original absorption features perfectly, including near-complete absorption at resonance as well as strong robustness to wide incident angles. Specifically, the full width at half-maximum and quality factor of the absorption resonances attenuate less than 26% and 8% with this new method, remaining in the ranges of ～0.03–0.04 THz and ～20–27 for two selected example MAs. More significantly, sensing capacities of this type of MA, in terms of maximum detection range (enhancing at least 9%), observable spectral modulation (increasing at least 6.3%), and refractive index sensitivity, are improved to a large extent because of more intense coupling between resonant field and matter in the case of surface-relief MAs. This stronger coupling results from exposing more spots of the resonantly high field to direct contact with an approaching analyte, which is illustrated by field profiles of the MAs at resonance in this work. Additionally, other desirable absorber features are also explored with such MAs, like functioning as building blocks to configure multiband MAs and strong robustness against fabrication errors. Such new-style terahertz MAs shown in the paper, acting as good examples, not only prove that terahertz MAs can be fabricated by the proposed time- and cost-saving route in contrast to the traditional MA fabrication process, but also can serve as novel platforms to explore other intriguing terahertz photonic effects, such as the field enhancement effect.

The ability to sense dynamic biochemical reactions and material processes is particularly crucial for a wide range of applications, such as early-stage disease diagnosis and biomedicine development. Optical microcavities-based label-free biosensors are renowned for ultrahigh sensitivities, and the detection limit has reached a single nanoparticle/molecule level. In particular, a microbubble resonator combined with an ultrahigh quality factor (Q) and inherent microfluidic channel is an intriguing platform for optical biosensing in an aqueous environment. In this work, an ultrahigh Q microbubble resonator-based sensor is used to characterize dynamic phase transition of a thermosensitive hydrogel. Experimentally, by monitoring resonance wavelength shift and linewidth broadening, we (for the first time to our knowledge) reveal that the refractive index is increased and light scattering is enhanced simultaneously during the hydrogel hydrophobic transition process. The platform demonstrated here paves the way to microfluidical biochemical dynamic detection and can be further adapted to investigating single-molecule kinetics.

Herein we propose a novel strategy to enhance surface plasmon resonance (SPR) by introducing a photonic cavity into a total-internal-reflection architecture. The photonic cavity, which is comprised of a highly reflective photonic crystal (PC), defect layers, and a gold (Au) film, enables Fabry–Perot (FP) resonances in the defect layers and therefore narrows the SPR resonance width in the metallic surface as well as increases the electric field intensity and penetration depth in the evanescent region. The fabricated sensor exhibits a 5.7-fold increase in the figure of merit and a higher linear coefficient as compared with the conventional Au-SPR sensor. The demonstrated PC/FP cavity/metal structure presents a new design philosophy for SPR performance enhancement.

Double-layer high-contrast subwavelength gratings that are separated by a dielectric space layer are investigated to achieve ultra-broadband reflection. The reflection phase of subwavelength gratings and the propagation phase shift between two gratings are manipulated to expand reflection bandwidth by properly stacking two reflective gratings. A reflector exhibiting a 99% reflectance bandwidth of ～1080 nm in the near-infrared is designed. Then this reflector is prepared using laser interference lithography and ion beam planarization, and an ultra-broadband reflection is achieved with reflectance exceeding 97% over a wavelength range of 955 nm in the near-infrared region.

A fabrication process allowing for the production of periodically poled lithium niobate (PPLN) photonic devices with any domain pattern and unit size down to 200 nm is developed by combining semiconductor fabrication techniques and piezo-force-microscopy tips polarization. Based on this fabrication process, PPLN microdisk resonators with quality factors of 8×104 were fabricated from a Z-cut lithium niobate film. Second-harmonic generation (SHG) utilizing d33 in the whole cavity was demonstrated in a PPLN microdisk with a 2 μm-spatial-period radial domain pattern. The SHG conversion efficiency was measured to be 1.44×10?5 mW?1. This work paves the way to fabricate complex PPLN photonic devices and to obtain efficient nonlinear optical effects that have wide applications in both classical and quantum optics.

