The sensing spatial resolution and signal-to-noise ratio (SNR) of Raman distributed optical fiber sensors are limited by the pulse width and weak Raman scattering signals. Notably, the sensing spatial resolution cannot exceed the order of meters at several kilometers sensing distances. To break through this physical bottleneck, a novel, to our knowledge, Raman scattering model based on paired-pulse sensing is constructed. The fundamental origins of the observed limited spatial resolution of conventional schemes are analyzed, and a chaotic asymmetric paired-pulse correlation-enhanced scheme for Raman distributed fiber-optic sensing is proposed and experimentally demonstrated. The proposed scheme uses a chaotic asymmetric paired-pulse as the sensing signal and extracts the light intensity information of each data point of the sensing fiber, which carries the random undulation characteristics of chaotic time series, based on the time-domain differential reconstruction method. This scheme overcomes the pulse width limitation of spatial resolution via correlation and demodulation, enhances the correlation characteristics between the temperature-modulated Raman scattered light field and detection signal, and improves the SNR. Finally, a sensing performance of 10 km, a spatial resolution of 30 cm, and an SNR of 6.67 dB are realized in the experiment. This scheme provides a new research idea for a high-performance Raman distributed optical fiber sensing system.

Most optical information processors deal with text or image data, and it is very difficult to deal experimentally with acoustic data. Therefore, optical advances that deal with acoustic data are highly desirable in this area. In particular, the development of a voice or acoustic-signal authentication technique using optical correlation can open a new line of research in the field of optical security and could also provide a tool for other applications where comparison of acoustic signals is required. Here, we report holographic acoustic-signal authentication by integrating the holographic microphone recording with optical correlation to meet some of the above requirements. The reported method avails the flexibility of 3D visualization of acoustic signals at sensitive locations and parallelism offered by an optical correlator/processor. We demonstrate text-dependent optical voice correlation that can determine the authenticity of acoustic signal by discarding or accepting it in accordance with the reference signal. The developed method has applications in security screening and industrial quality control.

Metasurfaces have prompted the transformation from the investigation of scalar holography to vectorial holography and led various applications in vectorial optical field manipulation. However, the majority of previously demonstrated methods focused on the reconstruction of a vectorial holographic image located at a predefined individual image plane. The evolution of polarization transformation during propagation can provide more design freedoms for realizing three-dimensional holography with complicated polarization feature. Here, we demonstrated a Jones matrix framework to generate vectorial holographic images with continuously varied polarization distributions at multiple different image planes based on a height tunable metasurface. The proposed metasurface is composed of IP-L (a type of photoresist) nanofins with different lengths, widths, heights, as well as orientation angles fabricated by femtosecond laser direct writing. Such a fabrication method is in favor of 3D arbitrary structure processing, large area fabrication, as well as fabrication on curved substrates. Meanwhile, it is easy to fabricate structures that can be integrated with other devices, including optical fibers, photodetectors, and complementary metal–oxide semiconductors. Our demonstrated method provides a feasible way to generate high-dimensional vectorial fields with longitudinally varied features from the perspective of holography and can be used in the related areas including optical trapping, sensing, and imaging.

The utilization of multimode fibers (MMFs) displays significant potential for advancing the miniaturization of optical endoscopes. However, the imaging quality is constrained by the physical conditions of MMF, which is particularly serious in small-core MMFs because of the limited mode quantity. To break this limitation and enhance the imaging ability of MMF to the maximum, we propose a mode modulation method based on the singular value decomposition (SVD) of MMF’s transmission matrix (TM). Before injection into the MMF, a light beam is modulated by the singular vectors obtained by SVD. Because the singular vectors couple the light field into eigenchannels during transmission and selectively excite the modes of different orders, the optimal distribution of the excited modes in MMF can be achieved, thereby improving the imaging quality of the MMF imaging system to the greatest extent. We conducted experiments on the MMF system with 40 μm and 105 μm cores to verify this method. Deep learning is utilized for image reconstruction. The experimental results demonstrate that the properties of the output speckle pattern were customized through the selective excitation of optical modes in the MMF. By applying singular vectors for mode modulation, the imaging quality can be effectively improved across four different types of scenes. Especially in the ultrafine 40 μm core MMF, the peak signal-to-noise ratio can be increased by up to 7.32 dB, and the structural similarity can be increased by up to 0.103, indicating a qualitative performance improvement of MMF imaging in minimally invasive medicine.

