Huygens metasurfaces have demonstrated remarkable potential in perfect transmission and precise wavefront modulation through the synergistic integration of electric resonance and magnetic resonance. However, prevailing active or reconfigurable Huygens metasurfaces, based on all-optical systems, encounter formidable challenges associated with the intricate control of bulk dielectric using laser equipment and the presence of residual thermal effects, leading to limitations in continuous modulation speeds. Here, we present an ultrafast electrically driven terahertz Huygens metasurface that comprises an artificial microstructure layer featuring a two-dimensional electron gas (2DEG) provided by an AlGaN/GaN heterojunction, as well as a passive microstructure layer. Through precise manipulation of the carrier concentration within the 2DEG layer, we effectively govern the current distribution on the metasurfaces, inducing variations in electromagnetic resonance modes to modulate terahertz waves. This modulation mechanism achieves high efficiency and contrast for terahertz wave manipulation. Experimental investigations demonstrate continuous modulation capabilities of up to 6 GHz, a modulation efficiency of 90%, a transmission of 91%, and a remarkable relative operating bandwidth of 55.5%. These significant advancements substantially enhance the performance of terahertz metasurface modulators. Importantly, our work not only enables efficient amplitude modulation but also introduces an approach for the development of high-speed and efficient intelligent transmissive metasurfaces.

The dielectric confinement effect plays an essential role in optoelectronic devices. Existing studies on the relationship between the dielectric confinement and the photoelectric properties are inadequate. Herein, three organic spacers with different dielectric constants are employed to tune the exciton dynamics of quasi-two-dimensional (quasi-2D) Ruddlesden–Popper perovskite films. Femtosecond transient absorption spectroscopy reveals that the small dielectric constant ligand enables a weak dynamic disorder and a large modulation depth of the coherent phonons, resulting in a more complete energy transfer and the inhibition of a trap-mediated nonradiative recombination. Additionally, the increase in the bulk-ligand dielectric constant reduces the corresponding exciton binding energy and then suppresses the Auger recombination, which is beneficial for high-luminance light-emitting diodes. This work emphasizes the importance of dielectric confinement for regulating the exciton dynamics of layered perovskites.

Yttrium iron garnet (YIG) is a promising material for various terahertz applications due to its special optical properties. At present, a high-quality YIG wafer is the desire of terahertz communities and it is still challenging to prepare substrate-free YIG single crystal films. In this work, we prepared wafer-level substrate-free La:YIG single crystal films, for the first time, to our knowledge. Terahertz optical and magneto-optical properties of La:YIG films were characterized by terahertz time domain spectroscopy (THz-TDS). Results show that the as-prepared La:YIG film has an insertion loss of less than 3 dB and a low absorption coefficient of less than 10 cm-1 below 1.6 THz. Benefitting from the thickness of the substrate-free YIG films and low insertion loss, their terahertz properties could be further manipulated by simply using a wafer-stacking technique. When four La:YIG films were stacked, there was an insertion loss of less than 10 dB in the range of 0.1-1.2 THz. The Faraday rotation angle of the four-layer-stacked La:YIG films reached 19°, and the isolation could reach 17 dB. By further increasing the stacking number to eight pieces, a remarkable Faraday rotation angle of 45° was achieved with an isolation of 23 dB, which is important for practical application in the THz band. This material may provide a milestone opportunity to make various non-reciprocal devices, such as isolators and phase shifters.

Scintillators are widely utilized in high-energy radiation detection in view of their high light yield and short fluorescence decay time. However, constrained by their current shortcomings, such as complex fabrication procedures, high temperature, and difficulty in the large scale, it is difficult to meet the increasing demand for cost-effective, flexible, and environment-friendly X-ray detection using traditional scintillators. Perovskite-related cesium copper halide scintillators have recently received multitudinous research due to their tunable emission wavelength, high photoluminescence quantum yield (PLQY), and excellent optical properties. Herein, we demonstrated a facile solution-synthesis route for indium-doped all-inorganic cesium copper iodide (Cs3Cu2I5) powders and a high scintillation yield flexible film utilizing indium-doped Cs3Cu2I5 powders. The large area flexible films achieved a PLQY as high as 90.2% by appropriately adjusting the indium doping concentration, much higher than the undoped one (73.9%). Moreover, benefiting from low self-absorption and high PLQY, the Cs3Cu2I5:In films exhibited ultralow detection limit of 56.2 nGy/s, high spatial resolution up to 11.3 lp/mm, and marvelous relative light output with strong stability, facilitating that Cs3Cu2I5:In films are excellent candidates for X-ray medical radiography. Our work provides an effective strategy for developing environment-friendly, low-cost, and efficient scintillator films, showing great potential in the application of high-performance X-ray imaging.

Janus metasurface holography with asymmetric transmission characteristics provides new degrees of freedom for multiplexing technologies. However, earlier metasurfaces with asymmetrical transmission faced limitations in terms of tunability and multifunctionality. In this study, we propose a metasurface color holographic encryption scheme with dynamic switching and asymmetric transmission at visible frequencies using a low-loss nonvolatile optical phase-change material, Sb2S3. Using a modified holographic optimization strategy, we achieved high-fidelity asymmetric holographic imaging of a nanostructured metasurface. By controlling the incident direction and wavelength of visible light, as well as the level of crystallization of Sb2S3, this reconfigurable metasurface enables the precise manipulation of tunable color holographic image displays. In particular, in the semi-crystalline state of Sb2S3, the encoded information can be securely encrypted using a two-channel color-holographic image, whereas only a preset camouflaged image is displayed in the crystalline or amorphous state of Sb2S3. The proposed multiencrypted Janus metasurface provides a potential approach for dynamic holographic displays with ultrahigh capacity, holographic encryption, and information storage.

Circular dichroism (CD) is extensively used in various material systems for applications including biological detection, enantioselective catalysis, and chiral separation. This paper introduces a chiral absorptive metasurface that exhibits a circular polarization-selective effect in dual bands—positive and negative CD peaks at short wavelengths and long wavelengths, respectively. Significantly, we uncover that this phenomenon extends beyond the far-field optical response, as it is also observed in the photothermal effect and the dynamics of thermally induced fluid motion. By carefully engineering the metasurface design, we achieve two distinct CD signals with high g factors (∼1) at the wavelengths of 877 nm and 1045 nm, respectively. The findings presented in this study advance our comprehension of CD and offer promising prospects for enhancing chiral light–matter interactions in the domains of nanophotonics and optofluidics.

Dynamic infrared thermal camouflage technology has attracted extensive attention due to its ability to thermally conceal targets in various environmental backgrounds by tuning thermal emission. The use of phase change materials (PCMs) offers numerous advantages, including zero static power, rapid modulation rate, and large emissivity tuning range. However, existing PCM solutions still encounter several practical application challenges, such as temperature uniformity, amorphization achievement, and adaptability to different environments. In this paper, we present the design of an electrically controlled metal-insulator-metal thermal emitter based on a PCM metasurface, and numerically investigate its emissivity tunability, physical mechanisms, heat conduction, and thermal camouflage performance across different backgrounds. Furthermore, the influence of the quench rate on amorphization was studied to provide a guidance for evaluating and optimizing device structures. Simulation results reveal that the thermal emitter exhibits a wide spectral emissivity tuning range between 8 and 14 μm, considerable quench rates for achieving amorphization, and the ability to provide thermal camouflage across a wide background temperature range. Therefore, it is anticipated that this contribution will promote the development of PCM-based thermal emitters for practical dynamic infrared thermal camouflage technology with broad applications in both civilian and military domains.

We report on deep-to-near-UV transient absorption spectra of core-shell Au/SiO2 and Au/TiO2 nanoparticles (NPs) excited at the surface plasmon resonance of the Au core, and of UV-excited bare anatase TiO2 NPs. The bleaching of the first excitonic transition of anatase TiO2 at ∼3.8 eV is a signature of the presence of electrons/holes in the conduction band (CB)/valence band (VB) of the material. We find that while in bare anatase TiO2 NPs, two-photon excitation does not occur up to the highest used fluences (1.34 mJ/cm2), it takes place in the TiO2 shell at moderate fluences (0.18 mJ/cm2) in Au/TiO2 core-shell NPs, as a result of an enhancement due to the plasmon resonance. We estimate the enhancement factor to be of the order of ∼108–109. Remarkably, we observe that the bleach of the 3.8 eV band of TiO2 lives significantly longer than in bare TiO2, suggesting that the excess electrons/holes in the conduction/valence band are stored longer in this material.

Optical metamaterials offer the possibility of controlling the behavior of photons similarly to what has been done about electrons in semiconductors. However, most optical metamaterials are narrowband, and they achieve negative refraction within a small window of incident angles, making them impractical for common visible light systems that operate effectively over a wide range of frequencies and directions. Considerable resistive loss at the resonant frequency of these metamaterials further prevents them from being deployed in the real world. Here, we develop a novel metamaterial randomly assembled by a list of narrowband, omnidirectional, and ultralow-loss meta-cluster systems using a bottom-up approach. Weak interactions among numerous meta-cluster sets greatly broaden the effective bandwidth of the overall structure, exhibiting frequency selectivity and spatial modulation when responding to white-light illumination. We observe negative refraction in the 490–730 nm band, and observe an inverse Doppler effect at green, yellow, and red frequencies, across most of the visible spectrum. Our method allows for low-cost fabrication of sizable broadband omnidirectional three-dimensional metamaterial samples, which opens the door to the rapid development of optical metamaterials, micro–nano assembly and preparation, tunable optical device engineering, etc.

Metasurfaces have provided an unprecedented degree of freedom (DOF) in the manipulation of electromagnetic waves. A geometric phase can be readily obtained by rotating the meta-atoms of a metasurface. Nevertheless, such geometric phases are usually spin-coupled, with the same magnitude but opposite signs for left- and right-handed circularly polarized (LCP and RCP) waves. To achieve independent control of LCP and RCP waves, it is crucial to obtain spin-decoupled geometric phases. In this paper, we propose to obtain completely spin-decoupled geometric phases by engineering the surface current paths on meta-atoms. Based on the rotational Doppler effect, the rotation manner is first analyzed, and it is found that the generation of a geometric phase lies in the rotation of the surface current paths on meta-atoms. Since the induced surface current paths under the LCP and RCP waves always start oppositely and are mirror-symmetrical with each other, it is natural that the geometric phases have the same magnitude and opposite signs when the meta-atoms are rotated. To obtain spin-decoupled geometric phases, the induced surface current under one spin should be rotated by one angle while the current under the other spin is rotated by a different angles. In this way, LCP and RCP waves can acquire different geometric phase changes. Proof-of-principle prototypes were designed, fabricated, and measured. Both the simulation and experiment results verify spin-decoupled geometric phases. This work provides a robust means to obtain a spin-dependent geometric phase and can be readily extended to higher frequency bands such as the terahertz, IR, and optical regimes.

Lead halide perovskite microlasers have shown impressive performance in the green and red wavebands. However, there has been limited progress in achieving blue-emitting perovskite microlasers. Here, blue-emitting perovskite-phase rubidium lead bromide (RbPbBr3) microcubes were successfully prepared by using a one-step chemical vapor deposition process, which can be utilized to construct optically pumped whispering gallery mode microlasers. By regulating the growth temperature, we found that a high-temperature environment can facilitate the formation of the perovskite phase and microcubic morphology of RbPbBr3. Notably, blue single-mode lasing in a RbPbBr3 microcubic cavity with a narrow linewidth of 0.21 nm and a high-quality factor (∼2200) was achieved. The obtained lasing from RbPbBr3 microlasers also exhibited an excellent polarization state factor (∼0.77). By modulating the mixed-monovalent cation composition, the wavelength of the microlaser could be tuned from green (536 nm) to pure blue (468 nm). Additionally, the heat stability of the mix-cation perovskite was better than that of conventional CsPbBr3. The stable and high-performance blue single-mode microlasers may thus facilitate the application of perovskite lasers in blue laser fields.

Pseudospin is an angular momentum degree of freedom introduced in analogy to the real electron spin in the effective massless Dirac-like equation used to describe wave evolution at conical intersections such as the Dirac cones of graphene. Here, we study a photonic implementation of a chiral borophene allotrope hosting a pseudospin-2 conical intersection in its energy–momentum spectrum. The presence of this fivefold spectral degeneracy gives rise to quasiparticles with pseudospin up to ±2. We report on conical diffraction and pseudospin–orbit interaction of light in photonic chiral borophene, which, as a result of topological charge conversion, leads to the generation of highly charged optical phase vortices.

Although the effective “stealth” of space vehicles is important, current camouflage designs are inadequate in meeting all application requirements. Here, a multilayer wavelength-selective emitter is demonstrated. It can realize visible light and dual-band mid-infrared camouflage with thermal control management in two application scenarios, with better effect and stronger radiation cooling capability, which can significantly improve the stealth and survivability of space vehicles in different environments. The selective emitter demonstrated in this paper has the advantages of simple structure, scalability, and ease of large-area fabrication, and has made a major breakthrough in driving multiband stealth technology from simulation research to physical verification and even practical application.

We present a hybrid coupling scheme of a magnetic toroidal and electric dipole metasurface with suppressed radiation loss, which can produce the tunable plasmon-induced transparency (PIT) with an enhanced slow-light effect in the terahertz regime. The terahertz metasurface is constructed by nesting a dual-split ring resonator (DSRR) inside a ring resonator (RR) to exploit the destructive coherence of hybrid electromagnetic mode coupling at the PIT resonance. The polarization-dependence excitation performs the active tunability of a PIT-induced group slowing down by rotating the polarization angle, experimentally achieving a maximum group delay of 3.5 ps. Furthermore, the modified terahertz metasurface with a four-split ring resonator (FSRR) nested in an RR is prepared on photoconductive silicon, demonstrating the pump-controllable group delay effect at the PIT resonance. The large group delay from 2.2 to 0.9 ps is dynamically tunable by adjusting the pump power. The experimental results are in good accord with the theoretical simulations.

