Matte surfaces with broadband transparency enabled by highly asymmetric diffusion of white light.

The long-standing paradox between matte appearance and transparency has deprived traditional matte materials of optical transparency. Here, we present a solution to this centuries-old optical conundrum by harnessing the potential of disordered optical metasurfaces. Through the construction of a random array of meta-atoms tailored in asymmetric backgrounds, we have created transparent matte surfaces that maintain clear transparency regardless of the strength of disordered light scattering or their matte appearances. This remarkable property originates in the achievement of highly asymmetric light diffusion, exhibiting substantial diffusion in reflection and negligible diffusion in transmission across the entire visible spectrum. By fabricating macroscopic samples of such metasurfaces through industrial lithography, we have experimentally demonstrated transparent windows camouflaged as traditional matte materials, as well as transparent displays with high clarity, full color, and one-way visibility. Our work introduces an unprecedented frontier of transparent matte materials in optics, offering unprecedented opportunities and applications.

Realizing high-efficiency third harmonic generation via accidental bound states in the continuum.

The bound states in the continuum (BICs) have attracted much attention in designing metasurface due to their high Q-factor and effectiveness in suppressing radiational loss. Here we report on the realization of the third harmonic generation (THG) at a near-ultraviolet wavelength (343 nm) via accidental BICs in a metasurface. The absolute conversion efficiency of the THG reaches 1.13 × 10-5 at a lower peak pump intensity of 0.7 GW/cm2. This approach allows the generation of an unprecedentedly high nonlinear conversion efficiency with simple structures.

Tuning Smith-Purcell radiation by rotating a metallic nanodisk array.

Smith-Purcell radiation (SPR) refers to the far-field, strong, spike radiation generated by the interaction of the evanescent Coulomb field of the moving charged particles and the surrounding medium. In applying SPR for particle detection and nanoscale on-chip light sources, wavelength tunability is desired. Here we report on tunable SPR achieved by moving an electron beam parallel to a two-dimensional (2D) metallic nanodisk array. By in-plane rotating the nanodisk array, the emission spectrum of the SPR splits into two peaks, with the shorter-wavelength peak blueshifted and the longer-wavelength one redshifted by increasing the tuning angle. This effect originates from the fact that the electrons fly effectively over a one-dimensional (1D) quasicrystal projected from the surrounding 2D lattice, and the wavelength of SPR is modulated by quasiperiodic characteristic lengths. The experimental data are in agreement with the simulated ones. We suggest that this tunable radiation provides free-electron-driven tunable multiple photon sources at the nanoscale.

Breaking the limitation of polarization multiplexing in optical metasurfaces with engineered noise.

Noise is usually undesired yet inevitable in science and engineering. However, by introducing the engineered noise to the precise solution of Jones matrix elements, we break the fundamental limit of polarization multiplexing capacity of metasurfaces that roots from the dimension constraints of the Jones matrix. We experimentally demonstrate up to 11 independent holographic images using a single metasurface illuminated by visible light with different polarizations. To the best of our knowledge, it is the highest capacity reported for polarization multiplexing. Combining the position multiplexing scheme, the metasurface can generate 36 distinct images, forming a holographic keyboard pattern. This discovery implies a new paradigm for high-capacity optical display, information encryption, and data storage.

Flip-component metasurfaces for camouflaged meta-domes.

Allowing microwaves to transmit through without changing the wavefront is one of the essential requirements of the dome structures of antenna arrays like radars. Here, we demonstrate a microwave metasurface as an array of two types of meta-atoms, which are the flip counterparts to each other. Due to the reciprocity and space-inversion symmetry, the wavefront in the transmission is unchanged by the metasurface in a broad spectrum; while at the same time, the wavefront in reflection can be manipulated independently by changing the arrangement of the meta-atoms. Specifically, a random-flip metasurface that produces diffuse reflection is realized, enabling a camouflaged meta-dome. The broadband, wide-angle, and polarization-independent diffuse reflection and undistorted transmission are numerically and experimentally verified. Our finding enables a unique meta-dome structure that has camouflage functionality.

Improving Photoelectric Conversion with Broadband Perovskite Metasurface.

The miniaturization and integration of optoelectronic devices require progressive size reduction of active layers, resulting in less optical absorption and lower quantum efficiency. In this work, we demonstrate that introducing a metasurface made of hybrid organic-inorganic perovskite (HOIP) can significantly enhance broadband absorption and improve photon-to-electron conversion, which roots from exciting Mie resonances together with suppressing optical transmission. On the basis of the HOIP metasurface, a broadband photodetector has been fabricated where photocurrent boosts more than 10 times in the frequency ranging from ultraviolet to visible. The device response time is less than 5.1 µs at wavelengths 380, 532, and 710 nm, and the relevant 3 dB bandwidth is over 0.26 MHz. Moreover, this photodetector has been applied as a signal receiver for transmitting 2D color images in broadband optical communication. These results accentuate the practical applications of HOIP metasurfaces in novel optoelectronic devices for broadband optical communication.