Fast and sensitive air-coupled ultrasound detection is essential for many applications such as radar, ultrasound imaging, and defect detection. Here we present a novel approach based on a digital optical frequency comb (DOFC) technique combined with high-Q optical microbubble resonators (MBRs). DOFC enables precise spectroscopy on resonators that can trace the ultrasound pressure with its resonant frequency shift with femtometer resolution and sub-microsecond response time. The noise equivalent pressure of air-coupled ultrasound as low as 4.4 mPa/√Hz is achieved by combining a high-Q (～3×107) MBR with the DOFC method. Moreover, it can observe multi-resonance peaks from multiple MBRs to directly monitor the precise spatial location of the ultrasonic source. This approach has a potential to be applied in 3D air-coupled photoacoustic and ultrasonic imaging.

The photonic spin Hall effect plays an important role in photonic information technologies, especially in on-chip spin Hall devices. However, conventional devices suffer from low efficiency or narrow bandwidth, which prevents their practical application. Here, we introduce a spin Hall device using inverse design to achieve both high efficiency and broadband. Spin-dependent light separation is enabled by a 2.4 μm circular device with 100 nm pixels. The photonic spin Hall element is fabricated on a silicon-on-insulator wafer compatible with a standard integrated photonic circuit. The spin light is detected and emitted with an efficiency of up to 22% and 35%, respectively, over a 200 nm bandwidth at optical wavelength.

Dopamine (DA), as a neurotransmitter in human brain, plays a crucial role in reward motivation and motor control. An improper level of DA can be associated with neurological disorders such as schizophrenia and Parkinson’s disease. To quantify DA, optical DA sensors have emerged as an attractive platform due to their capability of high-precision and label-free measurement, and immunity to electromagnetic interference. However, the lack of selectivity, limited biocompatibility, and complex fabrication processes are challenges that hinder their clinical applications. Here, we report a soft and biocompatible luminescent hydrogel optical sensor capable of recognizing and quantifying DA with a simple and compact interrogation setup. The sensor is made of a hydrogel optical fiber (HOF) incorporated with upconversion nanoparticles (UCNPs). DA molecules are detected through the luminescence energy transfer (LET) between the UCNPs and the oxidation products of DA, while the light-guiding HOF enables both excitation and emission collection of the UCNPs. The hydrogel sensor provides an optical readout that shows a linear response up to 200 μmol/L with a detection limit as low as 83.6 nmol/L. Our results show that the UCNP-based hydrogel sensor holds great promise of serving as a soft and biocompatible probe for monitoring DA in situ.

Whispering-gallery microcavities have been used to realize a variety of efficient parametric nonlinear optical processes through the enhanced light–matter interaction brought about by supporting multiple high quality factor and small modal volume resonances. Critical to such studies is the ability to control the relative frequencies of the cavity modes, so that frequency matching is achieved to satisfy energy conservation. Typically this is done by tailoring the resonator cross section. Doing so modifies the frequencies of all of the cavity modes, that is, the global dispersion profile, which may be undesired, for example, in introducing competing nonlinear processes. Here, we demonstrate a frequency engineering tool, termed multiple selective mode splitting (MSMS), that is independent of the global dispersion and instead allows targeted and independent control of the frequencies of multiple cavity modes. In particular, we show controllable frequency shifts up to 0.8 nm, independent control of the splitting of up to five cavity modes with optical quality factors ?105, and strongly suppressed frequency shifts for untargeted modes. The MSMS technique can be broadly applied to a wide variety of nonlinear optical processes across different material platforms and can be used to both selectively enhance processes of interest and suppress competing unwanted processes.

Interaction of acoustic waves and microbubbles occurs in numerous biomedical applications including ultrasound imaging, drug delivery, lithotripsy treatment, and cell manipulation, wherein the acoustically driven microbubbles routinely act as active microscale oscillators or actuators. In contrast, microbubbles were utilized here as passive receivers to detect broadband ultrasound waves in aqueous environments. The microbubble was photothermally generated on a microstructured optical fiber (MOF) tip, forming a flexible Fabry–Pérot cavity whose gas–water interface was sensitive to ultrasound waves. The MOF severed as both a low-loss waveguide and a compact light condenser, allowing high-efficiency generation and stabilization of ultrasmall microbubbles. Integrated with all-fiber interferometry, a 10 μm diameter microbubble exhibited a low noise-equivalent pressure level of ～3.4 mPa/Hz1/2 and a broad bandwidth of ～0.8 MHz, capable of detecting weak ultrasounds emitted from red blood cells irradiated by pulsed laser light. With advantages of high sensitivity, compact size, and low cost, the microbubble-based ultrasound sensor has great potential in biomedical imaging and sensing applications.