Perfect optical vortices (POVs), characterized as a ring radius independent of topological charge (TC), possess extensive application in particle manipulation and optical communication. At present, the complex and bulky optical device for generating POVs has been miniaturized by leveraging the metasurface, and either spin-dependent or spin-independent POV conversions have been further accomplished. Nevertheless, it is still challenging to generate superposed POVs for incidences with orthogonal circular polarization. Here, a spin-multiplexed all-dielectric metasurface method for generating superposed POVs in the terahertz frequency range is proposed and demonstrated. By using the multiple meta-atom comprised structure as the basic unit, the complex amplitude of two superposed POVs is modulated, decoupled, and subsequently encoded to left- and right-handed circular polarization incidences. Furthermore, two kinds of metasurfaces are fabricated and characterized to validate this controlling method. It is demonstrated that the measured intensity and phase distributions match well with the calculation of the Rayleigh–Sommerfeld diffraction integral, and the radius of superposed POVs is independent of TCs. This work provides promising opportunities for developing ultracompact terahertz functional devices applied to complex structured light generation and terahertz communication, and exploring sophisticated spin angular momentum and orbital angular momentum interactions like the photonic spin-Hall effect.

Three-dimensional chiral materials with intrinsic chirality play a crucial role in achieving a strong chiral response and flexible light manipulation. Reconfigurable chirality through the 3D morphological transformation of chiral materials is significant for greater freedom in tailoring light but remains a challenge. Inspired by the unique 3D morphological memory capability of shape memory alloys (SMAs), we demonstrate and discuss a chiral resonator in the microwave regime that can realize reconfigurable chirality through 3D morphological transformation. The introduction of heating film realizes voltage control of SMA’s morphology for utilizing the temperature sensitivity of SMA better, enabling arbitrary control of circular dichroism (CD) flip and CD intensity. The qualitative and quantitative analysis of the surface current distribution of chiral enantiomers reveals that the chirality of meta-atoms originates from the surge of electric dipole px and electric quadrupole Q. It is worth mentioning that the proposed strategy to achieve reconfigurable chirality using 3D morphological transformations can be directly extended to other higher frequencies, such as visible, infrared, and terahertz bands. Significantly, our paradigm to study the relationship between complex 3D morphology and chirality holds potential for application in biosensing, spin detection, and spin-selective devices.

The modulation of thermal radiation in the infrared region is a highly anticipated method to achieve infrared sensing and camouflage. Here, a multiband metamaterial emitter based on the Al/SiO2/Al nanosandwich structure is proposed to provide new ideas for effective infrared and laser-compatible camouflage. By virtue of the intrinsic absorption and magnetic resonance property of lossy materials, the thermal radiation in the infrared region can be rationally modulated. The fabricated samples generally present low emissivity (ε3–5 μm=0.21, ε8–14 μm=0.19) in the atmospheric windows to evade infrared detection as well as high emissivity (ε5–8 μm=0.43) in the undetected band for energy dissipation. Additionally, the laser camouflage is also realized by introducing a strong absorption at 10.6 μm through the nonlocalized plasmon resonance of the SiO2 layer. Moreover, the fabricated emitter shows promising prospects in thermal management due to the good radiative cooling property that is comparable to the metallic Al material. This work demonstrates a multiband emitter based on the metasurface structure with compatible infrared-laser camouflage as well as radiative cooling properties, which is expected to pave new routes for the design of thermal radiation devices.

In this paper, a 3D meta-atom-based structure is constructed for the multifunctional compatible design of visible, infrared, and microwave. To achieve high performance, a novel dispersion tailoring strategy is proposed. Through the incorporation of multiple controllable losses within the 3D meta-atom, the dispersion characteristics are tailored to the desired target region. The effectiveness of the strategy is verified with an error rate of less than 5%. A proof-of-concept prototype is designed and fabricated, exhibiting high visible transparency, low infrared emission of 0.28, and microwave ultra-broadband absorption with a fractional bandwidth of 150% under 2.7 to 18.7 GHz. This work contributes a novel design strategy for the development of high-performance multispectral stealth materials with wide applications.

Optical helicity provides us with an effective means to control the helicity-dependent photocurrent in the spin-momentum-locked surface states of topological insulators (TIs). Also, the TIs show potential in polarization detection as an intrinsic solid-state optical chirality detector for easier integration and fabrication. However, the complex photoresponses with the circular photogalvanic effect, the linear photogalvanic effect, and the photon drag effect in the TIs prevent them from direct chirality detection of the elliptically polarized light. Here, by fitting with the theoretical models to the measured photocurrents, the microscopic origin of different components of the helicity-dependent photocurrent has been demonstrated. We show a comprehensive study of the helicity-dependent photocurrent in (Bi1-xSbx)2Te3 thin films of different thicknesses as a function of the light incident angle and the gate-tuned chemical potential. The observation of the light incident angle dependence of the helicity-dependent photocurrent provides us with a polarization detection strategy using a TI thin film without the use of any additional optical elements, and the detection accuracy can be enhanced by gate tuning. Additionally, the Stokes parameters can be extracted by arithmetic operation of photocurrents measured with different incident angles and gating voltages for complete characterization of the polarization states of a light beam. Using this means, we realize the polarization detection and the Stokes parameters analysis with a single device. Our work provides an alternative solution to develop miniaturized intrinsic polarization-sensitive photodetectors.

Graphene quantum dots (GQDs), fascinating semiconductors with stable photoluminescence (PL), have important potential applications in the fields of biology, medicine, and new semiconductor devices. However, it is still challenging to overcome the weak PL intensity. Here, we report a strategy for selective resonance enhancement of GQD fluorescence using gold nanoparticles (AuNPs) as plasmas. Interestingly, the addition of low concentration AuNP makes AuNP/GQDs exhibit significant fluorescence enhancement of 2.67 times in the visible range. The addition of high concentration AuNP leads to the formation of an excitation peak at 421 nm and selectively enhances certain radiation modes. We concluded that the main reason for the selective enhancement of PL intensity in high concentration AuNP is the transfer of generous hot electrons at high energy states from AuNP to GQD and relaxation to the ground state. The electron resonance of low concentration AuNP transfers to GQD and relaxes to lower energy levels, exhibiting an overall enhancement of PL intensity. We apply it for detection of the heavy metal ion Cr3+, and verify that it has a correlation coefficient of 97.36%. We believe AuNP/GQDs can be considered excellent candidates for heavy metal detection and high fluorescence bio-imaging.

The kinetics of photoinduced changes, namely, photobleaching and photodarkening in sputtered ternary Ge29Sb8Se63 thin films, was studied. The study of time evolution of the absorption coefficient Δα(t) upon room-temperature near-bandgap irradiation revealed several types of photoinduced effects. The as-deposited films exhibited a fast photodarkening followed by a dominative photobleaching process. Annealed thin films were found to undergo photodarkening only. The local structure studied by Raman scattering spectroscopy showed significant structural changes upon thermal annealing, which are presumably responsible for a transition from the photobleaching observed in as-deposited and reversible photodarkening in annealed thin films. Moreover, a transient photodarkening process was observed in both as-deposited and annealed thin films. The influence of the initial film thickness and laser optical intensity on the kinetics of photoinduced changes is discussed.

The integration of a single III-V semiconductor quantum dot with a plasmonic nanoantenna as a means toward efficient single-photon sources (SPEs) is limited due to its weak, wide-angle emission, and low emission rate. These limitations can be overcome by designing a unique linear array of plasmonic antenna structures coupled to nanowire-based quantum dot (NWQD) emitters. A linear array of a coupled device composed of multiple plasmonic antennas at an optimum distance from the quantum dot emitter can be designed to enhance the directionality and the spontaneous emission rate of an integrated single-photon emitter. Finite element modeling has been used to design these compact structures with high quantum efficiencies and directionality of single-photon emission while retaining the advantages of NWQDs. The Purcell enhancement factor of these structures approaches 66.1 and 145.8, respectively. Compared to a single NWQD of the same diameter, the fluorescence was enhanced by 1054 and 2916 times. The predicted collection efficiencies approach 85% (numerical aperture, NA=0.5) and 80% (NA=0.5), respectively. Unlike single-photon emitters based on bulky conventional optics, this is a unique nanophotonic single-emission photon source based on a line-array configuration that uses a surface plasmon-enhanced design with minimum dissipation. The designs presented in this work will facilitate the development of SPEs with potential integration with semiconductor optoelectronics.

1T-polytype tantalum disulfide (1T-TaS2), an emerging strongly correlated material, features a narrow bandgap of 0.2 eV, bridging the gap between zero-bandgap graphene and large-bandgap 2D nonlinear optical (NLO) materials. Combined with its intense light absorption, high carrier concentration, and high mobility, 1T-TaS2 shows considerable potential for applications in broadband optoelectronic devices. However, its NLO characteristics and related applications have rarely been explored. Here, 1T-TaS2 nanosheets are prepared by chemical vapor deposition. The ultrafast carrier dynamics in the 400–1100 nm range and broadband NLO performance in the 515–2500 nm range are systematically studied using femtosecond lasers. An obvious saturable absorption phenomenon is observed in the visible to IR range. The nonlinear absorption coefficient is measured to be -22.60±0.52 cm MW-1 under 1030 nm, which is larger than that of other typical 2D saturable absorber (SA) materials (graphene, black phosphorus, and MoS2) under similar experimental conditions. Based on these findings, using 1T-TaS2 as a new SA, passively Q-switched laser operations are successfully performed at 1.06, 1.34, and 1.94 μm. The results highlight the promise of 1T-TaS2 for broadband optical modulators and provide a potential candidate material system for mid-IR nonlinear optical applications.

Recent moiré configurations provide a new platform for tunable and sensitive photonic responses, as their enhanced light–matter interactions originate from the relative displacement or rotation angle in a stacking bilayer or multilayer periodic array. However, previous findings are mostly focused on atomically thin condensed matter, with limitations on the fabrication of multilayer structures and the control of rotation angles. Structured microwave moiré configurations are still difficult to realize. Here, we design a novel moiré structure, which presents unprecedented capability in the manipulation of light–matter interactions. Based on the effective medium theory and S-parameter retrieval process, the rotation matrix is introduced into the dispersion relation to analyze the underlying physical mechanism, where the permittivity tensor transforms from a diagonal matrix to a fully populated one, whereas the permeability tensor evolves from a unit matrix to a diagonal one and finally becomes fully filled, so that the electromagnetic responses change drastically as a result of stacking and rotation. Besides, the experiment and simulation results reveal hybridization of eigenmodes, drastic manipulation of surface states, and magic angle properties by controlling the mutual rotation angles between two isolated layers. Here, not only a more precisely controllable bilayer hyperbolic metasurface is introduced to moiré physics, the findings also open up a new avenue to realize flat bands at arbitrary frequencies, which shows great potential in active engineering of surface waves and designing multifunctional plasmonic devices.

Metasurfaces, especially tunable ones, have played a major role in controlling the amplitude, phase, and polarization of electromagnetic waves and attracted growing interest, with a view toward a new generation of miniaturized devices. However, to date, most existing reconfigurable devices are bounded in volatile nature with sustained external energy to maintain and single functionality, which restrict their further applications. Here, we demonstrate for the first time, to our knowledge, nonvolatile, reconfigurable, and dynamic Janus metasurfaces by incorporating phase-change material Ge2Se2Te5 (GST) in the terahertz (THz) regime. First, we experimentally show the reversible switching characteristic of GST on large areas by applying a single nanosecond laser pulse, which exhibits excellent contrast of THz properties in both states. Then, we present a multiplex metasurface scheme. In each metasurface, three sets of structures are adopted, in which two sets integrate GST. The effective structures can be reversely modulated by the amorphization and crystallization of GST. As a proof of concept, the dynamic beam splitter, bifocal metalens, dual-mode focusing optical vortex generators, and switchable metalens/focusing optical vortex generators are designed, fabricated, and experimentally characterized, and can be switched reversibly and repeatedly with the help of optical and thermal stimuli. Our scheme will pave the way toward the development of multifunctional and compact THz devices and may find use for applications in THz imaging, sensing, and communications.

MAPbI3 perovskite has attracted widespread interests for developing low-cost near infrared semiconductor gain media. However, it faces the instability issue under operation conditions, which remains a critical challenge. It is found that the instability of the MAPbI3 nanoplatelet laser comes from the thermal-induced degradation progressing from the surface defects towards neighboring regions. By using PbI2 passivation, the defect-initiated degradation is significantly suppressed and the nanoplatelet degrades in a layer-by-layer way, enabling the MAPbI3 laser to sustain for 4500 s (2.7×107 pulses), which is nearly three times longer than that of the nanoplatelet laser without passivation. Meanwhile, the PbI2 passivated MAPbI3 nanoplatelet laser with the nanoplatelet cavity displays a maximum quality factor up to ∼7800, the highest reported for all MAPbI3 nanoplatelet cavities. Furthermore, a high stability MAPbI3 nanoplatelet laser that can last for 8500 s (5.1×107 pulses) is demonstrated based on a dual passivation strategy, by retarding the defect-initiated degradation and surface-initiated degradation simultaneously. This work provides in-depth insights for understanding the operating degradation of perovskite lasers, and the dual passivation strategy paves the way for developing high stability near infrared semiconductor laser media.

Both absorption and diffuse reflection can effectively suppress microwave backward reflection. However, the challenge of designing wideband absorptive elements with anti-phase reflection hinders the simultaneous working of the two principles. With aid of the wideband characteristic of bilateral complementary structure, we propose a strategy to design wideband absorptive elements with large reflection phase differences. For proof of concept, the proposed elements are arranged in a rectangular grid by optimizing scattering field distribution. The proposed diffusion metabsorber achieves over 20-dB scattering field reduction in the range of 8.5–20.3 GHz with good polarization stability and high angular insensitivity of up to ±40°, which has been verified by real experiments. Furthermore, the proposed design strategy exhibits the potential to further reduce electromagnetic wave reflection, and the optical transparent characteristic is promising for window applications.

