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Control of the angular momentum of light at the nanoscale is critical for many applications of subwavelength photonics, such as high-capacity optical communications devices, super-resolution imaging and optical trapping. However, conventional approaches to generate optical vortices suffer from either low efficiency or relatively large device footprints. Here we show a new strategy for vortex generation at the nanoscale that surpasses single-pixel phase control. We reveal that interaction between neighbouring nanopillars of a meta-quadrumer can tailor both the intensity and phase of the transmitted light. Consequently, a subwavelength nanopillar quadrumer is sufficient to cover a 2lπ phase change, thus efficiently converting incident light into high-purity optical vortices with different topological charges l. Benefiting from the nanoscale footprint of the meta-quadrumers, we demonstrate high-density vortex beam arrays and high-dimensional information encryption, bringing a new degree of freedom to many designs of meta-devices.

To quantify variations in the components of soil respiration (Rs), elucidate the impact of warming on heterotrophic respiration (Rh) in subalpine coniferous forest (LJ) and in subtropical evergreen broadleaf forest (ALS), and discern differences in the dominant regulating factors of soil respiration components between the two forest ecosystems. This study used multichannel automated soil efflux chambers with three treatments: control (CK), trenching (NR), and trenching with warming (NRW) in LJ and ALS in Southwest China. The results reveal that Rswas higher in ALS than in LJ, and the ratio of Rhto Rswas significantly higher in LJ than in ALS. Although soil temperature (ST) and soil moisture (SM) also identified the main controlling factors of Rs, Rh, Ra, and the warming effect of Rhwhen considering carbon cycle modeling, the sensitivity of Rsto temperature (Q10) was not significantly different between Rs, Rh, and Rafor both forests. The greater warming effect on Rhin LJ than in ALS is due to the combination of lower temperatures, recalcitrant organic matter, and slower litter decomposition in subalpine coniferous forests. This study confirmed that Rs, Rhand Rahad positive correlation with STand SMfor two ecosystems. This study has proven that the greater the warming effect of Rhis, the greater the contribution of Rhto Rsbecomes, and the lesser the contribution of Rato Rsin LJ becomes compared to that of ALS.

Significant knowledge gaps exist regarding the emission of elemental mercury (Hg0) from the tropical forest floor, which limit our understanding of the Hg mass budget in forest ecosystems. In this study, biogeochemical processes of Hg0deposition to and evasion from soil in a Chinese tropical rainforest were investigated using Hg stable isotopic techniques. Our results showed a mean air–soil flux as deposition of −4.5 ± 2.1 ng m–2h–1in the dry season and as emission of +7.4 ± 1.2 ng m–2h–1in the rainy season. Hg re-emission, i.e., soil legacy Hg evasion, induces negative transitions of Δ199Hg and δ202Hg in the evaded Hg0vapor, while direct atmospheric Hg0deposition does not exhibit isotopic fractionation. Using an isotopic mass balance model, direct atmospheric Hg0deposition to soil was estimated to be 48.6 ± 13.0 μg m–2year–1. Soil Hg0re-emission was estimated to be 69.5 ± 10.6 μg m–2year–1, of which 63.0 ± 9.3 μg m–2year–1is from surface soil evasion and 6.5 ± 5.0 μg m–2year–1from soil pore gas diffusion. Combined with litterfall Hg deposition (∼34 μg m–2year–1), we estimated a ∼12.6 μg m–2year–1net Hg0sink in the tropical forest. The fast nutrient cycles in the tropical rainforests lead to a strong Hg0re-emission and therefore a relatively weaker atmospheric Hg0sink.

In-plane diffractive optical networks based on meta-surfaces are promising for on-chip application. The design constraints of regular antenna unit place ultimate limits on the functionalities of the meta-systems. This fundamental limitation has been reflected by the large footprints of cascaded meta-surfaces. Here, we propose a digital meta-lens with a large degree of design freedom, enabling significantly improved beam focusing, collimation, and deflection capabilities. A highly dispersive and compact diffractive optical system is constructed for spectrometer via five layers of meta-lenses in a folded configuration. The device only occupies a 100 μm × 100 μm chip area on a silicon photonic platform. Sparse and continuous spectra reconstruction is achieved over a 35 nm bandwidth. Fine spectral lines separated by 0.14 nm are resolved. In addition to such a compact and high-resolution on-chip spectrometer, it is also expected to be promising for imaging, optical computing, and other applications due to the great versatility of the digital lens design.