We design and present a switchable slow light rainbow trapping (SLRT) state in a strongly coupling topological photonic system made from a magneto-optical photonic crystal waveguide channel. The waveguide channel supports slow light states with extremely small group velocity (vg=2.1×10 6c), low group-velocity dispersion, and a broadband operation bandwidth (3.60–4.48 GHz, near 22% of bandwidth). These slow light states originate from the strong coupling between two counter propagating topological photonic states. Under a gradient magnetic field, different frequency components of a wave packet are separated and stored at different positions for a long temporal duration with high spatial precision (without crosstalk and overlap between the electric fields of different frequencies) to form SLRT. Besides, these SLRT states can be easily switched among the forbidden state, trapped state, and releasing state by tuning the external magnetic field. The results suggest that the topological photonic state can offer a precise route of spatial-temporal-spectral control upon a light signal and find applications for optical buffers, broadband slow light systems, optical filters, wavelength-division multiplexing, and other optical communication devices.

Active control of metamaterial properties with high tunability of both resonant intensity and frequency is essential for advanced terahertz (THz) applications, ranging from spectroscopy and sensing to communications. Among varied metamaterials, plasmon-induced transparency (PIT) has enabled active control with giant sensitivity by embedding semiconducting materials. However, there is still a stringent challenge to achieve dynamic responses in both intensity and frequency modulation. Here, an anisotropic THz active metamaterial device with an ultrasensitive modulation feature is proposed and experimentally studied. A radiative-radiative-coupled PIT system is established, with a frequency shift of 0.26 THz in its sharp transparent windows by polarization rotation. Enabled by high charge-carrier mobility and longer diffusion lengths, we utilize a straightforwardly spin-coated MAPbI3 film acting as a photoactive medium to endow the device with high sensitivity and ultrafast speed. When the device is pumped by an ultralow laser fluence, the PIT transmission windows at 0.86 and 1.12 THz demonstrate a significant reduction for two polarizations, respectively, with a full recovery time of 561 ps. In addition, we numerically prove the validity that the investigated resonator structure is sensitive to the optically induced conductivity. The hybrid system not only achieves resonant intensity and frequency modulations simultaneously, but also preserves the all-optical-induced switching merits with high sensitivity and speed, which enriches multifunctional subwavelength metamaterial devices at THz frequencies.

Ultra-high quality (Q) whispering gallery mode (WGM) microtoroid optical resonators have demonstrated highly sensitive biomolecular detection down to the single molecule limit; however, the lack of a robust coupling method has prevented their widespread adoption outside the laboratory. We demonstrate through simulation that a phased array of nanorods can enable free-space coupling of light both into and out of a microtoroid while maintaining a high Q. To simulate large nanostructured WGM resonators, we developed a new approach known as FloWBEM, which is an efficient and compact 3D wedge model with custom boundary conditions that accurately simulate the resonant Fano interference between the traveling WGM waves and a nanorod array. Depending on the excitation conditions, we find loaded Q factors of the driven system as high as 2.1×107 and signal-to-background ratios as high as 3.86%, greater than the noise levels of many commercial detectors. These results can drive future experimental implementation.

The active control of electromagnetic response in metamaterial and mutual coupling between resonant building blocks is of fundamental importance in realizing high-quality metamaterials. In this work, we propose and experimentally demonstrate the tunabilities of symmetry-broken metasurfaces made of orthogonal electric dipolar resonators. The metasurface with vertical and horizontal wires is integrated with a PIN diode for active control. It is found that the electromagnetically induced transparency (EIT)-like spectrum appears due to the destructive or constructive interferences between the two electric dipolar modes when the structural symmetry broken is introduced to the metasurface. Different from previous works on the EIT-like effect, there is only electric dipole response in our metasuface. The microscopic response of the metasurface is numerically calculated to illustrate the mode coupling between the orthogonal electric dipolar resonators. By applying temporal coupled-mode theory, the interaction between the electromagnetic wave and the symmetry-broken metasurface is described, and the characteristic parameters of the resonator system, which determine the electromagnetic response of the metasurface, are acquired.