3D printing technologies have expanded beyond the research laboratories where they were used solely for prototyping and have become widely used in several industries. The production of custom 3D objects has significant potential in optical applications. However, this necessitates extremely specific material properties, such as transparency, homogeneity, birefringence, and surface finish. Currently, the majority of optical objects are manufactured using plastics. Moreover, the 3D printing processes using polymers to produce optical objects have significant advantages, such as limited wastage, short manufacturing time, and easy customization. However, despite extensive efforts, no technology has achieved the production of objects perfectly suited for optical applications. The objective of this review is to summarize recent advances in the field of 3D printing for optics, with an emphasis on specific developments for dedicated applications, and to explore new candidate processes.

Broadband near-infrared (NIR)-emitting materials are crucial components of the next generation of smart NIR light sources based on blue light-emitting diodes (LEDs). Here, we report a Te cluster-doped borate glass, which exhibits ultra-broadband emission around 980 nm with a full-width at half-maximum (FWHM) of 306 nm under blue light excitation. We propose adjustments of glass chemistry and processing condition as a means for topo-chemical tailoring of the NIR photoemission characteristics in such materials. Through implementing strongly reducing conditions during glass melting, Te clusters with broad NIR photoluminescence can be generated and stabilized once the melt is vitrified to the glassy state. Tunability of the NIR emission peak over the wavelength range of 904 to 1026 nm is possible in this way, allowing for fine adjustments of spectral properties relative to the stretching vibrations of common chemical bonds, for example, in water, proteins, and fats. This potentially enables high sensitivity in NIR spectroscopy. We further demonstrate potential application of glass-converted LEDs in night vision.

Inorganic halide perovskites (IHPs) have received substantial attention due to their unique optoelectronic properties. Among all the intriguing performance, the efficient luminescence of IHPs enables the practical application of white light-emitting diodes (WLEDs) for lighting. During the last decade, IHP-based white lighting sources with a high luminesce and a broad color gamut have been developed as strong competitors to conventional and classic WLEDs based on rare-earth phosphors and blue LED chips. Thus, it inspires us to give an overview of the emerging progress of IHP WLEDs that can function as lighting sources. Here, in this review, the generation of luminescent properties and white light in IHPs are first presented. Then, both photoluminescence and electroluminescence WLEDs with IHPs emitters, including both lead-based and lead-free IHPs, are synthetically discussed to exhibit their advantages. Furthermore, the efforts on the optical performance enhancement of IHPs in WLEDs are demonstrated and summarized. Apart from WLEDs, visible light communication based on IHPs featuring efficient luminescence is proposed to highlight their promising potential in lighting communication. Finally, some perspectives on the evolution and challenges are described, followed by an inspirational outlook on their future development.

The interaction between light and matter has always been the focus of quantum science, and the realization of truly strong coupling between an exciton and the optical cavity is a basis of quantum information systems. As a special semiconductor material, colloidal quantum dots have fascinating optical properties. In this study, the photoluminescence spectra of colloidal quantum dots are measured at different collection angles in microcavities based on hybrid refractive-index waveguides. The photon bound states in the continuum are found in the low–high–low refractive-index hybrid waveguides in the appropriate waveguide width region, where the photoluminescence spectra of colloidal quantum dots split into two or more peaks. The upper polaritons and lower polaritons avoid resonance crossings in the systems. The Rabi splitting energy of 96.0 meV can be obtained. The observed phenomenon of vacuum Rabi splitting at room temperature is attributed to the strong coupling between quantum dots and the bound states in the continuum.

Soft photonic crystals are appealing due to their self-assembly ability, wide tunability, and multistimuli-responsiveness. However, their response time is relatively slow, ranging from milliseconds to minutes. Here, we report submicrosecond switching of chiral liquid crystals (LCs) with 1D photonic microstructures, where electric fields modify the orientational order of molecules and quench their fluctuations, rather than altering the orientation. Thus, the adjusted refractive indices result in a fast shift of the photonic bandgap, on the order of 100 ns, which is four orders of magnitude faster than conventional electro-optic switching in cholesterics. This work offers tremendous opportunities for soft photonic applications.

In recent years, all-inorganic halide perovskite quantum dots (QDs) have drawn attention as promising candidates for photodetectors, light-emitting diodes, and lasing applications. However, the sensitivity and instability of perovskite to moisture and heat seriously restrict their practical application to optoelectronic devices. Recently, a facile ligand-engineering strategy to suppress aggregation by replacing traditional long ligands oleylamine (OAm) during the hot injection process has been reported. Here, we further explore its thermal stability and the evolution of photoluminescence quantum yield (PLQY) under ambient environment. The modified CsPbBr3 QDs film can maintain 33% of initial PL intensity, but only 17% is retained in the case of unmodified QDs after 10 h continuous heating. Further, the obtained QDs with higher initial PLQY (91.8%) can maintain PLQY to 39.9% after being continuously exposed in air for 100 days, while the PLQY of original QDs is reduced to 5.5%. Furthermore, after adhering CsPbBr3 QDs on the surface of a micro SiO2 sphere, we successfully achieve the highly-efficient upconversion random laser. In comparison with the unmodified CsPbBr3 QDs, the laser from the modified CsPbBr3 QDs presents a decreased threshold of 79.81 μJ/cm2 and higher quality factor (Q) of 1312. This work may not only provide a facile strategy to synthesize CsPbBr3 QDs with excellent photochemical properties but also a bright prospect for high-performance random lasers.

Organic–inorganic halide metal perovskites are an exciting class of two-dimensional (2D) materials that have sparked renewed interest for next-generation optoelectronics. In particular, the self-trapped excitons (STEs) in 2D perovskite with excellent optical properties suggest great potential in display and narrowband detection. A prerequisite of understanding STEs’ properties is correct identification of the underlying interaction that leads to STEs. Here, the optical properties of STEs in (iso-BA)2PbI4 are characterized through laser spectroscopy at various temperatures and excitation intensities. It is found that STEs are related to the octahedral distortion caused by strong electron–phonon interaction. Trapping and detrapping between STEs and free excitons (FEs) are clearly observed. With the increase in temperature, STEs and FEs will gain enough energy and migrate to each other. Moreover, by characterizing the thickness-dependent and two-photon excitation emission, it is confirmed that STEs exist inside the material because of their weak absorption. Our findings are of great significance for not only the fundamental understanding of STEs, but also the design and optimization of 2D-perovskite-based electronic and optoelectronic devices.

As a leader of long persistent luminescence (LPL) materials, the optical properties of aluminate phosphor have remained unsurpassed for many years. As a powder material, its practical application will always be limited to the field of security signs. In this paper, the SrAl2O4:Eu2+, Dy3+ inorganic solid material with comparable LPL properties to powder materials was obtained. The crystallization mechanism and crystallite micro-morphology of inorganic glass materials have been studied, and a new opinion is put forward that the large-size SrAl2O4 crystallites in the glass matrix are stacked by rod-shaped crystals arranged in a regular direction. In addition, the SrAl2O4:Eu2+, Dy3+ glass obtained cannot only collect high-energy photons but also is sensitive to low-energy sunlight. The results show that the material exhibits superior performance in LPL, thermoluminescence, and photostimulable luminescence. Based on this property, a new application of this material in the field of information storage was explored. This paper has a certain reference value for the development and application of aluminate LPL materials in the field of smart optical information storage.

Semipolar III-nitrides have attracted increasing attention in applications of optoelectronic devices due to the much reduced polarization field. A high-quality semipolar AlN template is the building block of semipolar AlGaN-based deep-ultraviolet light emitting diodes (DUV LEDs), and thus deserves special attention. In this work, a multi-step in situ interface modification technique is developed for the first time, to our knowledge, to achieve high-quality semipolar AlN templates. The stacking faults were efficiently blocked due to the modification of atomic configurations at the related interfaces. Coherently regrown AlGaN layers were obtained on the in situ treated AlN template, and stacking faults were eliminated in the post-grown AlGaN layers. The strains between AlGaN layers were relaxed through a dislocation glide in the basal plane and misfit dislocations at the heterointerfaces. In contrast, high-temperature ex situ annealing shows great improvement in defect annihilation, yet suffers from severe lattice distortion with strong compressive strain in the AlN template, which is unfavorable to the post-grown AlGaN layers. The strong enhancement of luminous intensity is achieved in in situ treated AlGaN DUV LEDs. The in situ interface modification technique proposed in this work is proven to be an efficient method for the preparation of high-quality semipolar AlN, showing great potential towards the realization of high-efficiency optoelectronic devices.

An ultrawideband, polarization-insensitive, metamaterial absorber for oblique angle of incidence is presented using characteristic mode analysis. The absorber consists of conductive meander square loops and symmetric bent metallic strips, which are embedded with lumped resistors. With the aid of modal currents and modal weighting coefficients, the positions of the lumped resistors are determined. After that, the equivalent circuit (EC) model and admittance formula are proposed and analyzed to further understand the working principle and ultrawide bandwidth. The proposed absorber measures an absorption bandwidth of 4.3–26.5 GHz (144.1% in fractional bandwidth) for 90% absorptivity under normal incidence. At the oblique angle of incidence of 45°, the bandwidth of 90% absorptivity is still 5.1–21.3 GHz (122.72%) for transverse electric (TE) polarization, and 6.8–29.5 GHz (125.07%) for transverse magnetic (TM) polarization. The good agreement among simulation, measurement, and EC calculation demonstrates the validity of the proposed method and indicates that the method can be applied to other microwave and optical frequency bands. The proposed metamaterial absorber can be widely applied in electromagnetic compatibility, electromagnetic interference, radar stealth, and biomedical detection.

The development of efficient and cost-effective micromachines is a challenge for applied and fundamental science, given their wide fields of usage. Light is a suitable tool to move small objects in a noncontact way, given its capabilities in exerting forces and torques. However, when complex manipulation is required, micro-objects with proper architecture could play a specific role. Here we report on the rotational dynamics of core-shell particles, with a polymeric nematic core of ellipsoidal shape capped by Au nanoparticles. They undergo a peculiar synchronous spinning and orbital motion when irradiated by a simple Gaussian beam, which originates from the coupling of the metallic nanoparticles’ optical response and the core anisotropies. The rotation capabilities are strongly enhanced when the trapping wavelength lies in the plasmonic resonance region: indeed, the spin kinetic energy reaches values two orders of magnitude larger than the one of bare microparticles. The proposed strategy brings important insights into optimizing the design of light controlled micro-objects and might benefit applications in microfluidics, microrheology, and micromachining involving rotational dynamics.

Controlling quantum dots’ emission, nanostructure, and energy level alignment to achieve stable and efficient blue emission is of great significance for electroluminescence devices but remains a challenge. Here, a series of blue ZnCdSeS/ZnS quantum dots was optimized in preparation by taming their nucleation and growth kinetics. Controlling anion precursor reactive properties to modulate quantum dots’ nucleation and growth tailors their alloy core and continuous gradient energy band nanostructure. These results not only elevate the thermal stability of blue quantum dots but also further enhance the injection/transportation of carriers and improve the radiative recombination efficiency in the device. The blue ZnCdSeS/ZnS quantum dots applied in light-emitting devices show superior performance, including maximum current efficiency and external quantum efficiency of, respectively, 8.2 cd/A and 15.8% for blue, 2.6 cd/A and 10.0% for blue-violet, and 10.9 cd/A and 13.4% for sky-blue devices. The blue and sky-blue devices exhibit lifetimes of more than 10,000 h. The proposed methodology for tailoring quantum dots is expected to pave new guidelines for further facilitating visible optoelectronic device exploration.

We demonstrate that multiple higher-order topological transitions can be triggered via the continuous change of the geometry in kagome photonic crystals composed of three dielectric rods. By tuning a single geometry parameter, the photonic corner and edge states emerge or disappear with higher-order topological transitions. Two distinct higher-order topological insulator phases and a normal insulator phase are revealed. Their topological indices are obtained from symmetry representations. A photonic analog of the fractional corner charge is introduced to distinguish the two higher-order topological insulator phases. Our predictions can be readily realized and verified in configurable dielectric photonic crystals.

We study the problem of a temporal discontinuity in the permittivity of an unbounded medium with Lorentzian dispersion. More specifically, we tackle the situation in which a monochromatic plane wave forward-traveling in a (generally lossy) Lorentzian-like medium scatters from the temporal interface that results from an instantaneous and homogeneous abrupt temporal change in its plasma frequency (while keeping its resonance frequency constant). In order to achieve momentum preservation across the temporal discontinuity, we show how, unlike in the well-known problem of a nondispersive discontinuity, the second-order nature of the dielectric function now gives rise to two shifted frequencies. As a consequence, whereas in the nondispersive scenario the continuity of the electric displacement D and the magnetic induction B suffices to find the amplitude of the new forward and backward wave, we now need two extra temporal boundary conditions. That is, two forward and two backward plane waves are now instantaneously generated in response to a forward-only plane wave. We also include a transmission-line equivalent with lumped circuit elements that describes the dispersive time-discontinuous scenario under consideration.

Chiro-optical effects offer a wide range of potential applications in nanophotonics, such as advanced imaging and molecular sensing and separation. Flat single-layer metasurfaces composed of subwavelength meta-atoms have gained significant attention due to their exceptional characteristics in light–matter interactions. Although metasurface-based devices have manipulated electromagnetic waves, the compact on-chip realization of giant chiro-optical effects remains a challenge at optical frequencies. In this work, we experimentally and numerically demonstrate an all-dielectric metasurface to realize large chiro-optical effects in the visible regime. Notably, the proposed strategy of utilizing achiral nanofins instead of conventional chiral structures provides an extra degree of design freedom. The mutual coupling between carefully engineered nanofins produces constructive and destructive interference, leading to the asymmetric transmission of 70% and average circular dichroism exceeding 60%. We investigate the underlying mechanism behind the chiro-optical effects using the theory of multipolar decomposition. The proposed design mechanism maximizes the chiro-optical response through a single-layer metasurface with potential applications in high-efficiency integrated ultrathin polarization rotators and shapers, chiral polarizers for optical displays, chiral beam splitters, and chiral sensors.