Light scattered or radiated from a material carries valuable information on the said material. Such information can be uncovered by measuring the light field at different angles and frequencies. However, this technique typically requires a large optical apparatus, hampering the widespread use of angle-resolved spectroscopy beyond the lab. Here we demonstrate compact angle-resolved spectral imaging by combining a tunable metasurface-based spectrometer array and a metalens. With this approach, even with a miniaturized spectrometer footprint of only 4 × 4 μm2, we demonstrate a wavelength accuracy of 0.17 nm, spectral resolution of 0.4 nm and a linear dynamic range of 149 dB. Moreover, our spectrometer has a detection limit of 1.2 fJ, and can be patterned to an array for spectral imaging. Placing such a spectrometer array directly at the back focal plane of a metalens, we achieve an angular resolution of 4.88 × 10−3rad. Our angle-resolved spectrometers empowered by metalenses can be employed towards enhancing advanced optical imaging and spectral analysis applications.

Driven by the demand to extend optical fiber communications wavelengths beyond the C?+?L band, the 2 µm wave band has proven to be a promising candidate. Extensive efforts have been directed into developing high-performance and functional photonic devices. Here we report an integrated silicon photonic arrayed waveguide grating (AWG) fabricated in a commercial foundry. The device has 64 channels with a spacing of approximately 50 GHz (0.7 nm), covering the bandwidth from 1967 nm to 2012 nm. The on-chip insertion loss of the AWG is measured to be approximately 5 dB. By implementing a TiN metal layer, the AWG spectrum can be thermally tuned with an efficiency of 0.27 GHz/mW. The device has a very compact configuration with a footprint of 2.3 mm?×?2 mm. The demonstrated AWG can potentially be used for dense wavelength division multiplexing in the 2 µm spectral band.

Optical metasurfaces is a rapidly developing research field driven by its exceptional applications for creating easy-to-integrate ultrathin planar optical devices. The tight confinement of the local electromagnetic fields in resonant photonic nanostructures can boost many optical effects and offer novel opportunities for the nanoscale control of light–matter interactions. However, once the structure-only metasurfaces are fabricated, their functions will be fixed, which limits it to make breakthroughs in practical applications. Recently, persistent efforts have led to functional multiplexing. Besides, dynamic light manipulation based on metasurfaces has been demonstrated, providing a footing ground for arbitrary light control in full space-time dimensions. Here, we review the latest research progress in multifunctional and tunable metasurfaces. Firstly, we introduce the evolution of metasurfaces and then present the concepts, the basic principles, and the design methods of multifunctional metasurface. Then with more details, we discuss how to realize metasurfaces with both multifunctionality and tunability. Finally, we also foresee various future research directions and applications of metasurfaces including innovative design methods, new material platforms, and tunable metasurfaces based metadevices.

Extending the optical communication wavelengths to 2 µm can significantly increase data capacity. Silicon photonics, which is a proven device integration technology, has made rapid progress at 2 µm recently. As a fundamental functional element in the photonic design kit, the 3 dB power splitter has been extensively studied in both the 1.55 µm and 2 µm regime. While the device is highly desirable to operate over both wave bands, the large waveguide dispersion in silicon makes it challenging. In this work, we demonstrate an ultra-broadband power splitter on silicon, which has a 0.2 dB bandwidth exceeding 520 nm from 1500 to 2020 nm according to simulations. The beam splitter is realized by a triple tapered Y-junction, and its operational bandwidth is greatly increased by subwavelength grating structure. The device has an ultra-compact footprint of only 3µm×2µm. Due to the limitations on the setup and coupling technique, we measure the device bandwidth in 1.55 µm and 2 µm wave bands. The device insertion loss is measured to be below 0.4 dB from 1500 to 1620 nm and from 1960 to 2020 nm, respectively. According to these results, the proposed device is believed to be capable of operating over a broadband from 1.55 µm and 2 µm wavelengths.

Lead halide perovskites have been promising materials for lasing applications. Despite that a series of perovskite microlasers have been reported, their lasing modes are confined by either the as-grown morphology or the etched boundary. The first one is quite random and incompatible with integration, whereas the latter one strongly spoils the laser performances. Herein, we propose and experimentally demonstrate a robust and generic mechanism to realize well-controlled perovskite microlasers without the etching process. By patterning a one-dimensional polymer grating onto a perovskite film, we show that the symmetry-protected bound states in the continuum (BICs) can be formed in it. The intriguing properties of BICs including a widely spread mode profile and high Qfactor, associated with the exceptional gain of perovskite, produce single-mode microlasers with high repeatability, controllability, directionality, and a polarization vortex. This mechanism can also be extended to two-dimensional nanostructures, enabling BIC lasers with different topological charges.

We conducted a three-month field experiment focusing on the physical and chemical characteristics of fog in a tropical rainforest in Xishuangbanna, Southwest China, in the winter of 2019. In general, the fog would form at midnight and persist because of the increased long-wave radiative cooling combined with the high relative humidity, gentle breeze, and a relatively low aerosol number concentration in the forest; the fog would dissipate before noon due to the increasing turbulence near the surface. This diurnal cycle is typical for radiation fog. The microphysical fog properties included a relatively low number concentration of the fog droplet, large droplet size, high liquid water content, narrow droplet number-size distribution, and high supersaturation. The chemical properties showed that the fog water was slightly alkaline with low electrical conductivity, whereas the highest proportions of anions and cations therein were Cl−and Ca2+, respectively; the chemical components were enriched in small fog droplets. In addition, we indirectly calculated the fog supersaturation according to the κ-Köhler theory. We found that condensation broadens the droplet number-size distribution at relatively low supersaturation, which is positively correlated with the fog-droplet number concentration and negatively correlated with the droplet mean-volume diameter; this affects the key microphysical processes of fog.