We designed a low-filling-factor and polarization-sensitive superconducting nanowire single photon detector (SNSPD), which can achieve a high absorption efficiency and counting rate simultaneously. Numerical simulations show that high absorption efficiency can be achieved by low-filling-factor SNSPDs with a silicon slot and silver reflector. The absorptance of the NbN nanowire for a transverse magnetic (TM) wave at the wavelength of 1550 nm can be 84.4% when the filling factor is only 16.5%, and the corresponding polarization extinction ratio (PER) is 562.9; the absorptance of the NbN nanowire for a transverse electric (TE) wave can be 67.0% when the filling factor is only 11.7%, and the PER is 7.4.

Refractive index (RI) sensing helps to identify biomolecules and chemicals in the mid-infrared range for drug discovery, bioengineering, and environmental monitoring. In this paper, we numerically demonstrate an electrically tunable RI sensor with ultrahigh sensitivity using a three-layer graphene nanoribbon array separated by hexagonal boron nitride (hBN). Unlike the weak resonance in single-layer graphene nanoribbons, a much stronger plasmon resonance featuring a higher-quality factor can be excited in the graphene/hBN few-layer ribbon array. Simultaneously, the high purity of graphene on hBN results in an outstanding charge mobility above 4×104 cm2·V 1·s 1 at 300 K, which allows a larger modulation depth. The interaction between the locally enhanced field around graphene ribbons and its surrounding analyte leads to ultrahigh sensitivity (4.207 μm/RIU), with the figure of merit reaching approximately 58. Moreover, this ultrasensitive detector could selectively work in different wavebands by controlling gate voltages applied to graphene. These merits of ultrahigh sensitivity and electrical tunability are major advances compared to previous RI sensors, paving a way toward ultrasensitive detection using graphene/hBN few-layer devices.

Ultrashort pulsed Bessel beams with intrinsic nondiffractive character and potential strong excitation confinement down to 100 nm can show a series of advantages over Gaussian beams in fabricating efficient Bragg grating waveguides (BGWs). In this work, we focus on parameter management for the inscription of efficient BGWs using the point-by-point method employing Bessel beams. Due to their high aspect ratio, the resulting one-dimensional void-like structures can section the waveguides and interact efficiently with the optical modes. Effective first-order BGWs with low birefringence can then be fabricated in bulk fused silica. By controlling the size and the relative location of grating voids via the Bessel pulse energy and scan velocities, the resonant behaviors of BGWs can be well regulated. A high value of 34 dB for 8 mm length is achieved. A simple predictive model for BGWs is proposed for analyzing the influences of processing parameters on the performance of BGWs. The technique permits multiplexing several gratings in the same waveguide. Up to eight grating traces were straightforwardly inscribed into the waveguide in a parallel-serial combined mode, forming the multiplex BGWs. As an application, the multiplex BGW sensor with two resonant peaks is proposed and fabricated for improving the reliability of temperature detection.

A unique all-fiber interferometric sensor was proposed and successfully demonstrated efficient low-refractive-index liquid sensing in the range from 1.33 to 1.37, which is compatible with those of bio-liquids. A special silica coreless optical fiber with an open V-groove was used as an optical sensing medium, which provided a high sensitivity for a minute liquid volume in the nanoliter scale. The V-groove fiber (VGF) was serially concatenated between two single-mode fibers (SMFs). The LP01 mode guided along the input SMF excited the higher-order modes in the VGF to generate multimode interference, whose spectrum was transmitted through the output SMF. A single liquid droplet with volume of ∼80 nanoliters wet the entire hydrophilic surface of the VGF, and the transmission spectra shifted corresponding to its refractive index in a very linear manner. The sensor also showed a negligible temperature cross-sensitivity in the range 25°C–75°C, which overlaps with the biological temperature window such that the sensitivity of 159.696 nm per refractive index unit (nm/RIU) remained independent of the temperature variation. Modal properties of VGF were thoroughly analyzed numerically, and detailed processes for the sensor fabrication and sensing experiments were reported.