Realizing transmissive and reflective focusing with an on-chip metalens.

A metalens made of compact planar metastructure exhibits an excellent capability of focusing. The high-quality transmissive and reflective focusing simultaneously provides Fourier transform (FT) operation for optical information processing. Here we show a transflective on-chip metalens (TOM) made of orthogonal nano-grooves (ONGs). The TOM simultaneously converges transmitted and reflected (T&R) waves to the designed focal points. By adjusting the phase gradient profiles provided by the ONGs, the focal lengths of the T&R in-plane waves can be independently tuned. Our simulations show that the TOM possesses the advantages of broadband (>400 nm bandwidth) and high-focusing-efficiency (∼60%) dual-focusing capability. Further, we utilize the TOM to build a one-to-two 4-f optical system. Two different spatial filtering operations based on FT can be simultaneously implemented in axial transmission and off-axis reflection channels for one input signal. We expect that the dual-focusing metalens approach can realize parallel optical processing in on-chip optical computing, spatial filtering, and beyond.

Multichannel Distribution and Transformation of Entangled Photons with Dielectric Metasurfaces.

Photonic quantum information processing relies on operating the quantum state of photons, which usually involves bulky optical components unfavorable for system miniaturization and integration. Here, we report on the transformation and distribution of polarization-entangled photon pairs with multichannel dielectric metasurfaces. The entangled photon pairs interact with metasurface building blocks, where the geometrical-scaling-induced phase gradients are imposed, and are transformed into two-photon entangled states with the desired polarization. Two metasurfaces, each simultaneously distributing polarization-entangled photons to spatially separated multiple channels M (N), may accomplish M×N channels of entanglement distribution and transformation. Experimentally we demonstrate 2×2 and 4×4 distributed entanglement states, including Bell states and superposition of Bell states, with high fidelity and strong polarization correlation. We expect this approach paves the way for future integration of quantum information networks.

Direct observation of terahertz topological valley transport.

Topological photonics offers the possibility of robust transport and efficiency enhancement of information processing. Terahertz (THz) devices, such as waveguides and beam splitters, are prone to reflection loss owing to their sensitivity to defects and lack of robustness against sharp corners. Thus, it is a challenge to reduce backscattering loss at THz frequencies. In this work, we constructed THz photonic topological insulators and experimentally demonstrated robust, topologically protected valley transport in THz photonic crystals. The THz valley photonic crystal (VPC) was composed of metallic cylinders situated in a triangular lattice. By tuning the relevant location of metallic cylinders in the unit cell, mirror symmetry was broken, and the degenerated states were lifted at the K and K' valleys in the band structure. Consequently, a bandgap of THz VPC was opened, and a nontrivial band structure was created. Based on the calculated band structure, THz field distributions, and valley Berry curvature, we verified the topological phase transition in such type of THz photonic crystals. Further, we showed the emergence of valley-polarized topological edge states between the topologically distinct VPCs. The angle-resolved transmittance measurements identified the bulk bandgap in the band structure of the VPC. The measured time-domain spectra demonstrated the topological transport of valley edge states between distinct VPCs and their robustness against bending and defects. Furthermore, experiments conducted on a topological multi-channel intersectional device revealed the valley-polarized characteristic of the topological edge states. This work provides a unique approach to reduce backscattering loss at the THz regime. It also demonstrates potential high-efficiency THz functional devices such as topologically protected beam splitters, low-loss waveguides, and robust delay lines.

Multiple-polarization-sensitive photodetector based on a perovskite metasurface.

Most polarization-sensitive photodetectors detect either linearly polarized (LP) or circularly polarized (CP) light. Here, we experimentally demonstrate a multiple-polarization photodetector based on a hybrid organic-inorganic perovskite (HOIP) metasurface, which is sensitive to both LP and CP light simultaneously. The perovskite metasurface is composed of a HOIP antenna array on a single-crystal HOIP film. Owing to the antenna anisotropy, the absorption of linearly polarized light at the metasurface depends on the polarization angle; also, due to the mirror asymmetry of the antenna elements, the metasurface is also sensitive to different circular polarizations. Polarization-dependent photocurrent responses to both LP and CP light are detected. Our results highlight the potential of perovskite metasurfaces for integrated photoelectric applications.

Pushing the Limits of Functionality-Multiplexing Capability in Metasurface Design Based on Statistical Machine Learning.