Micro- and nanolasers are miniaturized light sources with great potential in optical imaging, sensing, and communication. While various micro- and nanolasers have been synthesized, they are mostly linearly polarized and thus strongly restricted in many new applications, e.g., chiral resolution in synthetic chemistry, cancerous tissue imaging, information storage, and processing. Herein, we experimentally demonstrate the circularly polarized surface emitting perovskite lasers by integrating the as-grown perovskite microcrystals with an all-dielectric metalens. The perovskite microcrystal serves as an optical microcavity and produces linearly polarized laser emission, which is collected by a geometric phase based TiO2metalens. The left-handed circularly polarized components are collimated by the metalens into a directional laser beam with a divergent angle of <0.9°, whereas the right-handed components are strongly diverged by the same metalens. Consequently, the right-handed circularly polarized components are filtered out, and perovskite lasers with high directionality and pure circular polarization have been experimentally realized.

With their ultrathin characteristics as well as the powerful and flexible capabilities of wavefront modulation, optical metasurfaces have brought a new understanding of the interaction between light and matter and provided a powerful way to constrain and manage light. However, the unmodifiable structures and the immutable materials used in the construction lead to the unsatisfactory applications in most functional devices. The emergence of tunable optical metasurfaces breaks the aforementioned limitations and enables us to achieve dynamic control of the optical response. The work in recent years has focused on achieving tunability of optical metasurfaces through material property transition and structural reconfiguration. In this review, some tunable optical metasurfaces in recent years are introduced and summarized, as well as the advantages and limitations of various materials and mechanisms used for this purpose. The corresponding applications in functional devices based on tunability are also discussed. The review is terminated with a short section on the possible future developments and perspectives for future applications.

Mechanical stretching has been an effective way to achieve widely tunable optical response in artificial nanostructures. However, the typical stretchable optical devices produce exactly the reverse effects for two orthogonal linear polarizations, significantly hindering their practical applications in many emerging systems. Herein, we demonstrate an approach for a mechanically tunable all-dielectric metasurface with polarization insensitivity and full-spectrum response in the visible range from 450 to 650 nm. By embedding a TiO2metasurface in a polydimethylsiloxane substrate and stretching it in one direction, we find that the distinct reflection colors of two orthogonal linear polarizations can be tuned across the entire visible spectrum simultaneously. Encryption and display of information have also been realized with the same technique. The corresponding calculations show that the spectral responses of light with polarizations perpendicular and parallel to the strain are determined by two different mechanisms, that is, the near-field mutual interaction and the grating effects. This research shall shed light on stretchable and wearable photonics.

As emerging gain materials, lead halide perovskites have drawn considerable attention in coherent light sources. With the development of patterning and integration techniques, a perovskite laser array has been realized by distributing perovskite microcrystals periodically. Nevertheless, the packing density is limited by the crystal size and the channel gap distance. More importantly, the lasing performance for individual laser units is quite random due to variation of size and crystal quality. Herein an ultracompact perovskite nanoemitter array with uniform emission has been demonstrated. Individual emitters are formed via scattering evanescent components from a shared Fabry–Perot laser, ensuring uniform lasing emission in a unit cell with a side length of 160 nm and lattice constant of 400 nm. And the periodic silicon scatterers do not deteriorate the lasing threshold dramatically. In addition, the surface emitting efficiency increased significantly. The direct integration of a densely packed nanoemitter array with a silicon platform promises high-throughput sensing and high-capacity optical interconnects.

Journal of Lightwave Technology, vol. 27, no. 19, October 1, 2009 High-speed expansion of optical communications requires devices sizes to be smaller and smaller. Therefore, reducing cavity size attracted much [...]

As an ideal miniaturized light source, wavelength-tunable nanolasers capable of emitting a wide spectrum stimulate intense interests for on-chip optoelectronics, optical communications, and spectroscopy. However, realization of such devices remains a major challenge because of extreme difficulties in achieving continuously reversibly tunable gain media and high quality (Q)-factor resonators on the nanoscale simultaneously. Here, exploiting single bandgap-graded CdSSe NWs and a Fabry–Pérot/whispering gallery mode (FP/WGM) coupling cavity, a free-standing fiber-integrated reversibly wavelength-tunable nanolaser covering a 42 nm wide spectrum at room temperature with high stability and reproducibility is demonstrated. In addition, a 1.13 nm tuning spectral resolution is realized. The substrate-free device design enables integration in optical fiber communications and information. With reversible and wide, continuous tunability of emission color and precise control per step, our work demonstrates a general approach to nanocavity coupling affording high Q-factors, enabling an ideal miniaturized module for a broad range of applications in optics and optoelectronics, with optical fiber integration.