GaAs-based microdisk lasers with an active region representing a dense array of indium-rich islands (InGaAs quantum well-dots) were studied using direct small-signal modulation. We demonstrate that using dense arrays of InGaAs quantum well-dots enables uncooled high-frequency applications with a GHz-range bandwidth for microdisk lasers. A maximum 3?dB modulation frequency of 5.9?GHz was found in the microdisk with a radius of 13.5?μm operating without a heatsink for cooling. A modulation current efficiency factor of 1.5??GHz/mA1/2 was estimated.

An optical whispering gallery mode (WGM) resonator supports degenerate counter-propagating modes and the degeneracy is lifted as mode splitting due to Rayleigh scattering. However, quantitative analysis becomes difficult if the resonance experiences weak scattering. Here we develop a spectroscopical method to identify an arbitrary small scatterer using the Fano interference-induced spectral response modification. Scattering information can be revealed by fitting the responses as a function of the field’s phase and intensity. In addition, we show that this modified response helps achieve an ultra-low detection limit for the mode-splitting-based nanoparticle detection method. This approach may be promising in the characterization of high-Q-factor devices, novel sensing methods, and quantum coupling system investigation.

A black phosphorus (BP) functionalized optical fiber sensor based on a microfiber coil resonator (MCR) for Pb2+ ion detection in an aquatic environment is presented and experimentally demonstrated. The MCR-BP sensor is manufactured by winding a tapered microfiber on a hollow rod composed of a low-refractive-index polycarbonate (PC) resin with the BP deposited on the internal wall of the rod. Based on the propagation properties of the MCR, the chemical interaction between the Pb2+ ions and the BP alters the refractive index of the ambient environment and thus results in a detectable shift in the transmission spectrum. The resonance wavelength moves towards longer wavelengths with an increasing concentration of Pb2+ ions, and the sensor has an ultra-high detection resolution of 0.0285 ppb (parts per billion). The temperature dependence is 106.95 pm/°C due to the strong thermo-optic and thermal-expansion effect of the low-refractive-index PC resin. In addition, the sensor shows good stability over a period of 15 days. The local pH also influences the sensor, with the resonance wavelength shift increasing as pH approaches a value of 7 but then decreasing as the pH value increases further due to the effect of the BP layer by H+ and OH? ions. The sensor shows the potential for high-resolution detection of Pb2+ ions in a liquid environment with the particular advantages of having a simple structure, ease of fabrication, low cost, low loss, and simple interrogation.

We demonstrate inorganic halide perovskite quantum-dots-based white light-emitting diodes via three different geometries, including liquid, solid, and hybrid types. Problems of fast anion exchange and aggregation in the cases of liquid- and solid-type devices are discussed in detail and push us to move towards the fabrication of a hybrid-type device structure. The experiment results illustrate that a hybrid-type device has the highest luminance efficiency (51 lm/W) and a wide color gamut (122% of NTSC and 91% of Rec. 2020). Therefore, we conclude that a hybrid-type device can provide an outstanding color gamut for high color gamut display applications.

Open quantum and wave systems can exhibit non-Hermitian degeneracies called exceptional points, where both the eigenvalues and the corresponding eigenstates coalesce. Previously, such exceptional points have been investigated in dielectric microcavities in terms of optical modes which are well confined inside the cavity. However, beside these so-called “internal modes” with a relatively high quality factor, there exists another kind of mode called “external modes,” which have a large decay rate and almost zero intensity inside the cavity. In the present paper, we demonstrate the physical significance of the external modes via the occurrence of exceptional points of internal–external mode pairs for transverse electric polarization. Our numerical studies show that these exceptional points can be achieved by either a boundary deformation of the microdisk or by introducing absorption into a circular cavity.