As 2D metamaterials, metasurfaces provide an unprecedented means to manipulate light with the ability to multiplex different functionalities in a single planar device. Currently, most pursuits of multifunctional metasurfaces resort to empirically accommodating more functionalities at the cost of increasing structural complexity, with little effort to investigate the intrinsic restrictions of given meta-atoms and thus the ultimate limits in the design. In this work, it is proposed to embed machine-learning models in both gradient-based and nongradient optimization loops for the automatic implementation of multifunctional metasurfaces. Fundamentally different from the traditional two-step approach that separates phase retrieval and meta-atom structural design, the proposed end-to-end framework facilitates full exploitation of the prescribed design space and pushes the multifunctional design capacity to its physical limit. With a single-layer structure that can be readily fabricated, metasurface focusing lenses and holograms are experimentally demonstrated in the near-infrared region. They show up to eight controllable responses subjected to different combinations of working frequencies and linear polarization states, which are unachievable by the conventional physics-guided approaches. These results manifest the superior capability of the data-driven scheme for photonic design, and will accelerate the development of complex devices and systems for optical display, communication, and computing.

Formation of magnetic nanowire arrays by cooperative lateral growth.

Nanowires typically grow along their longitudinal axis, and the long-range order among wires sustains only when a template exists. Here, we report an unprecedented electrochemical growth of ordered metallic nanowire arrays from an ultrathin electrolyte layer, which is achieved by solidifying the electrolyte solution below the freezing temperature. The thickness of the electrodeposit is instantaneously tunable by the applied electric pulses, leading to parallel ridges on webbed film without using any template. An array of metallic nanowires with desired separation and width determined by the applied pulses is formed on the substrate with arbitrary surface patterns by etching away the webbed film thereafter. This work demonstrates a previously unrecognized fabrication strategy that bridges the gap of top-down lithography and bottom-up self-organization in making ordered metallic nanowire arrays over a large area with low cost.

Tunable valley polarization in Janus WSSe by magnetic proximity coupling to a CrI3 layer.

We investigate the electronic properties and valley physics of Janus monolayer WSSe on a CrI3 substrate layer based on first-principles calculations. It is shown that the K and K' valley degeneracy can be lifted which leads to valley polarization (VP) in the WSSe due to the magnetic proximity coupling to a magnetic substrate. The magnitude of VP is highly sensitive to the interfacial electronic properties and can be tuned by varying the stacking configurations of the heterostructure. Interestingly, the direction of VP can be altered by manipulating the layer alignment without reversing the magnetism orientation of the magnetic substrate CrI3. We suggest that the hybridization between the bands of WSSe and the substrate plays an important role. Meanwhile, the charge distributions have been mapped out to uncover the microscopic origin of the direction variable VP. In addition, large VP can be achieved by adjusting the interlayer spacing. Our investigations may have potential applications in the design of valleytronic devices.

Three-Dimensional Soundproof Acoustic Metacage.

In this Letter, we theoretically propose and experimentally demonstrate a three-dimensional soundproof acoustic cage structure, hereby denoted as an acoustic metacage. The metacage is composed of six acoustic metamaterial slabs with open holes and hidden bypass space coiling tunnels connected to the holes. Band structure analysis reveals a novel physical mechanism to open a low-frequency broad partial band gap via the band folding in other directions, which can also be interpreted by an effective medium with indefinite effective mass density and negative effective modulus. Transmission loss in simulations and in the acoustic impedance tube are administered. Strikingly, we prove that the soundproofing effect of the metacage is robust against the airflow perturbation induced by a fan. Our work paves a road for low-frequency airborne soundproof structures in the presence of ventilation.

Diffuse reflection and reciprocity-protected transmission via a random-flip metasurface.

Rough surfaces lead to diffused light in both reflection and transmission, thereby blurring the reflected and transmitted images. Here, we merge the traditionally incompatible diffuse reflection and undistorted transmission by introducing the concept of random-flip metasurfaces made of randomly flipped components. These metasurfaces have a globally random phase in reflection that leads to diffuse reflection, while the local space inversion and reciprocity principle ensure distortion-free transmission. Notably, the metasurface reflects like a rough surface yet transmits like a smooth one in a broad spectrum. On the basis of complementary random arrays of gold nanorods, we verified this functionality by both optical spectroscopy and imaging experiments over a broad range of frequencies from the visible to the infrared regime. This feature, which originates from breaking the phase correlation between reflection and transmission by the metasurface, could enable a range of new optical materials and display technology.

Invisible surfaces enabled by the coalescence of anti-reflection and wavefront controllability in ultrathin metasurfaces.