Random lasers have been ideal illumination sources for speckle-free and high-speed imaging. Despite their successes, the real applications of random lasers are facing a long-standing challenge, i.e., the cumbersome size of the illuminating system. Herein, we demonstrate perovskite-based surface emitting random lasers (SERLs) and explore their applications in speckle-free imaging. The random lasers are generated by multiple scattering in a perovskite polycrystalline film sandwiched by two distributed Bragg reflectors. Owing to the tight confinement in vertical direction and large number of random resonances, the wavevectors of random lasers are dominated by their vertical components, and thus, multimode SERLs with a divergence angle of ∼3–5° and low spatial coherence are produced. By directly illuminating the patterns with the SERLs, the notable speckle noises of conventional optical images have been dramatically suppressed. This research shall provide a strategy toward the integrated spectral-free imaging systems.

An ultra-compact polarization splitter–rotator with a discretized subwavelength nanostructure is demonstrated on the silicon-on-insulator platform. The device has a length of only 7.92 μm. For TE_0–TE_0, a low insertion loss (<0.2  dB), low crosstalk (<−20  dB), and high extinction ratio (>40  dB) are achieved over a broad wavelength range from 1500 nm to 1600 nm. For TM_0–TE_0, the insertion loss is lower than 1 dB over 40 nm bandwidth (1530–1570 nm). The crosstalk is lower than −25  dB, and the extinction ratio is larger than 20 dB, from 1500 nm to 1600 nm.

Nonlinear holographic metasurfaces have been intensively studied due to their potentials in practical applications. So far, nonlinear holographic metasurfaces have only been realized with plasmonic nanoantennas, suffering from high absorption loss and low damage threshold. Herein we propose and experimentally demonstrate a novel mechanism for nonlinear holographic metasurfaces. In contrast with conventional studies, the all-dielectric metasurface is composed of C-shaped Si nanoantennas. The incident laser is enhanced by their fundamental resonance, whereas the generated third-harmonic generation (THG) signals are redistributed to the air gap region via the higher order resonance, significantly reducing the absorption loss at short wavelength and resulting in an enhancement factor as high as 230. After introducing abrupt phase changes from 0 to 2π to the C elements, high-efficiency cyan and blue THG holograms have been experimentally generated with the Si metasurface for the very first time. This research shall shed light on the advances of nonlinear all-dielectric metasurfaces.

Nanoprint-based color display using either extrinsic structural colors or intrinsic emission colors is a rapidly emerging research field for high-density information storage. Nevertheless, advanced applications, e.g., dynamic full-color display and secure information encryption, call for demanding requirements on in situcolor change, nonvacuum operation, prompt response, and favorable reusability. By transplanting the concept of electrical/chemical doping in the semiconductor industry, we demonstrate an in situreversible color nanoprinting paradigm viaphoton doping, triggered by the interplay of structural colors and photon emission of lead halide perovskite gratings. It solves the aforementioned challenges at one go. By controlling the pumping light, the synergy between interlaced mechanisms enables color tuning over a large range with a transition time on the nanosecond scale in a nonvacuum environment. Our design presents a promising realization of in situdynamic color nanoprinting and will empower the advances in structural color and classified nanoprinting.

Very sharp 90°micro-bends for adiabatic optical wave propagation in multimode waveguides are demonstrated by an inverse design method, and the devices are fabricated on a silicon-on-insulator platform. The compact bending structures are based on digital waveguide metastructures with footprints as small as 2.6  μm×2.6  μm. For waveguides with widths of 2 μm and bending radii of only 1 μm, the TE_00 mode is able to turn around the sharp corner with 1 dB loss and >20  dB suppression ratio to higher-order modes. The transmission spectra of the devices are measured with ∼1  dB loss over a 40 nm bandwidth which are consistent with the numerical simulations.

High-quality (Q)-factor silicon microdisks are promising platforms for revolutionizing bio-sensing, medical diagnoses, and frequency combs. Nevertheless, their practical applications are hindered by the regular waveguide–resonator coupling configuration, which relies on sophisticated and high-cost nanofabrication. Here, we demonstrate a simple, cost-effective, and counterintuitive mechanism to couple light into a high-Q silicon microdisk. In contrast to the evanescent coupling, the incident light is injected into silicon microdisks through the waveguides directly connected to them. The end-fire injection coefficients and Q factors of waveguide-connected microdisks are around 57% and 2×10^5–7×10^5, comparable to conventional microdisks. Importantly, the end-fire injection configuration is quite robust to fabrication deviations and can be simply realized without using electron-beam lithography. Meanwhile, their applications in monitoring nanoparticles and tiny ambient changes have also been explored. This research will route a new way to on-chip biosensors and integrated photonic circuits.

