Observation of intrinsic chiral bound states in the continuum.

Photons with spin angular momentum possess intrinsic chirality, which underpins many phenomena including nonlinear optics1, quantum optics2, topological photonics3 and chiroptics4. Intrinsic chirality is weak in natural materials, and recent theoretical proposals5-7 aimed to enlarge circular dichroism by resonant metasurfaces supporting bound states in the continuum that enhance substantially chiral light-matter interactions. Those insightful works resort to three-dimensional sophisticated geometries, which are too challenging to be realized for optical frequencies8. Therefore, most of the experimental attempts9-11 showing strong circular dichroism rely on false/extrinsic chirality by using either oblique incidence9,10 or structural anisotropy11. Here we report on the experimental realization of true/intrinsic chiral response with resonant metasurfaces in which the engineered slant geometry breaks both in-plane and out-of-plane symmetries. Our result marks, to our knowledge, the first observation of intrinsic chiral bound states in the continuum with near-unity circular dichroism of 0.93 and a high quality factor exceeding 2,663 for visible frequencies. Our chiral metasurfaces may lead to a plethora of applications in chiral light sources and detectors, chiral sensing, valleytronics and asymmetric photocatalysis.

Compact angle-resolved metasurface spectrometer.

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 × 10-3 rad. Our angle-resolved spectrometers empowered by metalenses can be employed towards enhancing advanced optical imaging and spectral analysis applications.

Efficient and high-speed coupling modulation of silicon racetrack ring resonators at 2†µm waveband.

Significantly increased interests have been witnessed for the 2 µm waveband which is considered to be a promising alternative window for fiber and free-space optical communications. However, the less mature device technology at this wavelength range is one of the primary obstacles toward practical applications. In this work, we demonstrate an efficient and high-speed silicon modulator based on carrier depletion in a coupling tunable resonator. A benchmark high modulation efficiency of 0.75 V·cm is achieved. The 3-dB electro-optic bandwidth is measured to be 26 GHz allowing for up to 34 Gbit/s on-off keying modulation with a low energy consumption of ∼0.24 pJ/bit. It provides a solution for the silicon modulator with high-speed and low power consumption in the 2-µm waveband.

Folded Digital Meta-Lenses for on-Chip Spectrometer.

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.

Ultrahigh-Q Lead Halide Perovskite Microlasers.

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 CsPbBr3 microplate to demonstrate on-chip integrated perovskite microlasers with ultrahigh Q factors. By pattering polymer microdisks on CsPbBr3 microplates, 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 × 105 was 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.

Miniature Two-Photon Microscopic Imaging Using Dielectric Metalens.

Miniature two-photon microscopy has emerged as a powerful technique for investigating brain activity in freely moving animals. Ongoing research objectives include reducing probe weight and minimizing animal behavior constraints caused by probe attachment. Employing dielectric metalenses, which enable the use of sizable optical components in flat device structures while maintaining imaging resolution, is a promising solution for addressing these challenges. In this study, we designed and fabricated a titanium dioxide metalens with a wavelength of 920 nm and a high aspect ratio. Furthermore, a meta-optic two-photon microscope weighing 1.36 g was developed. This meta-optic probe has a lateral resolution of 0.92 µm and an axial resolution of 18.08 µm. Experimentally, two-photon imaging of mouse brain structures in vivo was also demonstrated. The flat dielectric metalens technique holds promising opportunities for high-performance integrated miniature nonlinear microscopy and endomicroscopy platforms in the biomedical field.

On-chip Y-junction with adaptive power splitting toward ultrabroad bandwidth.

Growing research interests have been directed to the emerging optical communication band at 2-µm wavelengths. The silicon photonic components are highly desired to operate over a broad bandwidth covering both C-band and the emerging 2-µm wave band. However, the dispersions of the silicon waveguides eventually limit the optical bandwidth of the silicon photonic devices. Here, we introduce a topology-optimized Y-junction with a shallow-etched trench and its utility to reverse the detrimental dispersion effect. The shallow trench enables the Y-junction to have an adaptive splitting capability over a broad spectral range. The 0.2-dB bandwidth of the power splitter exceeds 800 nm from 1400 nm to 2200 nm. The device has a compact footprint of 3 µm × 1.64 µm. The device is characterized at the C-band and 2-µm band with a measured excess loss below 0.4 dB for a proof-of-concept demonstration.

