Topological singularities, such as Weyl points (WPs) and Dirac points, can give rise to unidirectional propagation channels known as chiral zero modes (CZMs) when subject to a magnetic field. CZMs, as distinct zeroth Landau levels (bulk modes) with high degeneracy, are responsible for intriguing phenomena like the chiral anomaly in quantum systems. The propagation direction of each CZM is determined by both the applied magnetic field and the topological charge of the singularity point. While counterpropagating CZMs have been observed in 2D and 3D systems, the realization of copropagating CZMs has remained elusive. Here, we present the first experimental observation of copropagating CZMs in magnetic photonic crystals hosting a single pair of ideal Weyl points. By manipulating the crystal’s structural configuration and applying a uniform bias magnetic field, we spatially alter the locations of the WPs, creating pseudo-magnetic fields of opposite directions for different WPs. This arrangement results in a pair of CZMs that possess the same group velocity and copropagate. Our work opens up new possibilities for the topological manipulation of wave propagation and may lead to advancements in optical waveguides, switches, and various other applications.

Band singularities in the momentum space, such as Weyl points, play an important role in topological physics. Their nontrivial topological properties provide a platform to investigate various intriguing phenomena associated with wave propagation inside or at the surfaces of the medium. Here we show that the evanescent waves near the band singularities can host a variety of non-Hermitian behaviors, in the absence of material gain or loss. Different from commonly investigated parity-time (𝑃𝑇)–symmetric systems, one can manipulate the 𝑃𝑇-symmetric phase of the evanescent waves via tuning the temporal growing/decaying behavior of the excitation rather than the gain/loss or coupling. Interestingly, the distribution of non-Hermitian Berry curvature for the evanescent waves around a Weyl point can be confined inside an exceptional cone in the momentum space, which leads to the transition of the winding of the reflection phase around the Weyl point. Our findings provide a different way of manipulating the 𝑃𝑇-symmetric photonic system and insight into the topological behavior of band singularities.

Dynamically controlling electromagnetic waves at will is highly desired in many applications, but most previously realized mechanically reconfigurable metasurfaces are of restricted wave-control capabilities due to the limited tuning ranges of structural properties (e.g., lattice constant or meta-atoms). Here, we present mechanically reconfigurable metasurfaces in which both lattice constants and local reflection phases of constitutional meta-atoms can be synchronously controlled based on the kirigami rotation transformation, thereby exhibiting extended tuning ranges and thus wave-control capabilities. In particular, such metasurfaces can exhibit continuously varied and even re-formed reflection-phase profiles along with the kirigami rotation transformation, serving as ideal platforms to achieve reconfigurable beam steering in pre-designed manners. Using this concept, we design and fabricate two kirigami metasurfaces, working as a beam flipper and as a beam splitter for microwaves, respectively, and experimentally characterize their wave-manipulation functionalities. Experimental results are in good agreement with full-wave simulations. The proposed idea is so general that it can be applied to realize reconfigurable metasurfaces with different materials/configurations or in high frequency regimes, for controlling electromagnetic waves and other classical waves (e.g., acoustic waves).

On-chip devices for generating pre-designed vectorial optical fields (VOFs) under surface wave (SW) excitations are highly desired in integrated photonics. However, conventional devices are usually of large footprints, low efficiencies, and limited wave-control capabilities. Here, we present a generic approach to design ultra-compact on-chip devices that can efficiently generate pre-designed VOFs under SW excitations, and experimentally verify the concept in terahertz (THz) regime. We first describe how to design SW-excitation metasurfaces for generating circularly polarized complex beams, and experimentally demonstrate two meta-devices to realize directional emission and focusing of THz waves with opposite circular polarizations, respectively. We then establish a systematic approach to construct an integrated device via merging two carefully designed metasurfaces, which, under SW excitations, can separately produce pre-designed far-field patterns with different circular polarizations and generate target VOF based on their interference. As a proof of concept, we demonstrate experimentally a meta-device that can generate a radially polarized Bessel beam under SW excitation at ~0.4 THz. Experimental results agree well with full-wave simulations, collectively verifying the performance of our device. Our study paves the road to realizing highly integrated on-chip functional THz devices, which may find many applications in biological sensing, communications, displays, image multiplexing, and beyond.

Generating structured electromagnetic (EM) beams in pre-designed manners is highly desired in photonics applications, but conventional devices suffer from the issues of bulky sizes and restricted functionalities. Although metasurfaces have exhibited powerful capabilities in controlling EM waves, they usually work under propagating-wave (PW) excitations, being unfavorable for EM integration. Here, a generic scheme is proposed to design metasurfaces that can efficiently generate far-field (FF) vectorial beams with longitudinally varying polarizations under surface-wave (SW) excitations. Such metasurfaces consist of two sub-sets of Pancharatnam-Berry (PB) meta-atoms with specifically designed orientation angles, each sub-set can generate a circularly polarized FF beam with a particular spin and their interference forms the target vectorial beam. After experimentally demonstrating two benchmark metadevices for generating FF beams with homogeneously distributed polarizations, a microwave metasurface is designed and fabricated and experimentally demonstrate that it can generate a Bessel beam with longitudinally varying polarization under SW excitation. These findings significantly expand the capabilities of metasurfaces in controlling EM waves, paving the way to realize many applications in on-chip photonics, such as chip-integrated displays, encrypted holography, and augmented reality technology.