Hybrid plasmonic nanolasers are intensively studied due to their nanoscale mode confinement and potentials in highly integrated photonic and quantum devices. Until now, the characteristics of plasmonic nanolasers are mostly determined by the crystal facets of top semiconductors, such as ZnO nanowires or nanoplates. As a result, the spasers are isolated, and their lasing wavelengths are random and difficult to tune. Herein, we experimentally demonstrate the formation of lead halide perovskite (MAPbX3) based hybrid plasmonic nanolasers and nanolaser arrays with arbitrary cavity shapes and controllable lasing wavelengths. These spasers are composed of MAPbX3perovskite nanosheets, which are separated from Au patterns with a 10 nm SiO2spacer. In contrast to previous reports, here, the spasers are determined by the boundary of Au patterns instead of the crystal facets of MAPbX3nanosheets. As a result, whispering gallery mode based circular spasers and spaser arrays were successfully realized by patterning the Au substrate into circles and gratings, respectively. The standard wavelength deviation of spaser arrays is as small as 0.3 nm. Meanwhile, owing to the anion-exchangeable property of MAPbX3perovskite, the emission wavelengths of spasers were tuned more than 100 nm back and forth by changing the stoichiometry of perovskite postsynthetically.

Structural colors arising from all-dielectric nanostructures are very promising for high-resolution color nanoprinting and high-density optical storage. However, once the all-dielectric nanostructures are fabricated, their optical performances are usually static or change slowly, significantly limiting the practical applications in advanced displays. Herein, we experimentally demonstrate the real-time tunable colors with microfluidic reconfigurable all-dielectric metasurfaces. The metasurface is composed of an array of TiO2nanoblocks, which are embedded in a polymeric microfluidic channel. By injecting solutions with a different refractive index into the channel, the narrow band reflection peak and the corresponding distinct colors of a TiO2metasurface can be precisely controlled. The transition time is as small as 16 ms, which is orders of magnitude faster than the current techniques. By varying the lattice size of TiO2metasurfaces, the real-time tunable colors are able to span the entire visible spectrum. Meanwhile, the injection and ejection of solvent have also shown the capability of the erasion and the restoration of information encoded in TiO2metasurfaces. The combination of all-dielectric nanostructures with microfluidic channels shall boost their applications in functional color display, banknote security, anticounterfeiting, and point-of-care devices.

Two-dimensional (2D) methylammonium lead halide perovskites (MAPbX3) have gained intensive research attention in past two years. Due to their quantum and dielectric confinements, 2D MAPbX3exhibited giant exciton binding energy, increased bandgap, and blueshifts of photoluminescence. Some of these novel characteristics have been successfully utilized to improve the performances of optoelectronic devices. However, up to now, the studies of quantum confinements are restricted within the linear region. Lots of important effects in quantum systems such as the exciton-exciton interaction have never been considered. Herein we synthesized the 2D MAPbBr3nano-sheets with layer number n = 7–10, 5, 3, 1 and characterized their nonlinear optical properties under high excitation density. Due to the strong excitonic resonance, we have achieved significantly increased third-harmonic generation (THG) from the 2D MAPbBr3nano-sheets. The deduced χ3of few-layer MAPbBr3nano-sheets was more than 50 times larger than the bulk MaPbBr3microblocks. While the THG was strongly dependent on the excitonic resonance, we found that their peak wavelengths were different from the ones of excitons in linear absorption spectra. This kind of mismatching became much larger in 2D MAPbBr3with smaller layer number and high incident power. The experimentally demonstrated blueshift was as large as 150meV, which was more than an order of magnitude larger than the ones in conventional quantum wells. By solving the 2D exciton Schr ödinger equation with screened parameters, we found that the effective repulsion between carriers at high electron-hole pair density played an important role in the reduction of the exciton binding energy, and thus induced the increase (blueshift) of exciton transition energy. This research will help the understanding the quantum confinement effects in 2D materials and can pave a new route to 2D MAPbBr3nonlinear optical devices.

Whispering gallery mode based optical microcavities are important for highly sensitive optical sensing. However, the current experimental realizations are strongly dependent on high-resolution tunable lasers and evanescent coupling, which are too cumbersome and too expensive for portable devices. Herein we experimentally demonstrate a cost-effective and robust approach to detect and size a single nanoparticle with far-field laser emissions. By placing a limacon microdisk close to a spiral microdisk, chiral resonances have been successfully generated. In contrast to previous research, here the internal chirality is strongly correlated with the far-field patterns (FFPs) and thus can be transduced to far-field emissions. Once a nanoparticle is attached to the limacon microdisk, the asymmetrical backscattering at the notch of the spiral can be averaged by the symmetrical scattering of the nanoparticle. Consequently, the internal chirality and the corresponding FFPs are changed. By measuring a far-field directional laser emission, nanoparticles with a radius of ∼50  nm have been successfully detected and sized without employing any spectral information. As a narrow-linewidth tunable laser is not used in our experiment and microdisks lasers may be electrically driven, this research will provide a new path to cost-effective, portable, highly sensitive optical sensors.