The demand for low-cost, high-performance miniaturized optical imaging systems requires creating a new imaging paradigm. In this paper, we propose an imaging paradigm that achieves diffraction-limited imaging with a non-imaging spatial information transfer lens. The spatial information transfer lens realizes a perfect match between the space–bandwidth product (SBP) of the lens and that of the image sensor so that the collected spatial information from the object can be totally recorded and fully resolved by the image sensor. A backward wave propagation model is developed to reconstruct the object by propagating the light wave modulated by the information transfer lens back from the image space to object space. The proposed imaging paradigm breaks the point-to-point imaging structure and removes the focusing-distance constraint, allowing a flexible arrangement of the object and the image sensor along the optical axis with a compact form factor of the optical system. We experimentally demonstrate the versatility and effectiveness of the proposed imaging paradigm. The proposed imaging paradigm is low-cost, simple in configuration, flexible in arrangement, and diffraction limited with great potential applications in biomedical imaging.

Compensation of polarization-variance-related artifacts is required to steadily obtain high-quality optical coherence tomography (OCT) images at various experimental conditions. Since most OCT systems utilize optical fiber to transfer the light easily and a polarized light source, the polarization state is arbitrarily changed in every different condition. In this study, we proposed polarization-maintaining-fiber-based polarization-insensitive OCT (PM-PI-OCT) with a simple optical configuration and a simple compensation process. The proposed PM-PI-OCT is not only theoretically proved by mathematical derivations but also evaluated by quantitative analysis of various fiber twisting angles. Moreover, the applicability and robustness of the proposed PM-PI-OCT were proved by human retina imaging using the customized handheld probe. Our proposed polarization-insensitive OCT requires no pre-calibration, no post-processing procedure, and no computational load for implementation and is able to be applied to universal fiber-based OCT systems. We believe that our simple and robust polarization-insensitive OCT system is able to be applied to various existing OCT setups for polarization state variance compensation with high versatility and applicability.

Arrays of optically levitated nanoparticles with fully tunable light-induced dipole–dipole interactions have emerged as a platform for fundamental research and sensing applications. However, previous experiments utilized two optical traps with identical polarization, leading to an interference effect. Here, we demonstrate light-induced dipole–dipole interactions using two orthogonally polarized optical traps. Furthermore, we achieve control over the strength and polarity of the optical coupling by adjusting the polarization and propose a method to simultaneously and stably measure conservative and non-conservative coupling rates. Our results provide a new scheme for exploring entanglement and topological phases in arrays of levitated nanoparticles.

Professor Connie Chang-Hasnain discusses her career in semiconductor optoelectronics with her former student, Prof. Hao Sun.

Synchronously pumped optical parametric oscillators (OPOs) provide uniquely versatile platforms to generate ultrafast mid-infrared pulses within a spectral range beyond the access of conventional mode-locked lasers. However, conventional OPO sources based on bulk crystals have been plagued by complex optical alignment and large physical footprint. Here, we devise and implement two OPO variants based on a polarization-maintaining fiber-feedback cavity, which allow to robustly deliver sub-picosecond MIR pulses without the need of active stabilization. The first one integrates an erbium-doped fiber into the OPO cavity as the additional gain medium, which significantly reduces the pump threshold and allows stable optical pulse formation within a spectral range of 1553–1586 nm. The second one adopts a chirped poling nonlinear crystal in a passive-fiber cavity to further extend the operation spectral coverage, which facilitates broad tuning ranges of 1350–1768 nm and 2450–4450 nm for the signal and idler bands, respectively. Therefore, the presented mid-infrared OPO source is featured with high compactness, robust operation, and wide tunability, which would be attractive for subsequent applications such as infrared photonics, biomedical examination, and molecular spectroscopy.

Topological insulators are most frequently constructed using lattices with specific degeneracies in their linear spectra, such as Dirac points. For a broad class of lattices, such as honeycomb ones, these points and associated Dirac cones generally appear in non-equivalent pairs. Simultaneous breakup of the time-reversal and inversion symmetry in systems based on such lattices may result in the formation of the unpaired Dirac cones in bulk spectrum, but the existence of topologically protected edge states in such structures remains an open problem. Here a photonic Floquet insulator on a honeycomb lattice with unpaired Dirac cones in its spectrum is introduced that can support unidirectional edge states appearing at the edge between two regions with opposite sublattice detuning. Topological properties of this system are characterized by the nonzero valley Chern number. Remarkably, edge states in this system can circumvent sharp corners without inter-valley scattering even though there is no total forbidden gap in the spectrum. Our results reveal unusual interplay between two different physical mechanisms of creation of topological edge states based on simultaneous breakup of different symmetries of the system.