Reflection inherently occurs on the interfaces between different media. In order to perfectly manipulate waves on the interfaces, integration of antireflection function in metasurfaces is highly desired. In this work, we demonstrate an approach to realize exceptional metasurfaces that combine the two vital functionalities of antireflection and arbitrary phase manipulation in the deep subwavelength scale. Such ultrathin devices confer reflection-less transmission through impedance-mismatched interfaces with arbitrary wavefront shapes. Theoretically and experimentally, we demonstrate a three-layer antireflection metasurface that achieves an intriguing phenomenon: the simultaneous elimination of the reflection and refraction effects on a dielectric surface. Incident waves transmit straightly through the dielectric surface as if the surface turns invisible. We further demonstrate a wide variety of applications such as invisible curved surfaces, "cloaking" of dielectric objects, reflection-less negative refraction and flat axicons on dielectric-air interfaces, etc. The coalescence of antireflection and wavefront controllability in the deep subwavelength scale brings new opportunities for advanced interface optics with high efficiency and great flexibility.

Flexible Phase Change Materials for Electrically-Tuned Active Absorbers.

Phase change materials (PCMs), such as GeSbTe (GST) alloys and vanadium dioxide (VO2 ), play an important role in dynamically tunable optical metadevices. However, the PCMs usually require high thermal annealing temperatures above 700 K, but most flexible metadevices can only work below 500 K owing to the thermal instability of polymer substrates. This contradiction limits the integration of PCMs into flexible metadevices. Here, a mica sheet is chosen as the chemosynthetic support for VO2 and a smooth and uniformly flexible phase change material (FPCM) is realized. Such FPCMs can withstand high temperatures while remaining mechanically flexible. As an example, a metal-FPCM-metal infrared meta-absorber with mechanical flexibility and electrical tunability is demonstrated. Based on the electrically-tuned phase transition of FPCMs, the infrared absorption of the metadevice is continuously tuned from 20% to 90% as the applied current changes, and it remains quite stable at bending states. The metadevice is bent up to 1500 times, while no visible deterioration is detected. For the first time, the FPCM metastructures are significantly added to the flexible material family, and the FPCM-based metadevices show various application prospects in electrically-tunable conformal metadevices, dynamic flexible photodetectors, and active wearable devices.

Band engineering method to create Dirac cones of accidental degeneracy in general photonic crystals without symmetry.

Symmetry usually plays a key role in the formation of the Dirac cone in the band structure of triangular or hexagonal systems. In this work, we demonstrate a systematic method to create Dirac cones of accidental degeneracy in general photonic crystals without symmetry. With this method, a band gap can be closed gradually through a series of modification to the unit structure based on the eigenfields of the band edges, and consequently a Dirac point is formed with Dirac conical dispersions in its vicinity. The validity of this approach is demonstrated by three examples. We further show that the Dirac cones of accidental degeneracy have the same properties as the symmetry-induced Dirac cones, such as finite group velocity and pseudo-diffusive transmission. Our finding opens a route for the engineering of accidental degeneracy in general photonic crystals beyond the scope of high-symmetry ones.

Metasurface-assisted broadband optical absorption in ultrathin perovskite films.

Ultrathin hybrid organic-inorganic perovskite (HOIP) films have significant potential for use in integrated high-performance photoelectric devices. However, the relatively low optical absorption capabilities of thinner films, particularly in the long-wavelength region, pose a significant challenge to the further improvement of photoelectrical conversion in ultrathin HOIP films. To address this problem, we propose a combining of ultrathin HOIP film with plasmonic metasurface to enhance the absorption of the film effectively. The metasurface excites localized surface plasmon resonances and deflects the reflected light within the HOIP film, resulting in an obvious enhancement of film absorption. Finite-difference time-domain simulation results reveal that the far-field intensities, deflection angles, and electric field distributions can be effectively varied by using metasurfaces with different arrangements. Examination of the reflection and absorption spectra reveals that embedding a specifically designed metasurface into the HOIP film produces an obvious enhancement in broadband optical absorption compared with pure HOIP films. We further demonstrate that this broadband absorption promotion mechanism can be effective at a wide range of HOIP film thicknesses. Comparison of the absorption spectra at various incidence angles of ultrathin HOIP films with and without underlying metasurfaces indicates that the addition of a metasurface can effectively promote absorption under wide-angle incident light illumination. Moreover, by extending the metasurface structure to a two-dimensional case, absorption enhancements insensitive to the incident polarization states have also been demonstrated. This proposed metasurface-assisted absorption enhancement method could be applied in designing novel high-performance thin-film solar cells and photodetectors.

Non-Hermitian effective medium theory and complex Dirac-like cones.

In this work, we propose a non-Hermitian effective medium theory to interpret the spawning rings of exceptional points out of the Dirac cones in the band structures of photonic crystals with loss/gain. Based on this theory, we predict and demonstrate two unique types of band dispersions of fully passive photonic crystals. In one type, the exceptional ring shrinks into a complex Dirac point associated with a complex Dirac-like cone. In the other type, a point of quadratic degeneracy is realized in the imaginary frequency spectrum. Our theory provides a unified picture for the exceptional points in effective media and gives rise to novel concepts like complex Dirac-like cones in non-Hermitian photonics.