Lasing actions in organo-lead halide perovskite films have been heavily studied in the past few years. However, due to the disordered nature of synthesized perovskite films, the lasing actions are usually understood as random lasers that are formed by multiple scattering. Herein, we demonstrate the miscellaneous lasing actions in organo-lead halide perovskite films. In addition to the random lasers, we show that a single or a few perovskite microparticles can generate laser emissions with their internal resonances instead of multiple scattering among them. We experimentally observed and numerically confirmed whispering gallery (WG)-like microlasers in polygon shaped and other deformed microparticles. Meanwhile, owing to the nature of total internal reflection and the novel shape of the nanoparticle, the size of the perovskite WG laser can be significantly decreased to a few hundred nanometers. Thus, wavelength-scale lead halide perovskite lasers were realized for the first time. All of these laser behaviors are complementary to typical random lasers in perovskite film and will help the understanding of lasing actions in complex lead halide perovskite systems.

Recently, color generation in resonant nanostructures have been intensively studied. Despite of their exciting progresses, the structural colors are usually generated by the plasmonic resonances of metallic nanoparticles. Due to the inherent plasmon damping, such plasmonic nanostructures are usually hard to create very distinct color impressions. Here we utilize the concept of metasurfaces to produce all-dielectric, low-loss, and high-resolution structural colors. We have fabricated TiO2metasurfaces with electron-beam lithography and a very simple lift-off process. The optical characterizations showed that the TiO2metasurfaces with different unit sizes could generate high reflection peaks at designed wavelengths. The maximal reflectance was as high as 64% with full width at half-maximum (fwhm) around 30 nm. Consequently, distinct colors have been observed in bright field and the generated colors covered the entire visible spectral range. The detailed numerical analysis shows that the distinct colors were generated by the electric resonance and magnetic resonances in TiO2metasurfaces. Based on the unique properties of magnetic resonances, distinct colors have been observed in bright field when the metasurfaces were reduced to a 4 × 4 array, giving a spatial resolution around 16000 dpi. Considering the cost, stability, and CMOS-compatibility, this research will be important for the structural colors to reach real-world industrial applications.

Integrated optical power splitters are one of the fundamental building blocks in photonic integrated circuits. Conventional multimode interferometer-based power splitters are widely used as they have reasonable footprints and are easy to fabricate. However, it is challenging to realize arbitrary split ratios, especially for multi-outputs. In this Letter, an ultra-compact power splitter with a QR code-like nanostructure is designed by a nonlinear fast search method. The highly functional structure is composed of a number of freely designed square pixels with the size of 120×120  nm which could be either dielectric or air. The light waves are scattered by a number of etched squares with optimized locations, and the scattered waves superimpose at the outputs with the desired power ratio. We demonstrate 1×2 splitters with 1:1, 1:2, and 1:3 split ratios, and a 1×3 splitter with the ratio of 1:2:1. The footprint for all the devices is only 3.6×3.6  μm. Well-controlled split ratios are measured for all the cases. The measured transmission efficiencies of all the splitters are close to 80% over 30 nm wavelength range.

Lead halide perovskites have been promising platforms for micro- and nanolasers. However, the fragile nature of perovskites poses an extreme challenge to engineering a cavity boundary and achieving high-quality (Q) modes, severely hindering their practical applications. Here, we combine an etchless bound state in the continuum (BIC) and a chemically synthesized single-crystalline CsPbBr3microplate to demonstrate on-chip integrated perovskite microlasers with ultrahigh Q factors. By pattering polymer microdisks on CsPbBr3microplates, we show that record high-Q BIC modes can be formed by destructive interference between different in-plane radiation from whispering gallery modes. Consequently, a record high Q-factor of 1.04 × 105was achieved in our experiment. The high repeatability and high controllability of such ultrahigh Q BIC microlasers have also been experimentally confirmed. This research provides a new paradigm for perovskite nanophotonics.

The trapping of photons in low-refraction-index materials is thought to be prohibited in conventional photonic structures that employ total internal reflection. Specifically, the whispering gallery mode (WGM) microcavity, which is an important optical component, has to rely on a high contrast of the refraction index with the surrounding environment to manifest excellent light confinement. Here, we propose and demonstrate experimentally an optical microcavity structure consisting of a low-refraction-index silica microtoroid directly bonded on a high-refraction-index silicon substrate. The resonant structure supports high-Q fundamental WGMs in both visible and communication bands, while higher-order modes are suppressed significantly due to the strong leakage into the silicon substrate at long wavelengths. The highest measured quality factor exceeds ten million and low-threshold microcavity Raman lasing is also realized.