The emergence of metasurfaces provides a novel strategy to tailor the electromagnetic response of electromagnetic waves in a controlled manner by judicious design of the constitutive meta-atom. However, passive metasurfaces tend to perform a specific or limited number of functionalities and suffer from narrow-frequency-band operation. Reported reconfigurable metasurfaces can generally be controlled only in a 1D configuration or use p-i-n diodes to show binary phase states. Here, a 2D reconfigurable reflective metasurface with individually addressable meta-atoms enabling a continuous phase control is proposed in the microwave regime. The response of the meta-atom is flexibly controlled by changing the bias voltage applied to the embedded varactor diode through an elaborated power supply system. By assigning appropriate phase profiles to the metasurface through voltage modulation, complex beam generation, including Bessel beams, vortex beams, and Airy beams, is fulfilled to demonstrate the accurate phase-control capability of the reconfigurable metasurface. Both simulations and measurements are performed as a proof of concept and show good agreement. The proposed design paves the way toward the achievement of real-time and programmable multifunctional meta-devices, with enormous potential for microwave applications such as wireless communication, electromagnetic imaging, and smart antennas.

Lead halide perovskite quantum dots (PQDs) display remarkable photoelectric performance. However, defects such as weak stability in air and water environments limit the development of lead halide PQDs in solid-state light applications. Herein, centrifugal spinning is used for the fabrication of stable luminous CsPbBr3 PQD nanofibers. After immersion in water for 11 months, the PQD fibers still maintained considerable photoluminescence quantum yield, showing high stability in hostile environments. The water-stability mechanism of the fibers can be explained by the changing defect density, crystal growth of PQDs, and the molecular transformation at the fiber surface. The white LED based on the CsPbBr3 fibers exhibits satisfying color gamut performance (128% of National Television System Committee). Due to the short photoluminescence lifetime of CsPbBr3 PQDs, the communication potential is also considered. The CsPbBr3 fibers obtained by centrifugal spinning present a bandwidth of 11.2 MHz, showing promising performance for solid-state light and visible light communication applications.

Structured environments are employed in a plethora of applications to tailor dynamics of light–matter interaction processes by modifying the structure of electromagnetic fields. The promising example of such a system is antiresonant photonic crystal fibers (AR-PCFs), which allow light–analyte interactions in a very long channel. Here we probe contribution of microstructuring and nontrivial mode hierarchy on light–matter interactions in AR-PCFs by investigating lifetime shortening of perovskite (CsPbBr3) nanocrystals grown to fiber capillaries. The crystals have been deposited using a wet chemistry approach and then excited by a supercontinuum source in the 450–500 nm range. Emission spectra have been measured and analyzed via the time-correlated single photon counting (TCSPC) technique, unravelling contributions of core and cladding modes. Fluorescence lifetime imaging inside an AR-PCF enables mapping input of various electromagnetic channels into light–matter interaction processes. Our results pave the way for tailoring the dynamics of high-order quantum processes, promoting the concept of AR-PCF as a light-driven reactor.

Chalcogenide glass (ChG) is an attractive material for highly efficient nonlinear photonics, which can cover an ultrabroadband wavelength window from the near-visible to the footprint infrared region. However, it remains a challenge to implement highly-efficient and low-threshold optical parametric processes in chip-scale ChG devices due to thermal and light-induced instabilities as well as a high-loss factor in ChG films. Here, we develop a systematic fabrication process for high-performance photonic-chip-integrated ChG devices, by which planar-integrated ChG microresonators with an intrinsic quality (Q) factor above 1 million are demonstrated. In particular, an in situ light-induced annealing method is introduced to overcome the longstanding instability underlying ChG film. In high-Q ChG microresonators, optical parametric oscillations with threshold power as low as 5.4 mW are demonstrated for the first time, to our best knowledge. Our results would contribute to efforts of making efficient and low-threshold optical microcombs not only in the near-infrared as presented but more promisingly in the midinfrared range.

General relativity establishes the equality between matter-energy density and the Riemann curvature of spacetime. Therefore, light or matter will be bent or trapped when passing near the massive celestial objects, and Newton’s second law fails to explain it. The gravitational effect is not only extensively studied in astronomy but also attracts a great deal of interest in the field of optics. People have mimicked black holes, Einstein’s ring, and other fascinating effects in diverse optical systems. Here, with a gradient index lens, in the geometrical optics regime, we mimic the Schwarzschild precession in the orbit of the star S2 near the Galactic Center massive black hole, which was recently first detected by European Southern Observatory. We also find other series of gradient index lenses that can be used to mimic the possible Reissner–Nordstr?m metric of Einstein’s field equation and dark matter particle motion. Light rays in such gradient lenses will be closed in some cases, while in other cases it would be trapped by the center or keep dancing around the center. Our work presents an efficient toy model to help investigate some complex celestial behaviors, which may require long period detection by using high-precision astronomical tools. The induced gradient lenses enlightened by the gravitational effect also enrich the family of absolute optical instruments for their selective closed trajectories.

The resonant optical excitation of dielectric nanostructures offers unique opportunities for developing remarkable nanophotonic devices. Light that is structured by tailoring the vectorial characteristics of the light beam provides an additional degree of freedom in achieving flexible control of multipolar resonances at the nanoscale. Here, we investigate the nonlinear scattering of subwavelength silicon (Si) nanostructures with radially and azimuthally polarized cylindrical vector beams to show a strong dependence of the photothermal nonlinearity on the polarization state of the applied light. The resonant magnetic dipole, selectively excited by an azimuthally polarized beam, enables enhanced photothermal nonlinearity, thereby inducing large scattering saturation. In contrast, radially polarized beam illumination shows no observable nonlinearity owing to off-resonance excitation. Numerical analysis reveals a difference of more than 2 orders of magnitude in photothermal nonlinearity under two types of polarization excitations. Nonlinear scattering and the unique doughnut-shaped focal spot generated by the azimuthally polarized beam are demonstrated as enabling far-field high-resolution localization of nanostructured Si with an accuracy approaching 50 nm. Our study extends the horizons of active Si photonics and holds great potential for label-free superresolution imaging of Si nanostructures.

Residual infrared absorption is a key problem affecting the laser emission efficiency of Ti:sapphire crystal. In this paper, the origin of residual infrared absorption of Ti:sapphire crystal is systematically studied by using the first principles method. According to the contact conditions of O octahedron in the crystal structure of Al2O3, four Ti3+-Ti3+ ion pair models and three Ti4+-Ti3+ ion pair models were defined and constructed. For what we believe is the first time, the near-infrared absorption spectra consistent with the experimental results were obtained in specific theoretical models. The electronic structures, absorption spectra, and charge distributions calculated show that the line-contact Ti3+-Ti3+ ion pair with antiferromagnetic coupling and the face-contact Ti4+-Ti3+ ion pair are two main contributors to the residual infrared absorption of Ti:sapphire, while some other ion pair models provide a basis to explain more complex residual infrared absorption.

Two-photon excited fluorescence materials usually suffer from inefficient two-photon absorption (TPA) and nonradiative excited states. Here, upconversion fluorescence in an electron donor-acceptor (DA) exciplex doped with fluorescent emitters are systematically investigated. It has been found that the undoped DA exciplex exhibits enhancements of ～129% and ～365% in upconversion fluorescence compared to donor- and acceptor-only systems, respectively. Interestingly, photoluminescence quantum yields (PLQYs) up to ～98.65% were measured and immensely enhanced upconversion fluorescence was observed after doping various fluorescent emitters into the DA exciplex. Our results reveal the existence of two-photon excited energy harvesting in a thermally activated delayed fluorescence (TADF) DA exciplex doped with fluorescent emitters, via reverse intersystem crossing followed by rapid F?rster resonance energy transfer. Moreover, the additional gain mechanism related to intermolecular CT interaction that occurs at the TPA stage is found in the TADF DA exciplex system.

Recently, we have witnessed an extraordinary spurt in attention toward manipulating electromagnetic waves by metasurfaces. Particularly, tailoring of circular polarization has attracted great amounts of interest in both microwave and optics regimes. Circular dichroism, an exotic chiroptical effect of natural molecules, has aroused discussion about this issue, yet it is still in its infancy. Herein, we initiate circular dichroism followed by controlling spin-selective wavefronts via chiral metasurfaces. An N-shaped chiral resonator loaded with two lumped resistors is proposed as the meta-atom producing an adequate phase gradient. Assisted by the ohmic dissipation of the introduced resistors, the effect of differential absorption provides an auxiliary degree of freedom for developing circularly polarized waves with a designated spin state. A planar corner reflector that can achieve retro-reflection and absorption for right- and left-handed circularly polarized incidence is theoretically simulated and experimentally observed at microwave frequency. Thus, our effort provides an alternative approach to tailoring electromagnetic waves in a circular dichroitic manner and may also find applications in multi-functional systems in optics and microwave regimes.

In this paper, a controllable one-step doping method has been successfully adopted in the cesium copper iodide perovskite’s luminescence, a high-quality white-light emission with Commission Internationale de l′Eclairage (CIE) coordinates of (0.3397, 0.3325), and a color rendering index (CRI) reaching up to 90 was realized in a convenient way. Through adding impurities into the Cs3Cu2I5 system, high efficiency and stable CsCu2I3 was synthesized, and the coexistence of varied high luminescence phases realized the white lighting. Strikingly, blue-emitting Cs3Cu2I5 and yellow-emitting CsCu2I3 could coexist, and their respective luminescence was not interacted in the compound, which was beneficial for acquiring a single emission and highly efficient white lighting. This work carried out a deep exploration of the Cu-based metal halides, and would be favorable to the applications of lead-free perovskites.

Indium tin oxide (ITO) films have recently emerged as a new class of functional materials for nonlinear optical (NLO) devices due to their exotic properties around epsilon-near-zero (ENZ) wavelength. Here, we experimentally investigated and tailored the NLO absorption properties of ITO films. The NLO absorption response of ITO films is investigated by using the femtosecond Z-scan measurement technique at two different wavelengths of 1030 nm (out of ENZ region) and 1440 nm (within ENZ region). Interestingly, we observed conversion behavior from saturable absorption (SA) to reverse saturable absorption (RSA) at 1030 nm with the increasing incident laser intensity, whereas only SA behavior was observed at 1440 nm. We demonstrate that SA behavior was ascribed to ground-state free electrons bleaching in the conduction band, and RSA was attributed to three-photon absorption. Moreover, results reveal that ITO film shows more excellent SA performance at 1440 nm with a nonlinear absorption coefficient of ～-23.2 cm/GW and a figure of merit of ～1.22×10-16 esu·cm. Furthermore, we tailored the SA and RSA behaviors of ITO films at 1030 and 1440 nm wavelengths via post-annealing treatment. The modulatable NLO absorption was ascribed to the changing of free-carrier concentration in ITO films via annealing treatment. The experimental findings offered an inroad for researchers to tailor its NLO absorption properties by changing the free-carrier concentration through chemical modification such as annealing, oxidation, or defect implantation. The superior and tunable nonlinear optical response suggests that ITO film might be employed as a new class material with potential applications in novel optical switches or optical limiters to realize the all-optical information process.

Photonic nanostructures with resonant modes that can generate large electric field (EF) enhancements are applied to enhance light-matter interactions in nanoscale, bringing about great advances in both fundamental and applied science. However, a small hot spot (i.e., the regions with strong EF enhancements) and highly inhomogeneous EF distribution of the resonant modes usually hinder the enhancements of light-matter interactions in a large spatial scale. Additionally, it is a severe challenge to simultaneously generate multiple resonant modes with strong EF enhancements in a broadband spectral range, which greatly limits the capacity of a photonic nanostructure in boosting optical responses including nonlinear conversion, photoluminescence, etc. In order to overcome these challenges, we presented an arrayed hyperbolic metamaterial (AHMM). This AHMM structure is applied to simultaneously enhance the three-photon and four-photon luminescence of upconversion nanoparticles. Excitingly, the enhancement of the three-photon process is 1 order of magnitude larger than previous records, and for the enhancing four-photon process, we achieve an enhancement of 3350 times, greatly beneficial for overcoming the crucial problem of low efficiency in near infrared light upconversion. Our results demonstrated a promising platform for realizing giant enhancements of light-matter interactions, holding potential in constructing various photonics applications such as the nonlinear light sources.

Bulk scintillators that are with high density, low cost, and fine pulse-height energy spectral resolution, and are non-hygroscopic and user friendly, are desired for high-energy gamma-ray spectroscopy application. Recently, low-cost solution-processed perovskite nanoscintillators have been demonstrated with outstanding performances for indirect low-energy X-ray detection; however, the stability and thickness are not suitable for high-energy gamma-ray detection. Here, we report scintillation performances of a low-cost solution-processed bulk 0D Cs3Cu2I5 single crystal. The self-trapped exciton emission results in a large Stokes shift (109 nm) that is reabsorption free. A broad X-ray excited emission matches well with the sensitivity of a silicon photodiode. The unique Cs+ surrounded isolated [Cu2I5]3- cluster scintillator provides ultra-stability in air and strong radiation hardness under high-dose gamma-ray exposure from a 60Co source. This solution-processed Cs3Cu2I5 scintillator is expected with low-cost and has detection performances comparable to commercial alkali-halide scintillator products.

In this study, a quaternary blending strategy was applied in the fabrication of organic photovoltaic devices and large-area modules. As a result, the ultimate quaternary organic solar cells (OSCs) deliver 16.71% efficiency for small-area devices and 13.25% for large-area (19.34 cm2) modules (certified as 12.36%), which is one of the highest efficiencies for organic solar modules to date. Our results have proved the synergistic effects of multiple components in OSCs, providing an effective strategy for achieving high-performance organic photovoltaic devices and modules.