Higher-order topological insulators, originally proposed in quantum condensed matters, have provided a new avenue for localizing and transmitting light in photonic devices. Nontrivial band topology in crystals with certain symmetries can host robust topological edge states and lower dimensional topological corner states (TCS), making them a promising platform for photonics applications. Here, we have designed several types of TCS with only two specific C6v-symmetric photonic crystals with various seamless splicing boundaries, where all the supposed TCS with diverse electromagnetic characteristics are visualized via numerical simulations and experimental measurements. More interestingly, we have observed that those TCS overlapping in spectral and spatial space tend to interweaved, inducing spectrum division. Meanwhile, the equivalent corners appear to have TCS with a phase difference, which is critical for directional activation of pseudospin dependence. Our findings demonstrate that coupled TCS with phase difference at different nanocavities can be selectively excited by a chiral source, which indicates that the TCS at this time have pseudospin-dependent properties. We further design a specific splicing structure to prevent coupling between adjacent TCS. This work provides a flexible approach for space- and frequency-division multiplexing in photonic devices.

Responsivity is a critical parameter for sensors utilized in industrial miniaturized sensors and biomedical implants, which is typically constrained by the size and the coupling with external reader, hindering their widespread applications in our daily life. Here, we propose a highly-responsive sensing method based on Hamiltonian hopping, achieving the responsivity enhancement by 40 folds in microscale sensor detection compared to the standard method. We implement this sensing method in a nonlinear system with a pair of coupled resonators, one of which has a nonlinear gain. Surprisingly, our method surpasses the sensing performance at an exceptional point (EP)—simultaneous coalescence of both eigenvalues and eigenvectors. The responsivity of our method is notably enhanced thanks to the large frequency response at a Hamiltonian hopping point (HHP) in the strong coupling, far from the EP. Our study also reveals a linear HHP shift under different perturbations and demonstrates the detection capabilities down to sub-picofarad (<1 pF) of the microscale pressure sensors, highlighting their potential applications in biomedical implants.

Optical nonlinear response and its dynamics of wide-bandgap materials are key to realizing integrated nonlinear photonics and photonic circuit applications. However, those applications are severely limited by the unavailability of both dispersion and dynamics of nonlinear refraction (NLR) via conventional measurements. In this work, the broadband NLR dynamics with extremely high sensitivity (λ/1000) can be obtained from absorption spectroscopy in GaN:C using the refraction-related interference model. Both the absorption and refraction kinetics are found to be significantly modulated by the C-related defects. Especially, we demonstrate that the refractive index change Δn of GaN:C is negative and can be used to realize all-optical switching applications owing to the large NLR and ultrafast switching time. The NLR under different non-equilibrium carrier distributions originates from the capture of electrons by CN+ defect state, while the absorption modulation originates from the excitation of tri-carbon defects. We believe that this work provides a better understanding of the GaN:C nonlinear properties and an effective solution to broadband NLR dynamics of transparent thin films or heterostructure materials.

Within optical microresonators, the Kerr interaction of photons can lead to symmetry breaking of optical modes. In a ring resonator, this leads to the interesting effect that light preferably circulates in one direction or in one polarization state. Applications of this effect range from chip-integrated optical diodes to nonlinear polarization controllers and optical gyroscopes. In this work, we study Kerr-nonlinearity-induced symmetry breaking of light states in coupled resonator optical waveguides (CROWs). We discover, to our knowledge, a new type of controllable symmetry breaking that leads to emerging patterns of dark and bright resonators within the chains. Beyond stationary symmetry broken states, we observe Kerr-effect-induced homogeneous periodic oscillations, switching, and chaotic fluctuations of circulating powers in the resonators. Our findings are of interest for controlled multiplexing of light in photonic integrated circuits, neuromorphic computing, topological photonics, and soliton frequency combs in coupled resonators.

A Brillouin-assisted 80-GHz-spaced dual-comb source with a reconfigurable repetition frequency difference ranging from 48 MHz to 1.486 GHz is demonstrated. Two pairs of dual-pump seeds with an interval offset produce the corresponding dual Brillouin lasers in two fiber loops, and then the Brillouin lasers give rise to dual combs via the cavity-enhanced cascaded four-wave mixing effect. The repetition frequency difference is determined by the interval offset of the dual-pump seeds, which is induced by the Brillouin frequency shift difference between different fibers in a frequency shifter. Each comb provides 22 lasing lines, and the central 10 lines in a 20-dB power deviation feature high optical signal-to-noise ratios exceeding 50 dB. The linewidths of the dual-comb beating signals are less than 300 Hz, and the absolute linewidths of the comb lines are around 1.5 kHz. The dual-comb source enables substantial repetition frequency differences from 48 MHz to 1.486 GHz by changing the pluggable fibers in the frequency shifter.