Abstract

The synthesized perovskites are randomly distributed and their optical properties are fixed after synthesis. Here we demonstrate the tailoring of lasing properties of perovskite microwire via micromanipulation. One microwire has been lifted by a tungsten probe and repositioned on a nearby perovskite microplate with one end suspended in air. Consequently, the conventional Fabry–Perot lasers are completely suppressed and a single laser peak has been observed. The numerical calculations reveal that the single-mode laser is formed by the whispering-gallery mode in the transverse plane of microwire. Our research provides a simple way to tailor the properties of microwire postsynthesis.

Many breakthroughs in technologies are closely associated with the deep understanding and development of new material platforms. As the main material used in microelectronics, Si also plays a leading role in the development of integrated photonics. The indirect bandgap, absence of χ(2)nonlinearity and the parasitic nonlinear absorptions at the telecom band of Si imposed technological bottlenecks for further improving the performances and expanding the functionalities of Si microcavities in which the circulating light intensity is dramatically amplified. The past two decades have witnessed the burgeoning of the novel material platforms that are compatible with the complementary metal-oxide-semiconductor (COMS) process. In particular, the unprecedented optical properties of the emerging materials in the thin film form have resulted in revolutionary progress in microcavity photonics. In this review article, we summarize the recently developed material platforms for integrated photonics with the focus on chip-scale microcavity devices. The material characteristics, fabrication processes and device applications have been thoroughly discussed for the most widely used new material platforms. We also discuss open challenges and opportunities in microcavity photonics, such as heterogeneous integrated devices, and provide an outlook for the future development of integrated microcavities.

Chirality is one of the important phenomena at the vicinity of exceptional point (EP). The conventional understanding is that the chirality is determined by asymmetrical scattering efficiency (η), which reaches to zero only when the resonance approaches EP. Here we study the possibility to enhance the chirality in open systems with a more robust mechanism. By combining chirality with avoided resonance crossing, we show that the chirality and ηcan be dramatically modified. Taking a spiral shaped annular cavity as an example, we show that the chirality of optical resonances can be significantly improved when two sets of chiral states approach each other. The imbalance between counterclockwise (CCW) components and clockwise (CW) components has been enhanced by more than an order of magnitude. Our research provides a new route to tailor and control the chirality in open systems.

Here we explore the applications of a deformed microdisk as a passive photonic element by coupling it to a bus waveguide. We show that deformed microdisk-based resonant filters are able to have transmittance of more than 99%, and the dropped signals can be routed to two different ports for particular applications. Interestingly, our results show that the splitting ratio can be dynamically tuned by locally changing the refractive index of microdisk. Our research opens new opportunities for the applications of deformed microdisks.

Abstract: CO2 concentrations and related environmental factors were measured in an Asian tropical rainforest located in a small valley in Xishuangbanna, SW China, with the aim of investigating the CO2 pooling effect and its mechanism of formation. Pooling of CO2 was observed during the evening (1600–2200 hours local time); the accumulated CO2 subsequently flowed away after dusk. We consider that along-slope drainage flow, soil CO2 efflux, and temperature inversion contribute to the development of CO2 pooling. A new model is proposed to track the mechanism of the formation and dissipation of CO2 pooling (e.g., drainage flow, compensatory mechanisms). Given its influence on the storage term, we suggest that CO2 pooling and subsequent disappearance should be taken into account when calculating eddy covariance and other micrometeorological measurements of carbon flux for valley sites.

AbstractCO2concentrations and related environmental factors were measured in an Asian tropical rainforest located in a small valley in Xishuangbanna, SW China, with the aim of investigating the CO2pooling effect and its mechanism of formation. Pooling of CO2was observed during the evening (1600–2200 hours local time); the accumulated CO2subsequently flowed away after dusk. We consider that along-slope drainage flow, soil CO2efflux, and temperature inversion contribute to the development of CO2pooling. A new model is proposed to track the mechanism of the formation and dissipation of CO2pooling (e.g., drainage flow, compensatory mechanisms). Given its influence on the storage term, we suggest that CO2pooling and subsequent disappearance should be taken into account when calculating eddy covariance and other micrometeorological measurements of carbon flux for valley sites.

We demonstrate that the unique characteristics of random lasing in bone can be used to assess nanoscale structural alterations as a mechanical or structural biosensor, given that bone is a partially disordered biological nanostructure. In this proof-of-concept study, we conduct photoluminescence experiments on cortical bone specimens that are loaded in tension under mechanical testing. The ultra-high sensitivity, the large detection area, and the simple detection scheme of random lasers allow us to detect prefailure damage in bone at very small strains before any microscale damage occurs. Random laser-based biosensors could potentially open a new possibility for highly sensitive detection of nanoscale structural and mechanical alterations prior to overt microscale changes in hard tissue and biomaterials.