High-harmonic generation in the ultraviolet region is promising for wireless technology used for communications and sensing. However, small high-order nonlinear coefficients prevent us from obtaining high conversion efficiency and functional photonic devices. Here, we show highly efficient ultraviolet harmonic generation extending to the fifth order directly from an epsilon-near-zero indium tin oxide (ITO) film. The real part of the annealed ITO films was designed to reach zero around 1050 nm, matching with the central wavelength of an Yb-based fiber laser, and the internal driving electric field was extremely enhanced. A high energy conversion efficiency of 10-4 and 10-6 for 257.5 nm (fourth-order) and 206 nm (fifth-order) ultraviolet harmonic generation was obtained, which is at least 2 orders of magnitude higher than early reports. Our results demonstrate a new route for overcoming the inefficiency problem and open up the possibilities of compact solid-state high-harmonic generation sources at nanoscale.

In this work, we study how an epitaxial laser-like (or superluminescent diode-like) structure is modified by intentional changes of the substrate misorientation in the range of 0.5°–2.6°. The 40 μm×40 μm test structure with misorientation profiling was fabricated using multilevel photolithography and dry-etching. The local structural parameters were measured by synchrotron radiation microbeam X-ray diffraction, with the sampling area of below 1 μm×1 μm. We directly obtained the relation between the misorientation and indium content in the quantum well, changing from 9% to 18%, with a high resolution (small misorientation step). We also show a good agreement of local photoluminescence emission wavelength with simulation of transition energy based on synchrotron radiation microbeam X-ray diffraction (SR-XRD) data and estimated Stokes shift. We observe that the substrate misorientation influences also the InGaN waveguide and AlGaN cladding composition. Still, we showed through simulation of the optical confinement factor of a full laser diode structure that good light guiding properties should be preserved in the whole misorientation range studied here. This proves the usefulness of misorientation modification in applications like broadband superluminescent diodes or multicolor laser arrays.

Chiral ligand conjugated transition metal oxide nanoparticles (NPs) are a promising platform for chiral recognition, biochemical sensing, and chiroptics. Herein, we present chirality-based strategy for effective sensing of mercury ions via ligand-induced chirality derived from metal-to-ligand charge transfer (MLCT) effects. The ligand competition effect between molybdenum and heavy metal ions such as mercury is designated to be essential for MLCT chirality. With this know-how, mercury ions, which have a larger stability constant (Kf) than molybdenum, can be selectively identified and quantified with a limit of detection (LOD) of 0.08 and 0.12 nmol/L for D-cysteine and L-cysteine (Cys) capped MoO2 NPs. Such chiral chemical sensing nanosystems would be an ideal prototype for biochemical sensing with a significant impact on the field of biosensing, biological systems, and water research-based nanotoxicology.

Inorganic cesium lead halide (CsPbX3, X=Cl, Br, I) nanocrystals (NCs) attract extensive attention because of their excellent optoelectronic performance. However, the classic CsPbX3 NCs suffer from toxicity and instability, which impede their further applications in commercial fields. Here the inorganic lead-free cesium copper chlorine NCs are synthesized by a facile hot-injection method. The blue-emission 3D CsCu2Cl3 and green-emission 0D Cs3Cu2Cl5 NCs are prepared at 70°C and 120°C, respectively, suggesting that the reaction temperature may account for the final components. Owing to the self-trapped exciton effect, the unique optical properties, such as high photoluminescence (PL) quantum yield, broadband emission, large Stokes shift, and long PL decay time, are demonstrated for both cesium copper chlorine NCs. Moreover, highly efficient and stable warm white light-emitting diodes are fabricated with CsCu2Cl3 and Cs3Cu2Cl5 NCs. The study highlights the promising potential for lead-free cesium copper chlorine nanocrystals in nontoxic solid-state lighting applications.

Understanding the photophysical interactions between the components in organic-inorganic nanocomposites is a key factor for their efficient application in optoelectronic devices. In particular, the photophysical study of nanocomposites based on organic conjugated polymers is rare. We investigated the effect of surface plasmon resonance (SPR) of gold nanoparticles (Au NPs) on the photoluminescence (PL) property of a push-pull conjugated polymer (PBDB-T). We prepared the hybrid system by incorporating poly(3-hexylthiophene)-stabilized Au NPs (P3HT-Au NPs) into PBDB-T. The enhanced and blueshifted PL was observed in the hybrid system compared to PL in a neat PBDB-T system, indicating that the P3HT chains attached to the Au NPs suppressed charge-transfer from PBDB-T to the Au NPs and relayed the hot electrons to PBDB-T (the band-filling effect). This photophysical phenomenon limited the auto-dissociation of PBDB-T excitons. Thus, the radiative recombination of the excitons occurred more in our hybrid system than in the neat system.

As an emerging scintillation material, metal halide perovskite (CsPbX3) has been deemed the most potentially valuable candidate in X-ray detection and medical imaging. Nevertheless, it is a continuing challenge to implement efficient radioluminescence (RL) with high radiation stability and moisture resistance. Moreover, the optimized luminescence properties and excellent uniformity of CsPbX3 glass are also key points for obtaining perfect X-ray images. Herein, we have successfully precipitated Eu3+-doped CsPbBr3 nanocrystals (NCs) with improved photoluminescence quantum yield (≈58.6%) because partial Eu3+ entered the perovskite lattice in a robust borosilicate glass matrix by in situ crystallization. The small amount of Eu addition made the lattice of NCs shrink and promoted uniform distribution of CsPbBr3 NCs in the glass, which effectively reduced the light scattering of the sample. Subsequently, multimodal RL intensity of the CsPbBr3/CsPbBr3:xEu NCs glasses (CPB-0Eu/CPB-xEu) as a function of X-ray dose rate showed a superlinear relationship to the benefit of obtaining satisfactory X-ray images. Also, the outstanding radiation stability and water resistance of CPB-xEu were confirmed due to the protection of the robust glass matrix. Finally, an X-ray imaging system using a CPB-xEu scintillator was constructed, and the spring in the opaque sample was legibly detected under the motivation of X-rays, indicating that CsPbX3 glasses possess extensive application prospects in terms of X-ray detection and medical imaging.

The Pancharatnam–Berry geometric phase has attracted great interest due to the elegant phase control strategy via geometric transformation of optical elements. The commonly used geometric phase is associated with circular polarization states. Here, we show that by exploiting the geometric phase associated with the two elliptical eigen-polarization states in a racemic metallic helix array, exotic features including full range phase modulation for linear polarization states, diverse polarization conversion, and full complex amplitude modulation can be obtained with rotation of the helices. As a proof of concept, several devices for implementing polarization conversion, vortex beam generating, and lateral dual focusing are built with a racemic helix array in the microwave regime. The calculated and experimental results validate our proposals, which can stimulate various advanced metadevices.

The toxicity and instability of lead halide perovskite seriously limit its commercial application in lighting, although it has high photoluminescence (PL) efficiency and adjustable emission. Here, lead-free bismuth (Bi) and antimony (Sb) codoped Cs2SnCl6 (BSCSC) microcrystals (MCs) are prepared successfully by a solvothermal method. The PL spectrum is composed of dual emission bands with the peak at 485 and 650 nm, of which relative intensity can be tunable through the change of Bi and Sb feeding contents, respectively. Because of the phonon–electron interaction, the PL intensity is enhanced as the temperature rises within the range of 80–260 K. Then, the nonradiative transition is intensified until 380 K, which results in decrease in PL intensity. Simultaneously, combining with time-resolved PL, it is concluded that the emission peak at 485 nm is attributed to the [BiSn+VCl] as the luminescent centers with the lifetime of hundreds of nanoseconds, and the emission peak at 650 nm is attributed to microsecond-timescale self-trapped excitons. The maximum values of relative sensitivity (SR) and absolute sensitivity (SA) values obtained are 3.82% K-1 and 5.11 ns ·K-1, which for the first time to our knowledge demonstrate that BSCSC MCs can be novel luminescent materials for developing better optical thermometry. White-light-emitting diodes (WLEDs) are constructed using BSCSC MCs only combined with an LED chip, the Commission Internationale de L’Eclairage color coordinates of which are (0.30, 0.37). It provides a novel scheme for the lighting field to realize WLEDs without adding additional commercial phosphors.

Lanthanide (Ln)-doped nanoparticles have shown potential for applications in various fields. However, the weak and narrow absorption bands of the Ln ions (Ln3+), hamper efficient optical pumping and severely limit the emission intensity. Dye sensitization is a promising way to boost the near-infrared (NIR) emission of Er3+, hence promoting possible application in optical amplification at 1.5 μm, a region that is much sought after for telecommunication technology. Herein, we introduce the fluorescein isothiocyanate (FITC) organic dye with large absorption cross section as energy donor of small-sized (∼3.6 nm) Er3+-doped CaF2 nanoparticles. FITC molecules on the surface of CaF2 work as antennas to efficiently absorb light, and provide the indirect sensitization of Er3+ boosting its emission. In this paper, we employ photoluminescence and transient absorption spectroscopy, as well as density functional theory calculations, to provide an in-depth investigation of the FITC→Er3+ energy transfer process. We show that an energy transfer efficiency of over 89% is achieved in CaF2:Er3+@FITC nanoparticles resulting in a 28 times enhancement of the Er3+ NIR emission with respect to bare CaF2:Er3+. Through the multidisciplinary approach used in our work, we are able to show that the reason for such high sensitization efficiency stems from the suitable size and geometry of the FITC dye with a localized transition dipole moment at a short distance from the surface of the nanoparticle.

Here, Gd-doped CsPbCl1.5Br1.5 nanocrystals (NCs) in borosilicate glass matrix (B2O3-SiO2-ZnO) were prepared by melting quenching and in-situ crystallization. The optical performance of CsPbCl1.5Br1.5 NCs glasses under different heat-treatment temperatures and the content of Gd3+ were analyzed in detail. After CsPbCl1.5Br1.5 NCs glass is doped with Gd3+ ions, the photoluminescence intensity increases and the synthesized Gd-doped CsPbCl1.5Br1.5 NCs glasses have excellent water stability and thermal cycling performance. In addition, the influence of Gd-doped concentrations and heat-treatment temperatures on the amplified spontaneous emission (ASE) thresholds of CsPbCl1.5Br1.5 NCs glasses was studied, and the Gd-doped CsPbCl1.5Br1.5 NCs glasses achieve controllable ASE thresholds at room temperature. The ASE threshold can be as low as 0.39 mJ/cm2. This work offers a neoteric reference for the research in the application of metal ion-doped perovskite NCs and a new idea for the realization of controllable and low ASE thresholds on perovskite NCs.

All-inorganic perovskite micro/nanolasers are emerging as a class of miniaturized coherent photonic sources for many potential applications, such as optical communication, computing, and imaging, owing to their ultracompact sizes, highly localized coherent output, and broadband wavelength tunability. However, to achieve single-mode laser emission in the microscale perovskite cavity is still challenging. Herein, we report unprecedented single-mode laser operations at room temperature in self-assembly CsPbX3 microcavities over an ultrawide pumping wavelength range of 400–2300 nm, covering one- to five-photon absorption processes. The superior frequency down- and upconversion single-mode lasing manifests high multiphoton absorption efficiency and excellent optical gain from the electron–hole plasma state in the perovskite microcavities. Through direct compositional modulation, the wavelength of a single-mode CsPbX3 microlaser can be continuously tuned from blue-violet to green (427–543 nm). The laser emission remains stable and robust after long-term high-intensity excitation for over 12 h (up to 4.3×107 excitation cycles) in the ambient atmosphere. Moreover, the pump-wavelength dependence of the threshold, as well as the detailed lasing dynamics such as the gain-switching and electron–hole plasma mechanisms, are systematically investigated to shed insight into the more fundamental issues of the lasing processes in CsPbX3 perovskite microcavities.

Understanding and controlling defect in two-dimensional materials is important for both linear and nonlinear optoelectronic devices, especially in terms of tuning nonlinear optical absorption. Taking advantage of an atomic defect formed easily by smaller size, molybdenum disulfide nanosheet is prepared successfully with a different size by gradient centrifugation. Interestingly, size-dependent sulfur vacancies are observed by high-resolution X-ray photoelectron spectroscopy, atomic force microscopy, and transmission electron microscopy. The defect effect on nonlinear absorption is investigated by Z-scan measurement at the wavelength of 800 nm. The results suggest the transition from saturable absorption to reverse saturable absorption can be observed in both dispersions and films. First principle calculations suggest that sulfur vacancies act as the trap state to capture the excited electrons. Moreover, an energy-level model with the trap state is put forward to explain the role of the sulfur vacancy defect in nonlinear optical absorption. The results suggest that saturable absorption and reverse saturable absorption originate from the competition between the excited, defect state and ground state absorption. Our finding provides a way to tune the nonlinear optical performance of optoelectronic devices by defect engineering.

In this work, one kind of type II ZnSe/CdS/ZnS core/shell/shell nanocrystals (NCs) is synthesized, and their linear and nonlinear photophysical properties are investigated. Through measurements of the temperature-dependent photoluminescence spectra of NCs, their excitonic properties, including the coefficient of the bandgap change, coupling strength of the exciton acoustic phonons, exciton longitudinal optical (LO) phonons, and LO–phonon energy are revealed. Femtosecond transient absorption spectroscopy was employed to obtain insight into ultrafast processes occurring at the interface of ZnSe and CdS, such as those involving the injection of photo-induced electrons into the CdS shell, interfacial state bleaching, and charge separation time. At the end, their multiphoton absorption spectra were determined by using the z-scan technique, which yielded a maximum two-photon absorption cross section of 3717 GM at 820 nm and three-photon absorption cross section up to 3.9×10?77 cm6·s2·photon?2 at 1220 nm, respectively. The photophysical properties presented here may be important for exploiting their relevant applications in optoelectronic devices and deep-tissue bioimaging.