Quasi-bound states in the continuum (quasi-BICs) offer an excellent platform for the flexible and efficient control of light-matter interactions by breaking the structural symmetry. The active quasi-BIC device has great application potential in fields such as optical sensing, nonlinear optics, and filters. Herein, we experimentally demonstrate an active terahertz (THz) quasi-BIC device induced by the polarization conversion in a liquid crystal (LC)-integrated metasurface, which consists of a symmetrically broken double-gap split ring resonator (DSRR), an LC layer, and double graphite electrodes. In the process of LC orientation control under the external field, the device realizes the active control from the OFF state to the ON state. In the OFF state, the LC has no polarization conversion effect, and the device behaves in a non-resonant state; but for the ON state, the device exhibits obvious quasi-BIC resonance. Furthermore, we achieve asymmetric transmission based on polarization-induced quasi-BIC modulation precisely at the quasi-BIC resonance position, and its isolation can be controlled by the external field. The study on dynamic quasi-BIC by the LC-integrated metasurface introduces a very promising route for active THz devices, which guarantees potential applications for THz communications, switching, and sensing systems.

The optical regulation strategy of gold nanoparticles can significantly improve the performance of terahertz devices. We designed an all-dielectric double-layer honeycomb metamaterial absorber (MA) to demonstrate the broadband terahertz absorption characteristics in the presence or absence of gold nanoparticles. When it does not contain gold nanoparticles, MA exhibits a peak absorption efficiency of over 99% within the bandwidth range of ∼486 GHz. In particular, gold nanospheres (AuNPs), gold nanobipyramids (AuNBPs), and gold nanorods (AuNRs) are used to modulate the optical coupling effect of metamaterial absorbers, which improves their modulation performance. In the simulation, the effective medium theory (EMT) was applied to quantitatively calculate the optical response of a metamaterial absorber with an integrated gold nanoparticle equivalent gold layer. The integrated gold nanoparticle equivalent gold layer can achieve modulation enhancement of one order of magnitude. In the experiment, our process is compatible with CMOS technology, which may contribute to the development of terahertz detectors. In addition, the tunability and modulation enhancement characteristics demonstrated are beneficial for creating dynamic functional terahertz devices, such as THz modulators and switches.

The fiber-optic sensor is a great candidate in the field of metrology, developed to rely on the optical phase to convey valuable information. Some phase amplification methods have attracted wide attention due to their ability to improve measurement sensitivity; nevertheless, the precision is generally restricted in phase measurement. Here, we report a novel optoelectronic hybrid oscillating fiber-optic sensor by mapping the measurand loaded on the sensing fiber to the frequency shift of the microwave signal, which is generated by an all-electric oscillating cavity with a frequency conversion pair. Two branch signals assisted in twice frequency conversion are obtained by heterodyne interference, with the sensing information scaled up by two optical comb line frequencies contained, and then, the phase difference is cumulatively enhanced in the closed feedback loop. Thanks to the introduction of the oscillating cavity, a detection limit improvement of 42 dB at a 10 Hz frequency offset can be achieved in theory with a cavity delay of 1 μs. The sensing precision depends on the cavity noise limit and is independent of the instrument and cavity delay. A proof-of-concept experiment is carried out to demonstrate sensors with a sensitivity of 8.3 kHz/ps and 22.3 kHz/ps for a range of 50 ps, and 62 kHz/ps and 162 kHz/ps for a range of 6.7 ps. The minimum Allan deviation reaches 2.7 attoseconds at an averaging time of 0.2 s with a frequency interval of 150 GHz, indicating that the proposal may pave a new path for sensing interrogation systems, especially for high-precision measurement.