On-chip integrated orbital angular momentum (OAM) sorting is of great importance in tackling the severe challenge of exponential growth in data traffic. Despite the continuous success, current demultiplexing techniques either scarify efficiency dramatically or lose the compactness of a system. Here we experimentally demonstrate an ultracompact OAM sorter using TiO2metasurfaces integrated onto a complementary metal-oxide-semiconductor (CMOS) camera. By utilizing the propagation phases, we transfer the unitary transformation theory in bulky systems into two TiO2metasurfaces, responsible for the functions of log-polartransformation and fan-out beam copying and focusing as well as the functions of phase correction and Fourier transform. The flatform metasurface doublet enables one to integrate the OAM sorter onto a camera chip. Consequently, OAM beams with topological charges of m= −3 to 3 were separated by a CMOS camera with an average crosstalk of −6.43 dB. This approach shall shed light on next-generation OAM modes processing.

Multiphoton absorption and luminescence are fundamentally important nonlinear processes for utilizing efficient light–matter interaction. Resonant enhancement of nonlinear processes has been demonstrated for many nanostructures; however, it is believed that all higher-order processes are always much weaker than their corresponding linear processes. Here, we study multiphoton luminescence from structured surfaces and, combining multiple advantages of perovskites with the concept of metasurfaces, we demonstrate that the efficiency of nonlinear multiphoton processes can become comparable to the efficiency of the linear process. We reveal that the perovskite metasurface can enhance substantially two-photon stimulated emission with the threshold being comparable with that of the one-photon process. Our modeling of free-carrier dynamics and exciton recombination upon nonlinear photoexcitation uncovers that this effect can be attributed to the local field enhancement in structured media, a substantial increase of the mode overlap, and the selection rules of two-photon absorption in perovskites.

We report a simple and robust mechanism that can result in highly directional emission from long-lived resonances in microcavities. By placing a nanoparticle (NP) into the evanescent wave region of microcavities, highly directional outputs with divergence angle ∼1.9°–10° can be obtained in single or double directions. The perturbation of NP on evanescent waves preserves the high-quality (Q) factors, and the collimation of microcavities generates the highly directional outputs. Our numerical simulations show that this mechanism is very robust to the size of NP and the refractive index/separation distance/size of microcavities.

We report a spectroscopic method using coherent random lasers for a simple, yet nanoscale, sensing approach. Unique spectral properties of coherent random laser emission can be detectably altered when introducing nanoscale perturbations to a simple nanocomposite film that consists of dielectric nanospheres and laser-dye-doped polymer to serve as a transducer. Random lasing action provides a means to amplify subtle perturbations to readily detectable spectral shifts in multiple discrete emission peaks. Owing to several advantages, such as large-area detection, narrow and multiple emission peaks, straightforward detection, and simple fabrication, random laser spectroscopy has the potential for ultrasensitive, yet simple, biosensors in various applications.

Owing to the low-loss and high refractive index variations derived from the basic building block of bone structure, we, for the first time to our knowledge, demonstrate coherent random lasing action originated from the bone structure infiltrated with laser dye, revealing that bone tissue is an ideal biological material for random lasing. Our numerical simulation shows that random lasers are extremely sensitive to subtle structural changes even at nanoscales and can potentially be an excellent tool for probing nanoscale structural alterations in real time as a novel spectroscopic modality.

Here we report the laser actions inside an active organic-inorganic grating with some irregular perturbation. In an optical excitation experiment, band-edge and random-laser emissions were both observed. The laser thresholds of the two laser actions are very close, which shows that random laser sometimes can operate with a threshold comparable to band-edge emission. Owing to the diffraction of the grating, directional emission with a narrow divergence angle was also obtained.

Here we report electric-field-induced, wavelength-tunable random lasing action in a liquid-crystal infiltrated disordered planar microcavity. With the increasing of external voltage from 0 Vto30 V, the emission wavelength of the random laser was continuously tuned from 648 nmto597 nm. It was also tuned back to 648 nm again by reducing the voltage back to 0 V.

We demonstrate output wavelength and intensity switching in a three-element directly coupled microdisk device consisting of one spiral microdisk coupled to two semicircle microdisks. The gapless coupling mechanism used allows individual elements to achieve lasing while achieving optimal transfer of optical power between adjacent microdisks. By controlling the transparency of the center element via injection current, the edge elements can be allowed to exchange their amplified spontaneous emission. In this manner, on-off-on switching of the output intensity, as well as discontinuous shifts in the output wavelength, can be achieved as a function of increasing injection current.

Temperature-tunable directional laser emission from a dye-doped liquid-crystal-based planar random cavity laser is presented. The optically pumped nematic liquid crystal infiltrated planar random microcavity produces an ultranarrow linewidth (0.03nm, corresponding to Q>20,000), highly directional (1.4° divergence angle) laser emission. By increasing the temperature from 27°C to 34°C, the wavelength of an emitted polarized laser can be tuned between 605.8 and 608.5 (ordinary light) and 631.3 and 624.9 nm (extraordinary light). A simulation result from the transfer matrix method that matches the experimental results well is also presented.