Structural coloration techniques have improved display science due to their high durability in terms of resistance to bleaching and abrasion, and low energy consumption. Here, we propose and demonstrate an all-solid-state, large-area, lithography-free color filter that can switch structural color based on a doped semiconductor. Particularly, an indium-gallium-zinc-oxide (IGZO) thin film is used as a passive index-changing layer. The refractive index of the IGZO layer is tuned by controlling the charge carrier concentration; a hydrogen plasma treatment is used to control the conductivity of the IGZO layer. In this paper, we verify the color modulation using finite difference time domain simulations and experiments. The IGZO-based color filter technology proposed in this study will pave the way for charge-controlled tunable color filters displaying a wide gamut of colors on demand.

The electrical and structural properties of V/Al-based n-contacts on n‐AlxGa1-xN with an Al mole fraction x ranging from x=0.75 to x=0.95 are investigated. Ohmic n-contacts are obtained up to x=0.75 with a contact resistivity of 5.7×10-4 Ω·cm2 whereas for higher Al mole fraction the IV characteristics are rectifying. Transmission electron microscopy reveals a thin crystalline AlN layer formed at the metal/semiconductor interface upon thermal annealing. Compositional analysis confirmed an Al enrichment at the interface. The interfacial nitride-based layer in n-contacts on n‐Al0.9Ga0.1N is partly amorphous and heavily contaminated by oxygen. The role and resulting limitations of Al in the metal stack for n-contacts on n-AlGaN with very high Al mole fraction are discussed. Finally, ultraviolet C (UVC) LEDs grown on n‐Al0.87Ga0.13N and emitting at 232 nm are fabricated with an operating voltage of 7.3 V and an emission power of 120 μW at 20 mA in cw operation.

Self-powered and flexible ultrabroadband photodetectors (PDs) are desirable in a wide range of applications. The current PDs based on the photothermoelectric (PTE) effect have realized broadband photodetection. However, most of them express low photoresponse and lack of flexibility. In this work, high-performance, self-powered, and flexible PTE PDs based on laser-scribed reduced graphene oxide (LSG)/CsPbBr3 are developed. The comparison experiment with LSG PD and fundamental electric properties show that the LSG/CsPbBr3 device exhibits enhanced ultrabroadband photodetection performance covering ultraviolet to terahertz range with high photoresponsivity of 100 mA/W for 405 nm and 10 mA/W for 118 μm at zero bias voltage, respectively. A response time of 18 ms and flexible experiment are also acquired at room temperature. Moreover, the PTE effect is fully discussed in the LSG/CsPbBr3 device. This work demonstrates that LSG/CsPbBr3 is a promising candidate for the construction of high-performance, flexible, and self-powered ultrabroadband PDs at room temperature.

Q-switched fiber lasers are integral tools in science, industry, and medicine due to their advantages of flexibility, compactness, and reliability. All-optical strategies to generate ultrashort pulses have obtained considerable attention as they can modulate the intracavity Q factors without employing costly and complex electrically driven devices. Here, we propose a high-performance all-optical modulator for actively Q-switched pulse generation based on a microfiber knot resonator deposited with V2CTx MXene. Experimental results show that the obtained Q-switching pulses exhibit a wide adjustment range of repetition rate from 1 kHz to 20 kHz, a high signal-to-background contrast ratio of ～55 dB, and a narrow pulse width of 8.82 μs, indicating great potentials of providing a simple and viable solution in photonic applications.

All inorganic CsPbBr3 perovskite quantum dots (QDs) have been recognized as promising optical materials to fabricate green light emission devices because of their excellent optical performance. However, regular CsPbBr3 QDs with an oleic acid (OA) ligand show poor stability, which limits their practical application. We replaced the OA ligand in CsPbBr3 QDs with a 2-hexyldecanoic acid (DA) ligand and, in the synthesis, found that the new material has better optical properties than regular CsPbBr3 QDs (CsPbBr3-OA QDs). Due to the strong binding energy between the DA ligand and QDs, the ligand-modified CsPbBr3 QDs (CsPbBr3-DA QDs) show a high photoluminescence quantum yield (PLQY) of 96%, while the PLQY of CsPbBr3-OA QDs is 84%. Subsequently, the CsPbBr3 QDs coated on the blue light-emitting diode (LED) chips as green phosphors are demonstrated. The color conversion from blue to pure green is achieved by adding the CsPbBr3-OA QDs solution up to 60 μL, while the pure green emission devices only need 18 μL CsPbBr3-DA QDs solution under the same concentration. The ultrapure, highly efficient green light-emitting devices based on CsPbBr3-DA QDs exhibit a luminous efficiency of 43.6 lm/W with a CIE (0.2086, 0.7635) under a 15.3 mA driving current. In addition, the green emission wavelength of the devices based on CsPbBr3-DA QDs almost has no shift, even under a high injection current. These results highlight the promise of DA ligand-modified CsPbBr3 QDs for light-emitting devices and enrich the application field of ligand-modified CsPbBr3 QDs.

The effect of tin-oxide (SnO) nanoparticles, which are obtained by indium-tin-oxide (ITO) treatment, on the p-GaN surface of GaN-based flip-chip blue micro-light-emitting diode (μ-LED) arrays is investigated. A thin Ag layer is deposited on the ITO-treated p-GaN surface by sputtering. SnO nanoparticles originate from inhomogeneous Schottky barrier heights (SBHs) at Ag/p-GaN contact. Therefore, effective SBH is reduced, which causes carrier transport into the μ-LED to enhance. 10 nm thick ITO-treated μ-LEDs show better optoelectronic characteristics among fabricated μ-LEDs owing to improved ohmic contact and highly reflective p-type reflectors. Basically, SnO nanoparticles help to make good ohmic contact, which results in improved carrier transport into μ-LEDs and thus results in increased optoelectronic performances.

Indium phosphide (InP) nanowires (NWs) have attracted significant attention due to their exotic properties that are different from the bulk counterparts, and have been widely used for light generation, amplification, detection, modulation, and switching, etc. Here, high-quality InP NWs were directly grown on a quartz substrate by the Au-nanoparticle assisted vapor-liquid-solid method. We thoroughly studied their nonlinear optical absorption properties at 1.06 μm by the open-aperture Z-scan method. Interestingly, a transition phenomenon from saturable absorption (SA) to reverse saturable absorption (RSA) was observed with the increase of the incident laser intensity. In the analysis, we found that the effective nonlinear absorption coefficient (βeff～?102 cm/MW) under the SA process was 3 orders of magnitude larger than that during the RSA processes. Furthermore, the SA properties of InP NWs were experimentally verified by using them as a saturable absorber for a passively Q-switched Nd:YVO4 solid-state laser at 1.06 μm, where the shortest pulse width of 462 ns and largest single pulse energy of 1.32 μJ were obtained. Moreover, the ultrafast carrier relaxation dynamics were basically studied, and the intra-band and inter-band ultrafast carrier relaxation times of 8.1 and 63.8 ps, respectively, were measured by a degenerate pump–probe method with the probe laser of 800 nm. These results well demonstrate the nonlinear optical absorption properties, which show the excellent light manipulating capabilities of InP NWs and pave a way for their applications in ultrafast nanophotonic devices.

BST(Ba0.5Sr0.5TiO3)/PZT(Pb0.52Zr0.48TiO3) photonic crystals were fabricated by magnetron sputtering and annealed at 620°C?700°C. By controlling the crystallinity and the oxygen vacancies of the ferroelectric photonic crystals, the optically and electrically controllable terahertz wave modulations were realized. The variation in refractive index of the 680°C annealed sample showed the highest modulation to the optical pump and increased to 11.9 due to the highest absorption near 532 nm. In the optical pump, the electrons from Ti3+ 2p3/2 ions could be stimulated and captured by Ti4+ 2p3/2 ions, and the ratio of Ti3+/Ti4+ observed increased with the increasing annealing temperature, indicating the increasing oxygen vacancies concentration, which increased the 532 nm optical absorption and contributed to the improved optical modulation. The excess Pb migrating to the surface at higher annealing temperature might be one reason for the degradation of optical modulation. The increasing polarization and leakage current could contribute to the increasing permittivity and loss with the increasing annealing temperature. Two different results were observed on the sample annealed at 680°C when the order of applying external optical and electric fields was changed, due to the different migration mechanisms of excited carriers. This work provides a potentially effective approach to fabricate THz sensing, imaging, and communications devices with multi-function in the modulation of optical and electric multi-fields.

Single-nanowire solar cells with a unique light-concentration property are expected to exceed the Shockley–Queisser limit. The architecture of single nanowire is an important factor to regulate its optical performance. We designed a trilobal silicon nanowire (SiNW) with two equivalent scales that possesses superior light-absorption efficiency in the whole wavelength range and shows good tolerance for incident angle. The electric field distribution in this geometry is concentrated in the blade with small equivalent scale and pivot with large equivalent scale, respectively, in the short wavelength range and long wavelength range. Corresponding good light absorption of trilobal SiNW in the two wavelength ranges leads to stronger total light-absorption capacity than that of cylindrical SiNW. Trilobal single-nanowire solar cells can obtain a short-circuit current density (JSC) of 647 mA·cm?2, which provides a new choice for designing single nanowire with excellent light-capture capability.

We report on the carrier dynamic and electronic structure investigations on AlGaN-based deep-ultraviolet multiple quantum wells (MQWs) with lateral polarity domains. The localized potential maximum is predicted near the domain boundaries by first-principle calculation, suggesting carrier localization and efficient radiative recombination. More importantly, lateral band diagrams of the MQWs are proposed based on electron affinities and valance band levels calculated from ultraviolet (UV) photoelectron spectroscopy. The proposed lateral band diagram is further demonstrated by surface potential distribution collected by Kelvin probe microscopy and the density-of-state calculation of energy bands. This work illustrates that lateral polarity structures are playing essential roles in the electronic properties of III-nitride photonic devices and may provide novel perspective in the realization of high-efficiency UV emitters.

A full structure 290-nm ultraviolet light-emitting diode (UV-LED) with a nanoporous n-AlGaN underlayer was fabricated by top via hole formation followed by high-voltage electrochemical etching. The 20 to 120 nm nanopores were prepared in regular doped n-AlGaN by adjusting the etching voltage. The comparison between the Raman spectrum and the photoluminescence wavelength shows that the biaxial stress in the nanoporous material is obviously relaxed. The photoluminescence enhancement was found to be highly dependent on the size of the pores. It not only improves the extraction efficiency of top-emitting transverse-electric (TE)-mode photons but also greatly improves the efficiency of side-emitting transverse-magnetic (TM)-mode photons. This leads to the polarization change of the side-emitting light from ?0.08 to ?0.242. The intensity of the electroluminescence was increased by 36.5% at 100 mA, and the efficiency droop at high current was found to decrease from 61% to 31%.

Lead halide perovskites have drawn extensive attention over recent decades owing to their outstanding photoelectric performances. However, their toxicity and instability are big issues that need to be solved for further commercialization. Herein, we adopt a facile dry ball milling method to synthesize lead-free Cs3Cu2X5 (X=I, Cl) perovskites with photoluminescence (PL) quantum yield up to 60%. The optical features including broad emission spectrum, large Stokes shift, and long PL lifetime can be attributed to self-trapped exciton recombination. The as-synthesized blue emissive Cs3Cu2I5 and green emissive Cs3Cu2Cl5 lead-free perovskite powders have good thermal stability and photostability. Furthermore, UV-pumped phosphor-converted light-emitting diodes were obtained by using Cs3Cu2I5 and Cs3Cu2Cl5 as phosphors.

Photonic media containing hybrid noble metal–dielectric nanocrystals (NCs) represent a wonderland of nanophotonics, with a myriad of uncharted optical functions yet to be explored. Capitalizing on the unique phase separation and spontaneous formation of Au-metal NCs in a gallosilicate glass, we fabricated Ni2+-doped transparent nanoglass composites (GCs) containing Au-metal/γ-Ga2O3-dielectric NCs. Compared with GCs free of Au-metal NCs, the superbroadband near-infrared emission of Ni2+ with a full width at half-maximum over 280 nm is enhanced twice in the Au-metal/γ-Ga2O3 dual-phase GCs. A comparison is given as to the spontaneous emission (SPE) properties of Ni2+ in the dual-phase GCs when pumped resonantly and off-resonantly with the localized surface plasmon resonance band of the Au-metal NCs. The important role of the Au-metal NCs in the SPE enhancement is revealed by theoretical simulation based on the finite-element method. Combining the photonic engineering effect of hybrid Au-metal/γ-Ga2O3 NCs and the sensitization effect of Yb3+ on Ni2+, a record-high enhancement factor of over 10 of the Ni2+ NIR emission is achieved, and optical gain is demonstrated in the GCs at the fiber communication wavelength.

Investigating closely stacked GaN/AlN multiple quantum wells (MQWs) by means of cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope, we have reached an ultimate spatial resolution of σCL=1.8 nm. The pseudomorphically grown MQWs with high interface quality emit in the deep ultraviolet spectral range. Demonstrating the capability of resolving the 10.8 nm separated, ultra-thin quantum wells, a cathodoluminescence profile was taken across individual ones. Applying a diffusion model of excitons generated by a Gaussian-broadened electron probe, the spatial resolution of cathodoluminescence down to the free exciton Bohr radius scale has been determined.