Terahertz (THz) detectors with high sensitivity, fast response speed, room temperature operation, and self-powered feature are the key component for many THz applications. Microcavity resonators can effectively improve the sensitivity of THz detectors. However, it is difficult to precisely evaluate the microcavity resonator induced such improvement in experiment. Here, we realize a configurable microcavity–antenna-integrated graphene photothermoelectric (PTE) THz detector. Through the microcavity–antenna hybrid structure, THz radiations are localized and enhanced at one end of the graphene channel, and the temperature difference along the channel is greatly increased, resulting in the strong enhancement of PTE response. At the resonant frequency, the device has a high responsivity (976 V/W), low noise equivalent power (2.87 pW/Hz1/2), and fast response speed (300 ns) at room temperature and in zero-bias operation mode. The microcavity-induced peak enhancement factor of 13.14 is accurately extracted. The microcavity–antenna introduced enhancement is further confirmed by using a two-temperature heat transfer model. The strategy of using a configurable microcavity is useful for further optimizing THz detectors by introducing the critical coupling mechanism.

Optical manipulation of objects at the nanometer-to-micrometer scale relies on the precise shaping of a focused laser beam to control the optical forces acting on them. Here, we introduce and experimentally demonstrate surface-shaped laser traps with conformable phase-gradient force field enabling multifunctional optical manipulation of nanoparticles in two dimensions. For instance, we show how this optical force field can be designed to capture and move multiple particles to set them into an autonomous sophisticated optical transport across any flat surface, regardless of the shape of its boundary. Unlike conventional laser traps, the extended optical field of the surface laser trap makes it easier for the particles to interact among themselves and with their environment. It allowed us to optically transport multiple plasmonic nanoparticles (gold nanospheres) while simultaneously enabling their electromagnetic interaction to form spinning optically bound (OB) dimers, which is the smallest case of optical matter system. We have experimentally demonstrated, for the first time, the creation of stable spinning OB dimers with control of their rotational and translational motion across the entire surface. These traveling OB dimers guided by the phase-gradient force work as switchable miniature motor rotors, whose rotation is caused by the combined effects of optical binding forces and optical torque induced by a circularly polarized surface laser trap. The degree of customization of the surface laser traps provides a versatility that can boost the study and control of complex systems of interacting particles, including plasmonic structures as the optical matter ones of high interest in optics and photonics.

Nondiffracting beams (NDBs) have presented significant utility across various fields for their unique properties of self-healing, anti-diffraction, and high-localized intensity distribution. We present a versatile and flexible method for generating high-order nondiffracting beams predicated on the Fourier transformation of polymorphic beams produced by the free lenses with tunable shapes. Based on the tunability of the digital free lenses, we demonstrate the experimental generation of various long-distance nondiffracting beams, including Bessel beams, polymorphic generalized nondiffracting beams, tilted nondiffracting beams, asymmetric nondiffracting beams, and specially structured beams generated by the superposition of Bessel beams. Our method achieves efficiency of up to about seven times compared with complex beam shaping methods. The generated NDBs exhibit characteristics of extended propagation distance and high-quality intensity profiles consistent with the theoretical predictions. The proposed method is anticipated to find applications in laser processing, optical manipulation, and other fields.

Combining resonant excitation with Purcell-enhanced single quantum dots (QDs) stands out as a prominent strategy for realizing high-performance solid-state single-photon sources. However, optimizing photon extraction efficiency requires addressing the challenge of effectively separating the excitation laser from the QDs’ emission. Traditionally, this involves polarization filtering, limiting the achievable polarization directions and the scalability of polarized photonic states. In this study, we have successfully tackled this challenge by employing spatially orthogonal resonant excitation of QDs, deterministically coupled to monolithic Fabry–Perot microcavities. Leveraging the planar microcavity structure, we have achieved spectral filter-free single-photon resonant fluorescence. The resulting source produces single photons with a high extraction efficiency of 0.87 and an indistinguishability of 0.963(4).

The orbital angular momentum (OAM) of photons provides a pivotal resource for carrying out high-dimensional classical and quantum information processing due to its unique discrete high-dimensional nature. The cyclic transformation of a set of orthogonal OAM modes is an essential building block for universal high-dimensional information processing. Its realization in the quantum domain is the universal quantum Pauli-X gate. In this work, we experimentally demonstrate a cyclic transformation of six OAM modes with an averaged efficiency higher than 96% by exploiting a nonreciprocal Mach–Zehnder interferometer. Our system is simple and can, in principle, be scaled to more modes. By improving phase stabilization and inputting quantum photonic states, this method can perform universal single-photon quantum Pauli-X gate, thus paving the way for scalable high-dimensional quantum computation.