Ultracompact Orbital Angular Momentum Sorter on a CMOS Chip.

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 TiO2 metasurfaces 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 TiO2 metasurfaces, responsible for the functions of log-polar transformation 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.

Enhanced Multiphoton Processes in Perovskite Metasurfaces.

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.

Metasurface for Structured Light Projection over 120° Field of View.

Structured light projection is a widely adopted approach for depth perception in consumer electronics and other machine vision systems. Diffractive optical element (DOE) is a key component for structured light projection that redistributes a collimated laser beam to a spot array with uniform intensity. Conventional DOEs for laser spot projection are binary-phase gratings, suffering from low efficiency and low uniformity when designed for a large field of view (FOV). Here, by combining vectorial electromagnetic simulation and interior-point method for optimization, we experimentally demonstrate polarization-independent silicon-based metasurfaces that can project a collimated laser beam to a spot array in the far-field with an exceedingly large FOV over 120° × 120°. The metasurface DOE with large FOV may benefit a number of depth perception-related applications such as face-unlock and motion sensing.

Trichromatic and Tripolarization-Channel Holography with Noninterleaved Dielectric Metasurface.

Metasurfaces hold great potentials for advanced holographic display with extraordinary information capacity and pixel sizes in an ultrathin flat profile. A dual-polarization channel to encode two independent phase profiles or spatially multiplexed meta-holography by interleaved metasurfaces are captivated popular solutions to projecting multiplexed and vectorial images. However, the intrinsic limit of orthogonal polarization-channels, their crosstalk due to coupling between meta-atoms, and interleaving-induced degradation of efficiency and reconstructed image quality set great barriers for sophisticated meta-holography from being widely adopted. Here we report a noninterleaved TiO2 metasurface holography, and three distinct phase profiles are encoded into three orthogonal polarization bases with almost zero crosstalk. The corresponding three independently constructed intensity profiles are therefore assigned to trichromatic (RGB) beams, resulting in high-quality and high-efficiency vectorial meta-holography in the whole visible regime. Our strategy presents an unconventionally advanced holographic scheme by synergizing trichromatic colors and tripolarization channels, simply realized with a minimalist noninterleaved metasurface. Our work unlocks the metasurface's potentials on massive information storage, polarization optics, polarimetric imaging, holographic data encryption, etc.

Breakup and Recovery of Topological Zero Modes in Finite Non-Hermitian Optical Lattices.

The topological edge state (TES) in a one-dimensional optical lattice has exhibited robust field localization or waveguiding against the structural perturbations that would give rise to fault-tolerant photonic integrations. However, the zero mode as a kind of TES usually deviates from the exact zero-energy state in a finite Hermitian lattice due to the coupling between these edge states, which inevitably weaken the topological protection. Here, we first show such a breakup of zero modes in finite Su-Schriffer-Heeger optical lattices and then reveal their recovery by introducing non-Hermitian degeneracies with parity-time (PT) symmetry. We carry out experiments in a finite silicon waveguide lattice, where a passive-PT symmetry was implemented with carefully controlled lossy silicon waveguides. The experimental results are fully compatible with the theoretical prediction. Our results show that the topological property of an open system can be tuned by non-Hermitian lattice engineering, which offers a route to enhance the topological protection in a finite system.

Nonlinear Holographic All-Dielectric Metasurfaces.

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.

Very sharp adiabatic bends based on an inverse design.

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 TE00 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.

Transmissive structural color filters using vertically coupled aluminum nanohole/nanodisk array with a triangular-lattice.

We demonstrate a configuration to generate transmissive structural colors through triangular-lattice square nanohole arrays in aluminum (Al) film with Al nanodisks on the bottom of the nanoholes. By using a simple nanofabrication process, colors covering the entire visible light with different brightness and saturation are achieved by tuning both the period of arrays and the size of nanoholes. The optical behaviors of the structures are systematically investigated by both experimental and theoretical methods. The results indicate that the localized surface plasmon resonance of nanohole arrays plays the key role in the extraordinary transmission and meanwhile the coupling of disks and holes is also of importance for the enhanced transmission. With the wide color gamut, these kinds of vertically coupled Al nanohole/nanodisk arrays show the capabilities for high-resolution full-color printing. Compared to existing transmissive plasmonic color filters, the configuration in this work has the advantages of a simple fabrication process and using cheap aluminum materials.