Supercapacitors (SCs) have broad applications in wearable electronics (e.g., e-skin, robots). Recently, graphene-based supercapacitors (G-SCs) have attracted extensive attention for their excellent flexibility and electrochemical performance. Laser fabrication of G-SCs exhibits obvious superiority because of the simple procedures and integration compatibility with future electronics. Here, we comprehensively summarize the state-of-the-art advancements in laser-assisted preparation of G-SCs, including working mechanisms, fabrication procedures, and unique characteristics. In the working mechanism section, electric double-layer capacitors and pseudo-capacitors are introduced. The latest advancements in this field are comprehensively summarized, including laser reduction of graphene oxides, laser treatment of graphene prepared from chemical vapor deposition, and laser-induced graphene. In addition, the unique characteristics of laser-enabled G-SCs, such as structured graphene, graphene hybrids, and heteroatom doping graphene-related electrodes, are presented. Subsequently, laser-enabled miniaturized, stretchable, and integrated G-SCs are also discussed. It is anticipated that laser fabrication of G-SCs holds great promise for developing future energy storage devices.

We fabricated a three-dimensional microstructured optical waveguide (MOW) in a single-crystal using the femtosecond-laser writing and phosphoric acid etching techniques, and observed excellent midinfrared waveguiding performance with low loss of ～0.5 dB/cm. Tracks with a periodic arrangement were written inside the yttrium aluminum garnet (YAG) crystal via femtosecond laser inscription, and then etched by the phosphoric acid (H3PO4) to form the hollow structures. The evolution of the microstructure of tracks was investigated in detail. The function of the MOW was analyzed by different numerical methods, indicating the proposed MOW can effectively operate in quasi-single-mode pattern in the midinfrared wavelength range, which agrees well with our experiment results.

Recent advances in power scaling of fiber lasers are hindered by the thermal issues, which deteriorate the beam quality. Anti-Stokes fluorescence cooling has been suggested as a viable method to balance the heat generated by the quantum defect and background absorption. Such radiation-balanced configurations rely on the availability of cooling-grade rare-earth-doped gain materials. Herein, we perform a series of tests on an ytterbium-doped ZrF4–BaF2–LaF3–AlF3–NaF (ZBLAN) optical fiber to extract its laser-cooling-related parameters and show that it is a viable laser-cooling medium for radiation balancing. In particular, a detailed laser-induced modulation spectrum test is performed to highlight the transition of this fiber to the cooling regime as a function of the pump laser wavelength. Numerical simulations support the feasibility of a radiation-balanced laser, but they highlight that practical radiation-balanced designs are more demanding on the fiber material properties, especially on the background absorption, than solid-state laser-cooling experiments.

Due to their outstanding surface-to-volume ratio, highly smooth surface, and well-defined crystal boundary, semiconducting micro-/nanocrystals have been used as a pivotal platform to fabricate multifunctional optoelectronic devices, such as superresolution imaging devices, solar concentrators, photodetectors, light-emitting diodes (LEDs), and lasers. In particular, micro-/nanocrystals as key elements can be employed to tailor the fundamental optical and electronic transport properties of integrated hetero-/homostructures. Herein, ZnO microcrystal-decorated pre-synthesized Ga-doped ZnO microwire (ZnO@ZnO:Ga MW) was prepared. The single ZnO@ZnO:Ga MW can be used to construct optically pumped Fabry–Perot (F–P) mode microlasers, with the dominating lasing peaks centered in the violet spectral region. Stabilized exciton-polariton emissions from single ZnO@ZnO:Ga MW-based heterojunction diode can also be realized. The deposited ZnO microcrystals can facilitate the strong coupling of F–P optical modes with excitons, leading to the formation of exciton-polariton features in the ZnO@ZnO:Ga MW. Therefore, the waveguiding lighting behavior and energy-band alignment of ZnO microcrystal-sheathed ZnO:Ga MW radial structures should be extremely attractive for potential applications in semiconducting microstructure-based optoelectronic devices, such as micro-LEDs, laser microcavities, waveguides, and photodetectors.

We propose what we believe is a novel optical thermometry strategy (FIR-Ex) based on the fluorescence intensity ratio (FIR) between two radiations associated with the same emission peak but different excitation wavelengths, in contrast to the traditional approach (FIR-Em), which depends on the FIR at varying emission wavelengths. The temperature-dependent FIR within the FIR-Ex strategy arises from the different charge/energy evolution routes, rather than the distribution of thermally coupled levels within the FIR-Em strategy. Considerable diversity in thermal behaviors and luminescence mechanisms was demonstrated by analyzing the 618 nm red emission in Pr3+-doped congruent LiNbO3 (Pr:CLN) under 360 and 463 nm excitations. The temperature sensitivity was further improved via Mg2+ codoping due to the optimization of charge dynamics and energy transfer processes. Given its wide detection scope, relatively high absolute sensitivity at low temperature, and high tunability of temperature sensitivity, the FIR-Ex strategy is promising for developing optical temperature-sensing materials with high performance.

Nonlinear all-optical technology is an ultimate route for next-generation ultrafast signal processing of optical communication systems. New nonlinear functionalities need to be implemented in photonics, and complex oxides are considered as promising candidates due to their wide panel of attributes. In this context, yttria-stabilized zirconia (YSZ) stands out, thanks to its ability to be epitaxially grown on silicon, adapting the lattice for the crystalline oxide family of materials. We report, for the first time to the best of our knowledge, a detailed theoretical and experimental study about the third-order nonlinear susceptibility in crystalline YSZ. Via self-phase modulation-induced broadening and considering the in-plane orientation of YSZ, we experimentally obtained an effective Kerr coefficient of n^2YSZ=4.0±2×10?19 m2·W?1 in an 8% (mole fraction) YSZ waveguide. In agreement with the theoretically predicted n^2YSZ=1.3×10?19 m2· W?1, the third-order nonlinear coefficient of YSZ is comparable with the one of silicon nitride, which is already being used in nonlinear optics. These promising results are a new step toward the implementation of functional oxides for nonlinear optical applications.

Semiconductor lasers directly grown on silicon offer great potential as critical components in high-volume, low-cost integrated silicon photonics circuits. Although InAs/InP quantum dash (QDash) lasers on native InP substrate emitting at 1.5 μm (C-band) have demonstrated notable performance, the growth of InAs/InP QDash lasers on silicon remains undeveloped because of the 8% lattice mismatch between InP and silicon. Here we report advances of growth techniques leading to the first C-band room-temperature continuous-wave electrically pumped QDash lasers on CMOS standard (001) silicon substrates by metalorganic chemical vapor deposition. A correlation between various material characterizations and device performance is analyzed for different QDash laser structures grown on planar nominal (001) silicon. With the optimized QDash growth and improved fabrication process, the lowest threshold current density of 1.5 kA/cm2 was determined on an 8 μm×1.5 mm device on planar silicon with a single facet output power exceeding 14 mW. The device results illustrate the good material quality of the QDash lasers grown on silicon, suggesting potential applications for other active components of photonic integrated circuits, such as semiconductor optical amplifiers, modulators, and photodetectors.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) attain increasing attention due to their exceptional nonlinear optical efficiencies, which hold promising potential for on-chip photonics and advanced optoelectronic applications. Planar TMDs have been proven to support orders-higher third-order nonlinear coefficients in comparison with common nonlinear materials. Interestingly, stronger light–matter interaction could be motivated when curved features are introduced to 2D TMDs. Here, a type of inorganic fullerene-like WS2 nanoparticles is chemically synthesized using hard mesoporous silica. By using the spatial self-phase modulation (SSPM) method, the nonlinear refractive index n2 and third-order susceptibility χ(3) are investigated in the visible range. It is found that n2～10-5 cm2/W and χ(3)～10-7 esu, two orders higher than the counterparts of planar WS2 structures. Our experimental findings provide a fresh thinking in designing nonlinear optical materials and endow TMDs with new potentials in photonic integration applications.

Solution-processable, single-crystalline perovskite nanowires are ideal candidates for developing low-cost photodetectors, but their detectivities are limited due to a high level of unintentional defects. Through the surface-initiated solution-growth method, we fabricated high-quality, single-crystalline, defects-suppressed MAPbI3 nanowires, which possess atomically smooth side surfaces with a surface roughness of 0.27 nm, corresponding to a carrier lifetime of 112.9 ns. By forming ohmic MAPbI3/Au contacts through the dry contact method, high-performance metal–semiconductor–metal photodetectors have been demonstrated with a record large linear dynamic range of 157 dB along with a record high detectivity of 1.2×1014 Jones at an illumination power density of 5.5 nW/cm2. Such superior photodetector performance metrics are attributed to, first, the defects-suppressed property of the as-grown MAPbI3 nanowires, which leads to a quite low noise current in the dark, and second, the ohmic contact between MAPbI3 and Au interfaces, which gives rise to an improved responsivity compared with the Schottky contact counterpart. The realized high-performance MAPbI3 nanowire photodetector advances the development of low-cost photodetectors and has potential applications in weak-signal photodetection.

Zirconium carbide (ZrC) with layered structure and nanoparticle morphology was prepared by sonication in an ethyl alcohol solvent. The morphology and saturable absorption properties of the ZrC were systematically analyzed. By using ZrC nanoparticle coated substrates as saturable absorbers, stable Q-switched 3 μm Er:Lu2O3 lasers were realized. Pulse durations of 50 ns with pulse energies of 20 μJ and peak power of 0.4 kW are the shortest obtained with novel-material-based Q-switched lasers in the 3 μm wavelength range.

Two-dimensional (2D) tin diselenide (SnSe2), a novel layered material with excellent optical and electronic properties, has been extensively investigated in various promising applications, including photodetectors, optical switching, and ultrafast photonics. In this work, SnSe2 nanosheets have been obtained after pretreatment in an alkaloid, exhibiting high optical absorption and electron-enriched properties. Besides, the performances of the prepared SnSe2 in near-infrared (NIR) and mid-infrared (MIR) ultrafast photonics are presented. Notably, by employing the SnSe2-deposited microfiber device as a saturable absorber (SA) exhibiting typical nonlinear optical absorption properties, stable ultrashort pulses and rogue waves are realized in an erbium-doped fiber laser. Furthermore, the SnSe2-deposited SA device is also applied to a thulium-doped fiber laser to achieve stable ultrashort pulses. This study indicates that SnSe2 is expected to be a suitable candidate for ultrafast fiber lasers in the NIR and MIR regions.

All-inorganic cesium lead bromide (CsPbBr3) perovskite quantum dots (QDs) with excellent optical properties have been regarded as good gain materials for amplified spontaneous emission (ASE). However, the poor stability as the results of the high sensitivity to heat and moisture limits their further applications. Here, we report a facile one-pot approach to synthesize CsPbBr3@SiO2 QDs at room temperature. Due to the effective defects passivation using SiO2, as-prepared CsPbBr3@SiO2 QDs present an enhanced photoluminescence quantum yield (PLQY) and chemical stability. The PLQY of CsPbBr3@SiO2 QDs reaches 71.6% which is higher than 46% in pure CsPbBr3 QDs. The PL intensity of CsPbBr3@SiO2 QDs maintains 84% while remaining 24% in pure CsPbBr3 after 80 min heating at 60°C. The ASE performance of the films is also studied under a two-photon-pumped laser. Compared with the films using pure CsPbBr3 QDs, those with as-prepared CsPbBr3@SiO2 QDs exhibit a reduced threshold of ASE. The work suggests that room-temperature-synthesized SiO2-coated perovskites QDs are promising candidates for laser devices.

Bipolar phototransistors have higher optical responsivity than photodiodes and play an important role in the field of photoelectric conversion. Two-dimensional materials offer a good optical responsivity and have the potential advantages of heterogeneous integration, but mass-production is difficult. In this study, a bipolar phototransistor is presented based on a vertical Au/graphene/MoS2 van der Waals heterojunction that can be mass-produced with a silicon semiconductor process using a simple photolithography process. Au is used as the emitter, which is a functional material used not just for the electrodes, MoS2 is used for the collector, and graphene in used for the base of the bipolar phototransistor. In the bipolar phototransistor, the electric field of the dipole formed by the Au and graphene contact is in the same direction as the external electric field and thus enhances the photocurrent, and a maximum photocurrent gain of 18 is demonstrated. A mechanism for enhancing the photocurrent of the graphene/MoS2 photodiode by contacting Au with graphene is also described. Additionally, the maximum responsivity is calculated to be 16,458 A/W, and the generation speed of the photocurrent is 1.48×10?4 A/s.

A multifunctional photo-thermal therapeutic nano-platform Y2O3: Nd3+/Yb3+/Er3+@SiO2@Cu2S (YR-Si-Cu2S) was designed through a core–shell structure, expressing the function of bio-tissue imaging, real-time temperature detection, and photo-thermal therapy under 808 nm light excitation. In this system, the core Y2O3: Nd3+/Yb3+/Er3+ (YR) takes the responsibility of emitting optical information and monitoring temperature, while the shell Cu2S nano-particles carry most of the photo-thermal conversion function. The temperature sensing characteristic was achieved by the fluorescence intensity ratio using the thermally coupled energy levels (TCLs) S3/24/H211/2 of Er3+, and its higher accuracy for real-time temperature measurement in the bio-tissue than that of an infrared thermal camera was also proved by sub-tissue experiments. Furthermore, the photo-thermal effect of the present nano-system Y2O3: Nd3+/Yb3+/Er3+@SiO2@Cu2S was confirmed by Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) ablation. Results indicate that YR-Si-Cu2S has application prospect in temperature-controlled photo-thermal treatment and imaging in bio-tissues.