High-efficiency superconducting nanowire single-photon detectors (SNSPDs), which have numerous applications in quantum information systems, function by using the optical cavity and the ultrasensitive photon response of their ultra-thin superconducting nanowires. However, the wideband response of superconducting nanowires is limited due to the resonance of the traditional optical cavity. Here, we report on a supercontinuum SNSPD that can efficiently detect single photons over an ultra-broad spectral range from visible to mid-infrared light. Our detection approach relies on using multiple cavities with well-separated absorbed resonances formed by fabricating multilayer superconducting nanowires on metallic mirrors with silica acting as spacer layers. Thus, we are able to extend the absorption spectral bandwidth while maintaining considerable efficiency, as opposed to a conventional single-layer SNSPD. Our calculations show that the proposed supercontinuum SNSPD exhibits an extended absorption bandwidth with increased nanowire layers. Its absorption efficiency is greater than 70% over the entire range from 400 to 2500 nm (or 400 to 3000 nm), when using two-layer (or three-layer) nanowires. As a proof of principle, the SNSPD with bilayer nanowires is fabricated based on the proposed detector architecture with simplified geometrical parameters. The detector achieves broadband detection efficiency over 60% from 950 to 1650 nm. This type of detector may replace multiple narrow band detectors in a system and find uses in the emerging and rapidly advancing field of atomic and molecular broadband spectroscopy.

We fabricate a tapered fiber coupler, position it near an ultrahigh-Q resonator made from a microdroplet, and experimentally measure stimulated Raman emission. We then calculate the molecular vibrational mode associated with each of the Raman lines and present it in a movie. Our Raman laser lines show themselves at a threshold of 160 μW input power, the cold-cavity quality factor is 250 million, and mode volume is 23 μm3. Both pump and Raman laser modes overlap with the liquid phase instead of just residually extending to the fluid.

As one of the ideal tools for monitoring the formation and evolution of solar storms, the vacuum ultraviolet (VUV) detector should have both a fast temporal response and an array structure that enables image formation. Here, by combining a nontraditional graphene processing technique with traditional metal organic chemical vapor deposition epitaxy technology, we created hybrid heterostructure (HH) arrays of p-Gr/AlN/p-Si with VUV photovoltaic response capability and silicon integration potential. The HH arrays not only exhibit ultrafast temporal response (rise time of only 120 ns) and an extremely high Ion/Ioff ratio of 107, but also achieve the imaging demonstration of a VUV pattern for the first time. The HH technique provides a possible new path for the development of VUV imaging devices.

Digital alloy In0.52Al0.48As avalanche photodiodes exhibit lower excess noise than those fabricated from random alloys. This paper compares the temperature dependence, from 203 to 323 K, of the impact ionization characteristics of In0.52Al0.48As and Al0.74Ga0.26As digital and random alloys. These results provide insight into the low excess noise exhibited by some digital alloy materials, and these materials can even obtain lower excess noise at low temperature.

A nanowire (NW) structure provides an alternative scheme for deep ultraviolet light emitting diodes (DUV-LEDs) that promises high material quality and better light extraction efficiency (LEE). In this report, we investigate the influence of the tapering angle of closely packed AlGaN NWs, which is found to exist naturally in molecular beam epitaxy (MBE) grown NW structures, on the LEE of NW DUV-LEDs. It is observed that, by having a small tapering angle, the vertical extraction is greatly enhanced for both transverse magnetic (TM) and transverse electric (TE) polarizations. Most notably, the vertical extraction of TM emission increased from 4.8% to 24.3%, which makes the LEE reasonably large to achieve high-performance DUV-LEDs. This is because the breaking of symmetry in the vertical direction changes the propagation of the light significantly to allow more coupling into radiation modes. Finally, we introduce errors to the NW positions to show the advantages of the tapered NW structures can be projected to random closely packed NW arrays. The results obtained in this paper can provide guidelines for designing efficient NW DUV-LEDs.

We employed a metallic wire grating loaded with graphene and operating in total internal reflection (TIR) geometry to realize deep and broadband THz modulation. The non-resonant field enhancement effect of the evanescent wave in TIR geometry and in the subwavelength wire grating was combined to demonstrate a ～77% modulation depth (MD) in the frequency range of 0.2–1.4 THz. This MD, achieved electrically with a SiO2/Si gated graphene device, was 4.5 times higher than that of the device without a metal grating in transmission geometry. By optimizing the parameters of the metallic wire grating, the required sheet conductivity of graphene for deep modulation was lowered to 0.87 mS. This work has potential applications in THz communication and real-time THz imaging.