Integrated photonic power divider with arbitrary power ratios.

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.

Occurrence of Cryptosporidium and Giardia and the Relationship between Protozoa and Water Quality Indicators in Swimming Pools.

A total of 60 samples were collected from 35 swimming pools in Beijing, China, and the presence of Cryptosporidium and Giardia were investigated. The results showed that 16.7% and 15.0% of samples were positive for Cryptosporidium oocyst and Giardia cysts, respectively, with a mean concentration of 0.30 oocysts/10 L and 0.27 cysts/10 L. The oocysts and cysts were found to have higher rates of occurrence in August than in May. Genotyping confirmed the presence of Cryptosporidium hominis, C. parvum, and Giardia assemblages A and B, all of which were associated with human infections. The predominant species/assemblages were C. hominis and Giardia assemblage A. Analyses of the relationships between parasite oocysts/cysts, indicator bacteria, and physical-chemical parameters revealed that there was no correlation between 2 parasites and fecal bacterial indicators, whilst there was a significant correlation between protozoa and urea concentration, which indicates that urea concentration rather than fecal bacterial indicators might be an appropriate index for chlorine-resistant protozoa in swimming pools. This study provides useful information to improve the safety of swimming pool water and deduce the risk of protozoan infections.

Effects of pH and substrate concentrations on dark fermentative biohydrogen production from xylose by extreme thermophilic mixed culture.

Biohydrogen is considered as one of the most promising energy alternatives considering the climate and energy crisis. The dark fermentative hydrogen production from xylose at extreme thermophilic condition (70 °C) using mixed culture was conducted in this study. The effects of initial pH values (ranged from 5.0 to 10.0) and substrate concentrations (ranged from 2.5 to 15.0 g/L) on the hydrogen production, substrate degradation and metabolite distributions were investigated using batch-mode operations. Results showed that initial substrate pH values in the neutral region (6.0-7.0) were beneficial for hydrogen production. The fermentation at initial pH 7.0 and 7.5 g/L xylose reached an optimal hydrogen yield of 1.29 mol-H2/mol-xyloseconsumed. Ethanol, butyrate, and propionate were the major liquid metabolites. The xylose biodegradation efficiency of the mixed culture decreased sharply at high initial culture pH values. The increase of xylose concentration resulted in the accumulation of propionate and an obvious decrease in the final pH value, as well as a low hydrogen yield. Polymerase chain reaction-denaturing gradient gel electrophoresis analysis indicated that hydrogen producing bacteria were enriched by repeated culture under extreme thermophilic conditions. Also, the mixed culture was dominated with bacterial species related to Clostridium and Thermoanaerobacterium.

Formation of single-mode laser in transverse plane of perovskite microwire via micromanipulation.

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.

Loss-free and active optical negative-index metamaterials.

The recently emerged fields of metamaterials and transformation optics promise a family of exciting applications such as invisibility, optical imaging with deeply subwavelength resolution and nanophotonics with the potential for much faster information processing. The possibility of creating optical negative-index metamaterials (NIMs) using nanostructured metal-dielectric composites has triggered intense basic and applied research over the past several years. However, the performance of all NIM applications is significantly limited by the inherent and strong energy dissipation in metals, especially in the near-infrared and visible wavelength ranges. Generally the losses are orders of magnitude too large for the proposed applications, and the reduction of losses with optimized designs seems to be out of reach. One way of addressing this issue is to incorporate gain media into NIM designs. However, whether NIMs with low loss can be achieved has been the subject of theoretical debate. Here we experimentally demonstrate that the incorporation of gain material in the high-local-field areas of a metamaterial makes it possible to fabricate an extremely low-loss and active optical NIM. The original loss-limited negative refractive index and the figure of merit (FOM) of the device have been drastically improved with loss compensation in the visible wavelength range between 722 and 738 nm. In this range, the NIM becomes active such that the sum of the light intensities in transmission and reflection exceeds the intensity of the incident beam. At a wavelength of 737 nm, the negative refractive index improves from -0.66 to -1.017 and the FOM increases from 1 to 26. At 738 nm, the FOM is expected to become macroscopically large, of the order of 10(6). This study demonstrates the possibility of fabricating an optical negative-index metamaterial that is not limited by the inherent loss in its metal constituent.