The Cr4+-doped yttrium aluminum garnet (Cr4+:YAG) saturable absorber, a new generation of passively Q-switched solid-state laser material, faces a significant obstacle of low conversion rate of chromium from trivalent to tetravalent, degrading efficiency in a passively Q-switched laser. In this paper, highly transparent Cr4+:YAG ceramics were fabricated and committed to compare the laser performance with Cr4+:YAG crystals on a 1 μm passively Q-switched laser. Thanks to the grain boundary effect, the Cr4+ conversion efficiency of 0.05 at.% Cr4+:YAG transparent ceramics coated with high transparency (HT) films (T=86.46% at 1064 nm) was nine times higher than that of 0.1 at.% Cr4+:YAG single crystals coated with HT films (T=84.00% at 1064 nm). Differing from the counterpart Cr4+:YAG crystals, no absorption saturation tendency was observed for the 0.05 at.% Cr:YAG ceramics when the pump power exceeded ～1900 mW. Furthermore, the repetition frequency reached 217 kHz for 0.05 at.% Cr:YAG ceramics, which was a three-fold factor increase from that of the corresponding single crystal. The advantages of transparent ceramics over single crystals were proved through laser performance for the first time, to the best of our knowledge. This study also provided compelling evidence for replacing single crystals with ceramics for u

Optical microcavities, which support whispering gallery modes, have attracted tremendous attention in both fundamental research and potential applications. The emerging of two-dimensional materials offers a feasible solution to improve the performance of traditional microcavity-based optical devices. Besides, the integration of two-dimensional materials with microcavities will benefit the research of heterogeneous materials on novel devices in photonics and optoelectronics, which is dominated by the strongly enhanced light–matter interaction. This review focuses on the research of heterogeneous two-dimensional-material whispering-gallery-mode microcavities, opening a myriad of lab-on-chip applications, such as optomechanics, quantum photonics, comb generation, and low-threshold microlasing.

Perovskite quantum dots (QDs) are of great interest due to their outstanding optoelectronic properties and tremendous application potential. Improving photoluminescence (PL) spectra in all-inorganic perovskite QDs is of great importance for performance enhancement. In this work, the PL quantum yield of the CsPbBr3 perovskite QDs is enhanced from 70% to 95% with increasing radiation pressure. Such enhancement is attributed to the increased binding energy of self-trapped excitons (STEs) upon radiation pressure, which is consistent with its blue-shifted PL and other characterization results. Furthermore, we study ultrafast absorption spectroscopy and find that the dynamics of relaxation from free excitons to STEs in radiation pressure CsPbBr3 QDs is ascribed to stronger electron–phonon coupling in the contracted octahedral structure. It is further demonstrated that radiation pressure can boost the PL efficiency and explore effectively the relationship between the structure and optical properties.

Questions hovering over the modulation of bandgap size and excitonic effect on nonlinear absorption in two-dimensional transition metal dichalcogenides (TMDCs) have restricted their application in micro/nano optical modulator, optical switching, and beam shaping devices. Here, degenerate two-photon absorption (TPA) in the near-infrared region was studied experimentally in mechanically exfoliated MoS2 from single layer to multilayer. The layer-dependent TPA coefficients were significantly modulated by the detuning of the excitonic dark state (2p). The shift of the quasiparticle bandgap and the decreasing of exciton binding energy with layers were deduced, combined with the non-hydrogen model of excitons in TMDCs and the scaling rule of semiconductors. Our work clearly demonstrates the layer modulation of nonlinear absorption in TMDCs and provides support for layer-dependent nonlinear optical devices, such as optical limiters and optical switches.

Monolayer transition metal dichalcogenides (TMDs) are ideal materials for atomically thin, flexible optoelectronic and catalytic devices. However, their optoelectrical performance such as quantum yield and carrier mobility often shows below theoretical expectations due to the existence of defects. For monolayer TMD-based devices, finding a low-cost, time-efficient, and nondestructive technique to visualize the change of defect distribution in the space domain and the defect-induced change of the carrier’s lifetime is vital for optimizing their optoelectronic properties. Here, we propose a microscopic pump-probe technique to map the defect distribution of monolayer TMDs. It is found that there is a linear relationship between transient differential reflection intensity and defect density, suggesting that this technique not only realizes the visualization of the defect distribution but also achieves the quantitative estimation of defect density. Moreover, the carrier lifetime at each point can also be obtained by the technique. The technique used here provides a new route to characterize the defect of monolayer TMDs on the micro-zone, which will hopefully guide the fabrication of high-quality two-dimensional (2D) materials and the promotion of optoelectrical performance.

Transparent ceramics are emerging as future materials for lasers, scintillation, and illumination. In this paper, an interesting and surprising phenomenon in YAG transparent ceramics is reported. UV light leads to significant changes in the microstructure of open volume defects and nano clusters as well as in the optical properties. Light-induced lattice relaxation is suggested as the mechanism behind this intriguing behavior. The complex F-type color center with broad absorption bands is caused by the aliovalent sintering additives (Ca2+/Mg2+) and Fe ion impurities. Two individual peaks in the thermoluminescence spectra illustrate both shallow and deep level traps. From positron annihilation lifetime data, vacancy clusters and nanovoids are detected and characterized, although these free-volume defects could not be observed by high-resolution transmission electron microscopy. The solarization induced by UV irradiation is associated with a change in the structure and size of defect clusters due to lattice relaxation. Therefore, this work shows how UV irradiation leads not only to a change in the charge state of defects, but also to a permanent change in defect structure and size. It significantly affects the optical properties of YAG ceramics and their performance in lasers and other optical applications. These results are crucial for advancing transparent ceramics technology.

Black phosphorus (BP), a typical mono-elemental and two-dimensional (2D) material, has gathered significant attention owing to its distinct optoelectronic properties and promising applications, despite its main obstacle of long-term stability. Consequently, BP-analog materials with long-term chemical stability show additional potential. In this contribution, tin sulfide (SnS), a novel two-elemental and 2D structural BP-analog monochalcogenide, has been demonstrated to show enhanced stability under ambient conditions. The broadband nonlinear optical properties and carrier dynamics have been systematically investigated via Z-scan and transient absorption approaches. The excellent nonlinear absorption coefficient of 50.5×10 3 cm/GW, 1 order of magnitude larger than that of BP, endows the promising application of SnS in ultrafast laser generation. Two different decay times of τ1～873 fs and τ2～96.9 ps allow the alteration between pure Q switching and continuous-wave (CW) mode locking in an identical laser resonator. Both mode-locked and Q-switched operations have been experimentally demonstrated using an SnS saturable absorber at the telecommunication window. Femtosecond laser pulses with tunable wavelength and high stability are easily obtained, suggesting the promising potential of SnS as an efficient optical modulator for ultrafast photonics. This primary investigation may be considered an important step towards stable and high-performance BP-analog material-based photonic devices.

We report, to the best of our knowledge, the first demonstration of solid-state optical refrigeration of a Ho-doped material. A 1 mol% Ho-doped yttrium lithium fluoride (YLF) crystal is cooled by mid-IR laser radiation, and its external quantum efficiency and parasitic background absorption are evaluated. Using detailed temperature-dependent spectroscopic analysis, the minimum achievable temperature of a 1% Ho:YLF sample is estimated. Owing to its narrower ground- and excited-state manifolds, larger absorption cross section, and the coincidence of the optimum cooling wavelength of 2070 nm with commercially available high-power and highly efficient Tm-fiber lasers, Ho3+-doped crystals are superior to Tm3+-doped systems for mid-IR optical refrigeration. With further improvement in material purity and increased doping concentration, they offer great potential towards enhancing the cooling efficiency nearly two-fold over the best current Yb:YLF systems, achieving lower temperatures as well as for the realization of eye-safe mid-IR high-power radiation balanced lasers.

Bismuth (Bi)-doped photonic materials, which exhibit broadband near-infrared (NIR) luminescence (1000–1600 nm), are evolving into interesting gain media. However, the traditional methods have shown their limitations in enhancing Bi NIR emission, especially in the microregion. Consequently, the typical NIR emission has seldom been achieved in Bi-doped waveguides, which highly restricts the application of Bi-activated materials. Here, superbroadband Bi NIR emission is induced in situ instantly in the grating region by a femtosecond (fs) laser inside borosilicate glasses. A series of structural and spectroscopic characterizations are summoned to probe the generation mechanism. And we show how this novel NIR emission in the grating region can be enhanced significantly and erased reversibly. Furthermore, we successfully demonstrate Bi-activated optical waveguides. These results present new insights into Bi-doped materials and push the development of broadband waveguide amplification.

Semiconductor heterostructures based on layered two-dimensional transition metal dichalcogenides (TMDs) interfaced to gallium nitride (GaN) are excellent material systems to realize broadband light absorbers and emitters due to their close proximity in the lattice constants. The surface properties of a polar semiconductor such as GaN are dominated by interface phonons, and thus the optical properties of the vertical heterostructure are influenced by the coupling of these carriers with phonons. The activation of different Raman modes in the heterostructure caused by the coupling between interfacial phonons and optically generated carriers in a monolayer MoS2–GaN (0001) heterostructure is observed. Different excitonic states in MoS2 are close to the interband energy state of intraband defect state of GaN. Density functional theory (DFT) calculations are performed to determine the band alignment of the interface and revealed a type-I heterostructure. The close proximity of the energy levels and the excitonic states in the semiconductors and the coupling of the electronic states with phonons result in the modification of carrier relaxation rates. Modulation of the excitonic absorption states in MoS2 is measured by transient optical pump-probe spectroscopy and the change in emission properties of both semiconductors is measured by steady-state photoluminescence (PL) emission spectroscopy. There is significant red-shift of the C excitonic band and faster dephasing of carriers in MoS2. However, optical excitation at energy higher than the bandgap of both semiconductors slows down the dephasing of carriers and energy exchange at the interface. Enhanced and blue-shifted PL emission is observed in MoS2. GaN band-edge emission is reduced in intensity at room temperature due to increased phonon-induced scattering of carriers in the GaN layer. Our results demonstrate the relevance of interface coupling between the semiconductors for the development of optical and electronic applications.

The prominent third-order nonlinear optical properties of WTe2 films are studied through the Z-scan technique using a femtosecond pulsed laser at 1030 nm. Open-aperture (OA) and closed-aperture (CA) Z-scan measurements are performed at different intensities to investigate the nonlinear absorption and refraction properties of WTe2 films. OA Z-scan results show that WTe2 films always hold a saturable absorption characteristic without transition to reverse saturable absorption. Further, a large nonlinear absorption coefficient β is determined to be 3.37×103 cm/GW by fitting the OA Z-scan curve at the peak intensity of 15.603 GW/cm2. In addition, through the slow saturation absorption model, the ground state absorption cross section, excited state absorption cross section, and absorber’s density were found to be 1.4938×10 16 cm2, 1.2536×10 16 cm2, and 6.2396×1020 cm 3, respectively. CA Z-scan results exhibit a classic peak–valley shape of the CA Z-scan signal, which reveals a self-defocusing optical effect of WTe2 films under the measured environment. Furthermore, a considerable nonlinear refractive index value n2 can be obtained at 1.629×10 2 cm2/GW. Ultimately, the values of the real and imaginary parts of the third-order nonlinear s

This work reports the real-time observation of the interlayer lattice vibrations in bilayer and few-layer PtSe2 by means of the coherent phonon method. The layer-breathing mode and standing wave mode of the interlayer vibrations are found to coexist in such a kind of group-10 transition metal dichalcogenides (TMDCs). The interlayer breathing force constant standing for perpendicular coupling (per effective atom) is derived as 7.5 N/m, 2.5 times larger than that of graphene. The interlayer shearing force constant is comparable to the interlayer breathing force constant, which indicates that PtSe2 has nearly isotropic interlayer coupling. The low-frequency Raman spectroscopy elucidates the polarization behavior of the layer-breathing mode that is assigned to have A1g symmetry. The standing wave mode shows redshift with the increasing number of layers, which successfully determines the out-of-plane sound velocity of PtSe2 experimentally. Our results manifest that the coherent phonon method is a good tool to uncover the interlayer lattice vibrations, beyond the conventional Raman spectroscopy limit. The strong interlayer interaction in group-10 TMDCs reveals their promising potential in high-frequency (～terahertz) micro-mechanical resonators.

The mid-infrared (mid-IR) second-order optical nonlinearity of the barium titanate (BTO) thin films was characterized by second harmonic generation (SHG). The epitaxial BTO thin films were grown on strontium titanate substrates by pulsed-laser deposition. From the azimuthal-dependent polarized SHG measurements, the tensorial optical nonlinear coefficients, dij, and ferroelectric domain fraction ratio, δAY/δAz, were resolved. Strong SHG signals were obtained at the pumping laser wavelength λ between 3.0 and 3.6 μm. The SHG intensity was linearly dependent upon the square of the pumping laser power. The broadband mid-IR optical nonlinearity enables BTO thin films for applications in chip-scale quantum optics and nonlinear integrated photonic circuits.

Carbon nanodots (C-dots) with a uniform size of about 2 nm are synthesized via in situ pyrolysis of n-propylamine that is confined in the nanochannels of zeolite Linde Type A (LTA). The as-synthesized C-dots@LTA composite shows nonlinear optical saturable absorption properties in a broad wavelength band and can be used as saturable absorber (SA) to generate ultrafast pulsed fiber lasers. By inserting a zeolite LTA single crystal hosting C-dots into the fiber laser cavity, mode-locked fiber lasers with long-term operation stability at 1.5 μm and 1 μm are achieved. These results show that the C-dots@LTA are a promising SA material for ultrafast pulsed fiber laser generation in a broad wavelength band. To the best of our knowledge, this is the first demonstration of a C-dots@LTA-based mode-locked fiber laser.

Transition radiation (TR) induced by electron–matter interaction usually demands vast accelerating voltages, and the radiation angle cannot be controlled. Here we present a mechanism of direction controllable inverse transition radiation (DCITR) in a graphene-dielectric stack excited by low-velocity electrons. The revealed mechanism shows that the induced hyperbolic-like spatial dispersion and the superposition of the individual bulk graphene plasmons (GPs) modes make the fields, which are supposed to be confined on the surface, radiate in the stack along a special radiation angle normal to the Poynting vector. By adjusting the chemical potential of the graphene sheets, the radiation angle can be controlled. And owing to the excitation of bulk GPs, only hundreds of volts for the accelerating voltage are required and the field intensity is dramatically enhanced compared with that of the normal TR. Furthermore, the presented mechanism can also be applied to the hyperbolic stack based on semiconductors in the infrared region as well as noble metals in the visible and ultraviolet region. Accordingly, the presented mechanism of DCITR is of great significance in particle detection, radiation emission, and so on.