Phase change materials (PCMs), characterized by high optical contrast (Δn>1), nonvolatility (zero static power consumption), and quick phase change speed (∼ns), provide new opportunities for building low-power and highly integrated photonic tunable devices. Optical integrated devices based on PCMs, such as optical switches and optical routers, have demonstrated significant advantages in terms of modulation energy consumption and integration. In this paper, we theoretically verify the solution for a highly integrated nonvolatile optical switch based on the modulation of the topological interface state (TIS) in the quasi-one-dimensional photonic crystal (quasi-1D PC). The TIS exciting wavelength changes with the crystalline level of the PCM. The extinction ratio (ER) of the topological optical switch is over 18 dB with a modulation length of 9 μm. Meanwhile, the insertion loss (IL) can be controlled within 2 dB. Furthermore, we have analyzed the impact of fabrication errors on the device’s performance. The obtained results show that, the topological optical switch, which changes its “on/off” state by modulating TIS, exhibits enhanced robustness to the fabrication process. We provide an interesting and highly integrated scheme for designing the on-chip nonvolatile optical switch. It offers great potential for designing highly integrated on-chip optical switch arrays and nonvolatile optical neural networks.

The four-wave mixing (FWM) effect offers promise to generate or amplify light at wavelengths where achieving substantial gain is challenging, particularly within the mid-infrared (MIR) spectral range. Here, based on the commonly used 340 nm silicon-on-insulator (SOI) platform, we experimentally demonstrate high-efficiency and broadband wavelength conversion using the FWM effect in a high-Q silicon microring resonator pumped by a continuous-wave (CW) laser in the 2 μm waveband. The microring resonator parameters are carefully optimized for effective phase-matching to obtain high conversion efficiency (CE) with broad bandwidth. The loaded quality (Ql) factor of the fabricated microring resonator is measured to be 1.11×105, at a resonance wavelength of 1999.3 nm, indicating low propagation losses of 1.68 dB/cm. A maximum CE of -15.57 dB is achieved with a low input pump power of only 4.42 dBm, representing, to our knowledge, the highest on-chip CE demonstrated to date under the CW pump in the MIR range. Furthermore, broadband wavelength conversion can be observed across a 140.4 nm wavelength range with a CE of -19.32 dB, and simulations indicate that the conversion bandwidth is over 400 nm. This work opens great potential in exploiting widely tunable on-chip sources using high-efficiency wavelength conversion, particularly leveraging the advantages of the SOI platform in integrated photonics across the 2 μm MIR range.

Photonic computing has the potential to harness the full degrees of freedom (DOFs) of the light field, including the wavelength, spatial mode, spatial location, phase quadrature, and polarization, to achieve a higher level of computing parallelism and scalability than digital electronic processors. While multiplexing using the wavelength and other DOFs can be readily integrated on silicon photonics platforms with compact footprints, conventional mode-division multiplexed (MDM) photonic designs occupy areas exceeding tens to hundreds of microns for a few spatial modes, significantly limiting their scalability. Here, we utilize inverse design to demonstrate an ultracompact photonic computing core that calculates vector dot products based on MDM coherent mixing. Our dot-product core integrates the functionalities of two-mode multiplexers and one multimode coherent mixer within a nominal footprint of 5 μm×3 μm. We have experimentally demonstrated computing examples on the fabricated dot-product core, including complex number multiplication and motion estimation using optical flow. The compact dot-product core design enables large-scale on-chip integration in a parallel photonic computing primitive cluster for high-throughput scientific computing and computer vision tasks.

In the field of quantum metrology, transition matrix elements are crucial for accurately evaluating the black-body radiation shift of the clock transition and the amplitude of the related parity-violating transition, and can be used as probes to test quantum electrodynamic effects, especially at the 10-3–10-4 level. We developed a universal experimental approach to precisely determine the dipole transition matrix elements by using the shelving technique, for the species where two transition channels are involved, in which the excitation pulses with increasing duration were utilized to induce shelving, and the resulting shelving probabilities were determined by counting the scattered photons from the excited P1/22 state to the S1/22 ground state. Using the scattered photons offers several advantages, including insensitivity to fluctuations in magnetic field, laser intensity, and frequency detuning. An intensity-alternating sequence to minimize detection noise and a real-time approach for background photon correction were implemented in parallel. We applied this technique to a single Yb+ ion, and determined the 6p P1/22-5d D23/2 transition matrix element 2.9979(20) ea0, which indicates an order of magnitude improvement over existing reports. By combining our result with the 6p P1/22 lifetime of 8.12(2) ns, we extracted the 6s S1/22-6p P1/22 transition matrix element to be 2.4703(31) ea0. The accurately determined dipole transition matrix elements can serve as a benchmark for the development of high-precision atomic many-body theoretical methods.