Distributed Bragg reflectors (DBRs) are essential components for the development of optoelectronic devices. In this paper, we first report the use of the nanoporous GaN (NP-GaN) DBR as a template for regrowth of InGaN-based light-emitting diodes (LEDs). The wafer-scale NP-GaN DBR, which is fabricated by electrochemical etching in a neutral solution, has a smooth surface, high reflectivity (>99.5%), and wide spectral stop band width (>70 nm). The chemical composition of the regrown LED thin film is similar to that of the reference LED, but the photoluminescence (PL) lifetime, PL intensity, and electroluminescence intensity of the LED with the DBR are enhanced several times compared to those of the reference LED. The intensity enhancement is attributed to the light reflection effect of the NP-GaN DBR and improved crystalline quality as a result of the etching scheme, whereas the enhancement of PL lifetime is attributable to the latter.

An all-optical light–control–light functionality with the structure of a microfiber knot resonator (MKR) coated with tin disulfide (SnS2) nanosheets is experimentally demonstrated. The evanescent light in the MKR [with a resonance Q of ～59,000 and an extinction ratio (ER) of ～26 dB] is exploited to enhance light–matter interaction by coating a two-dimensional material SnS2 nanosheet onto it. Thanks to the enhanced light–matter interaction and the strong absorption property of SnS2, the transmitted optical power can be tuned quasi-linearly with an external violet pump light power, where a transmitted optical power variation rate ΔT with respect to the violet light power of ～0.22 dB/mW is obtained. In addition, the MKR structure possessing multiple resonances enables a direct experimental demonstration of the relationship between resonance properties (such as Q and ER), and the obtained ΔT variation rate with respect to the violet light power. It verifies experimentally that a higher resonance Q and a larger ER can lead to a higher ΔT variation rate. In terms of the operating speed, this device runs as fast as ～3.2 ms. This kind of all-optical light–control–light functional structure may find applications in future all-optical circuitry, handheld fiber sensors, etc.

We propose and demonstrate a tapered hollow annular core fiber (HACF) coupler for excitation of whispering-gallery modes (WGMs) of an embedded microsphere resonator. The coupler is simply fabricated by fusion splicing of a segment of HACF with the single-mode fiber (SMF), and then improved by tapering the splicing joint to reduce the cone-apex angle. Therefore, the coupling efficiency from the SMF to the HACF is enhanced to excite various WGMs via evanescent field coupling. Normal positive, negative symmetrical Lorentzian and asymmetric Fano line shapes can be obtained by varying the resonator size and location. Another interesting phenomenon is observed that a higher Q-factor mode in a lower Q-factor mode has a contrast as high as 58. Temperature sensing with good stability is also demonstrated. This embedded WGM microsphere resonator in the tapered HACF is expected to promote environmental adaptability in practical applications due to its simplicity and robustness.

Quantum dots (QDs) can achieve high quantum yields close to unity in liquid solutions, whereas they exhibit a decreased conversion efficiency after being integrated into solid-state polymer matrices for light-emitting diode (LED) devices, which is called the host matrix effect. In this study, we propose a solid–liquid hybrid-state QD-LED to solve this issue. The ethylene-terminated polydimethylsiloxane (ethylene-PDMS) is used to establish a solid-state cross-linked network, whereas the methyl-terminated PDMS (methyl-PDMS) is used in its liquid state. From a macroscopic level, the cured solid–liquid hybrid-state PDMS (SLHP) composites reach a solid state, which is stable and flexible enough to be used in LED devices. Compared with LEDs using conventional QD/solid PDMS composites at equal color conversion efficiency ranging from 40% to 60%, the luminous flux of LEDs with QD/SLHP composites is increased by 13.0% using an optimized methyl-PDMS concentration of 85 wt. %. As a result, high efficiency QD-LEDs using QDs as the only color convertor with luminous efficacy of 89.6 lm/W (0.19 A) were achieved, which show a working stability comparable with that using conventional solid-state structures at a harsh condition. Consequently, the novel approach shows great potential for achieving high efficiency and high stability QD-LEDs, which is also compatible with current structures used in illumination and display applications.