Surface plasmons have been given high expectations in terahertz (THz) on-chip photonics with highly bound integrated transmission and on-chip wavefront engineering. However, most surface plasmonic coupling strategies with tailorable polarization-dependent features are challenged in broadband propagation and dynamic manipulation. In this work, a liquid crystal (LC)-integrated surface plasmonic metadevice based on arc-arrayed pair-slit resonators (APSRs) is demonstrated. The mirror-symmetry structures of this metadevice achieve the spin-selective unidirectional achromatic focusing, of which the broadband characteristic is supported by containing multiple APSRs with slits of different sizes corresponding to different excitation frequencies. Moreover, arc radii are precisely designed to meet the phase matching condition of constructive interference, so that the operating frequency of this on-chip metadevice is broadened to 0.33–0.60 THz. Furthermore, the LC integration provides the active energy distribution between the left and right focal spots, and the actual modulation depth reaches up to 73%. These THz active, wideband, on-chip manipulation mechanisms and their devices are of great significance for THz-integrated photonic communication, information processing, and highly sensitive sensing.

Nanoparticle-based plasmonic optical fiber sensors can exhibit high sensing performance, in terms of refractive index sensitivities (RISs). However, a comprehensive understanding of the factors governing the RIS in this type of sensor remains limited, with existing reports often overlooking the presence of surface plasmon resonance (SPR) phenomena in nanoparticle (NP) assemblies and attributing high RIS to plasmonic coupling or waveguiding effects. Herein, using plasmonic optical fiber sensors based on spherical Au nanoparticles, we investigate the basis of their enhanced RIS, both experimentally and theoretically. The bulk behavior of assembled Au NPs on the optical fiber was investigated using an effective medium approximation (EMA), specifically the gradient effective medium approximation (GEMA). Our findings demonstrate that the Au-coated optical fibers can support the localized surface plasmon resonance (LSPR) as well as SPR in particular scenarios. Interestingly, we found that the nanoparticle sizes and surface coverage dictate which effect takes precedence in determining the RIS of the fiber. Experimental data, in line with numerical simulations, revealed that increasing the Au NP diameter from 20 to 90 nm (15% surface coverage) led to an RIS increase from 135 to 6998 nm/RIU due to a transition from LSPR to SPR behavior. Likewise, increasing the surface coverage of the fiber from 9% to 15% with 90 nm Au nanoparticles resulted in an increase in RIS from 1297 (LSPR) to 6998 nm/RIU (SPR). Hence, we ascribe the exceptional performance of these plasmonic optical fibers primary to SPR effects, as evidenced by the nonlinear RIS behavior. The outstanding RIS of these plasmonic optical fibers was further demonstrated in the detection of thrombin protein, achieving very low limits of detection. These findings support broader applications of high-performance NP-based plasmonic optical fiber sensors in areas such as biomedical diagnostics, environmental monitoring, and chemical analysis.

In this paper, the concept of anisotropic impedance holographic metasurface is proposed and validated by realizing holographic imaging with multipoint focusing techniques in near-field areas at the radio frequency domain. Combining the microwave holographic leaky-wave theory and near-field focusing principle, the mapped geometrical patterns can be constructed based on the correspondence between meta-atom structural parameters and equivalent scalar impedances in this modulated metasurface. Different from conventional space-wave modulated holographic imaging metasurfaces, this surface-wave-based holographic metasurface fed by monopole antenna embedded back on metal ground enables elimination of the misalignment error between the air feeding and space-wave-based metasurface and increase of the integration performance, which characterizes ultra-low profile, low cost, and easy integration. The core innovation of this paper is to use the classical anisotropic equivalent surface impedance method to achieve the near-field imaging effect for the first time. Based on this emerging technique, a surface-wave meta-hologram is designed and verified through simulations and experimental measurements, which offers a promising choice for microwave imaging, information processing, and holographic data storage.

Single-pixel imaging is a burgeoning computational imaging technique that utilizes a single detector devoid of spatial resolution to capture an image, offering great potential for creating cost-effective and simplified imaging systems. Nevertheless, achieving super-resolution with a single pixel remains a formidable challenge. Here, we introduce a single-pixel super-resolution imaging technique based on space–time modulation. The modulation parametrically mixes the incoming signals, enabling the space–time scattered signals of the object carrying finer details to be captured by the single-pixel imaging system. To validate our proposed technique, we designed and fabricated a computational metasurface imager that needs only a single transmitting port and a single receiving port. The achieved resolution surpasses the Abbe resolution limit. The principle of our proposed technique is well-suited for low-cost and compact imaging systems.

Mid-infrared frequency-comb spectroscopy enables measurement of molecules at megahertz spectral resolution, sub-hertz frequency accuracy, and microsecond acquisition speed. However, the widespread adoption of this technique has been hindered by the complexity and alignment sensitivity of mid-infrared frequency-comb sources. Leveraging the underexplored mid-infrared window of silica fibers presents a promising approach to address these challenges. In this study, we present the first, to the best of our knowledge, experimental demonstration and quantitative numerical description of mid-infrared frequency-comb generation in silica fibers. Our all-silica-fiber frequency comb spans over two octaves (0.8 μm to 3.4 μm) with a power output of 100 mW in the mid-infrared region. The amplified quantum noise is suppressed using four-cycle (25 fs) driving pulses, with the carrier-envelope offset frequency exhibiting a signal-to-noise ratio of 40 dB and a free-running bandwidth of 90 kHz. Our developed model provides quantitative guidelines for mid-infrared frequency-comb generation in silica fibers, enabling all-fiber frequency-comb spectroscopy in diverse fields such as organic synthesis, pharmacokinetics processes, and environmental monitoring.

Self-assembly of dissipative solitons arouses versatile configurations of molecular complexes, enriching intriguing dynamics in mode-locked lasers. The ongoing studies fuel the analogy between matter physics and optical solitons, and stimulate frontier developments of ultrafast optics. However, the behaviors of multiple constituents within soliton molecules still remain challenging to be precisely unveiled, regarding both the intramolecular and intermolecular motions. Here, we introduce the concept of “soliton isomer” to elucidate the molecular dynamics of multisoliton complexes. The time-lens and time-stretch techniques assisted temporal-spectral analysis reveals the diversity of assembly patterns, reminiscent of the “isomeric molecule”. Particularly, we study the fine energy exchange during the intramolecular motions, therefore gaining insights into the degrees of freedom of isomeric dynamics beyond temporal molecular patterns. All these findings further answer the question of how far the matter-soliton analogy reaches and pave an efficient route for assisting the artificial manipulation of multisoliton structures.

Resonant metasurfaces provide a promising solution to overcome the limitations of nonlinear materials in nature by enhancing the interaction between light and matter and amplifying optical nonlinearity. In this paper, we design an aluminum (Al) metasurface that supports surface lattice resonance (SLR) with less nanoparticle filling density but more prominent saturable absorption effects, in comparison to a counterpart that supports localized surface plasmon resonance (LSPR). In detail, the SLR metasurface exhibits a narrower resonance linewidth and a greater near-field enhancement, leading to a more significant modulation depth (9.6%) at a low incident fluence of 25 μJ/cm2. As an application example, we have further achieved wavelength-tunable Q-switched pulse generation from 1020 to 1048 nm by incorporating the SLR-based Al metasurface as a passive saturable absorber (SA) in a polarization-maintaining ytterbium-doped fiber laser. Typically, the Q-switched pulse with a repetition rate of 33.7 kHz, pulse width of 2.1 μs, pulse energy of 141.7 nJ, and signal-to-noise ratio (SNR) of greater than 40 dB at the fundamental frequency can be obtained. In addition, we have investigated the effects of pump power and central wavelength of the filter on the repetition rate and pulse width of output pulses, respectively. In spite of demonstration of only using the Al metasurface to achieve a passive Q-switched fiber laser, our work offers an alternative scheme to build planar, lightweight, and broadband SA devices that could find emerging applications from ultrafast optics to neuromorphic photonics, considering the fast dynamics, CMOS-compatible fabrication, and decent nonlinear optical response of Al-material-based nanoplasmonics.

The realization of spatiotemporal vortex structure of various physical fields with transverse orbital angular momentum (OAM) has attracted much attention and is expected to expand the research scope and open new opportunities in their respective fields. Here we present theoretically the first, to the best of our knowledge, study on the generation of attosecond pulse trains featuring a spatiotemporal optical vortex (STOV) structure by a two-color femtosecond light field, with each color carrying transverse OAM. Through careful optimization of relative phase and intensity ratio, we validate the efficient upconversion of the infrared pulse into its tens of order harmonics, showing that each harmonic preserves a corresponding intact topological charge. This unique characteristic enables the synthesis of an extreme ultraviolet attosecond pulse train with transverse OAM. In addition, we reveal that ionization depletion plays an outsize role therein. Our studies pave the way for the generation and utilization of light fields with STOV in the attosecond regime.