A narrowband tunable antireflection optical filter is proposed and numerically studied. The structure is a metasurface based on plasmonic nanoparticles on an electro-optic film in a three-layer configuration of metal-dielectric-metal (MDM) in the visible near-infrared range. By tuning the voltage and thus tuning the refractive index of the dielectric LiNbO3, one can shift the wavelength of minimum reflection as desired. The parameters of gold nanoparticles and other elements used for the filter design and refractive index of the dielectric are obtained by the finite-element method (FEM). An analytical theory is presented to explain the FEM simulation results, and they agree well with each other. It is found that the frequency of the plasmonic resonance wave on the metasurface should be equal to that of the Fabry–Perot resonator formed by the MDM to have a good filtering property. Theoretical spectra obtained by FEM simulation show that the structure has extensive potential for the design of tunable narrow-band filters for modulators, displayers, and color extraction for imaging.

In this study, a full-color emission red–green–blue (RGB) quantum-dot (QD)-based micro-light-emitting-diode (micro-LED) array with the reduced optical cross-talk effect by a photoresist mold has been demonstrated. The UV micro-LED array is used as an efficient excitation source for the QDs. The aerosol jet technique provides a narrow linewidth on the micrometer scale for a precise jet of QDs on the micro-LEDs. To reduce the optical cross-talk effect, a simple lithography method and photoresist are used to fabricate the mold, which consists of a window for QD jetting and a blocking wall for cross-talk reduction. The cross-talk effect of the well-confined QDs in the window is confirmed by a fluorescence microscope, which shows clear separation between QD pixels. A distributed Bragg reflector is covered on the micro-LED array and the QDs’ jetted mold to further increase the reuse of UV light. The enhanced light emission of the QDs is 5%, 32%, and 23% for blue, green, and red QDs, respectively.

An all-fiber acousto-optic modulator (AOM), which features a compact structure and a low-driving voltage, is experimentally demonstrated for the active mode-locking of a fiber laser. The proposed AOM is based on the short length of the cladding-etched fiber, the ends of which are fixed on a slide glass. On top of the cladding-etched fiber, a piezoelectric transducer was overlaid. A chemical wet-etching technique, which is based on a mixed solution of NH4F and (NH4)2SO4, is used to reduce the fiber diameter down to ～25 μm, and the length of the etched section is only 0.5 cm. The fabricated device exhibited a modulation depth of 73.10% at an acoustic frequency of 918.9 kHz and a peak-to-peak electrical voltage of 10 V, while a laser beam was coupled at 1560 nm. By using the prepared AOM within an erbium-doped-fiber ring cavity, the mode-locked pulses with a temporal width of 2.66 ps were readily obtained at a repetition rate of 1.838 MHz.

Waveguide superlattices, a special type of waveguide arrays, can be designed to achieve very low cross talk at submicrometer/subwavelength pitches. The theoretical framework and design rationales for such waveguide superlattices will be presented in depth. Waveguide sidewall roughness can help to deter the coherent coupling between identical waveguides in nearby supercells, but it also induces random fluctuation of transmission. Statistical behavior of the transmission due to roughness in a waveguide superlattice is systematically treated. Complex transmission characteristics due to spectral oscillation and random roughness will be presented, and their evolution with the superlattice length will be analyzed.Institutions.

A 459 nm Faraday anomalous dispersion optical filter (FADOF) working at the side wings of the cesium 6S1∕2 → 7P1∕2 transition with weak oscillator strength is achieved. The transmittance of the higher side wing reaches 98% at a temperature of 179°C and magnetic field above 323 G. The experimental results coincide with the theoretical predictions in 1982 and 1995, which were not realized in experiments for over three decades. Due to its high transmittance, high accuracy, and narrow linewidth, the 459 nm FADOF can be applied in underwater optical communications, the building of active optical clocks, and laser frequency stabilization in active optical clocks.

In this paper, an ultrathin metalens has been proposed based on a holographic metasurface that consists of elongated apertures in 40 nm gold film, which exhibit intriguing properties such as on- and off-axis focusing and also can concentrate light into multiple, discrete spots for circularly polarized incident lights. First, the spatial transmission phase distributions of the designed metalens with arbitrary focusing can be obtained by computergenerated holography. Then, the discrete phase distributions can be continuously encoded by subwavelength nanoapertures with spatially varying orientations in gold film. The simulation results show that our designed metalens can work efficiently for different types of focusing. Finally, our metasurface shows superior broadband characteristics between 670 and 810 nm, and the corresponding focal lengths of the designed lenses also can be efficiently modulated with the incident lights at different wavelengths.