Holography is a highly desired technology in modern photonics, yet setups for traditional generating methods suffer from complexity and bulky sizes. While metasurface-based holography exhibits advantages of compactness and easy-fabrication, most meta-holograms realized so far exhibit only single functionality, with a few multifunctional ones suffering from imbalances of efficiency and device-thickness. Here, we propose a generic approach to design ultra-thin metasurfaces for realization of multiple holographic images with high efficiencies, and experimentally verify the concept in the telecom regime. We first design a series of high-efficiency reflective meta-atoms exhibiting incident-spin-delinked reflection phases governed by geometric and resonant mechanisms, and experimentally characterize their optical properties at wavelengths around 1,064 nm. We next experimentally demonstrate a single-functional meta-hologram as a benchmark test. Finally, we employ the designed meta-atoms to construct a metasurface with the thickness ∼1/4λ, and experimentally demonstrate its capability of generating two distinct holographic images under illuminations of circularly polarized light beams with different helicities, possessing generation efficiencies ∼48.08 %. Our work provides a highly-efficient and ultra-compact platform to generate multifunctional holographic images, which may inspire numerous applications in integration optics.

All-dielectric optical nano-resonators have emerged as low-loss, versatile and highly adaptable components in nanophotonic structures for manipulating electromagnetic waves and enhancing light–matter interactions. However, achieving full three-dimensional characterization of near fields within dielectric nano-resonators poses great experimental challenges. Here we develop a technique to image near-field wave patterns inside dielectric optical nano-resonators using high-order sideband generation. By exploiting the phase sensitivity of various harmonic orders, which enables the detection of near-field distributions at distinct depths, we achieve three-dimensional tomographic and near-field imaging with a transverse resolution of ~920 nm and a longitudinal resolution of ~130 nm inside a micrometre-thick silicon anapole resonator. Our method offers high-contrast polarization sensitivity and phase-resolving capabilities, providing comprehensive vectorial near-field information and could be applied to diverse dielectric metamaterials.

Optical imaging has evolved from capturing light intensity to recording high-resolution, multi-dimensional images with various optical parameters, such as amplitude, phase, polarization, and wavelength. Optical multiparameter imagers are capable of providing detailed insights into objects and scenes by measuring multiple optical parameters. Traditional optical multiparameter imaging systems, such as imaging polarimeters and spectrometers, are bulky and limited in time resolution. Metasurfaces have emerged as a compact solution for multiparameter imaging by enabling the flexible manipulation of light fields. In this review, we highlight recent fundamental advances in optical metasurface multiparameter imaging, including imaging polarimeters, imaging spectrometers, and quantitative phase/depth imagers, as well as their applications in imaging technologies. We also discuss current trends and challenges of applications relying on these imaging technologies, and offer parting thoughts about promising ways to overcome them for the advancement of imaging technologies.

Developing switchable and multifunctional metasurfaces is essential for high-integration photonics. However, most previous studies encountered challenges such as limited degrees of freedom, simple tuning of predefined functionality, and complicated control systems. Here, we develop a general strategy to construct switchable and multifunctional metasurfaces. Two spin-modulated wave-controls are enabled by the proposed high-efficiency metasurface, which is designed using both resonant and geometric phases. Furthermore, the switchable wavefront tailoring can also be achieved by flexibly altering the lattice constant and reforming the phase retardation of the metasurfaces based on the “rotating square” (RS) kirigami technique. As a proof of concept, a kirigami metasurface is designed that successfully demonstrates dynamic controls of three-channel beam steering. In addition, another kirigami metasurface is built for realizing tri-channel complex wavefront engineering, including straight beam focusing, tilted beam focusing, and anomalous reflection. By altering the polarization of input waves as well as transformation states, the functionality of the metadevice can be switched flexibly among three different channels. Microwave experiments show good agreement with full-wave simulations, clearly demonstrating the performance of the metadevices. This strategy exhibits advantages such as flexible control, low cost, and multiple and switchable functionalities, providing a new pathway for achieving switchable wavefront engineering.

Plasmons in graphene are highly tunable, mainly by leveraging the carrier density and Drude scattering. Here, we introduce another scheme, i.e., accessing the retardation regime (originated from the finite speed of light), to engineer the lifetime and dispersion of plasmons in graphene in the terahertz regime. We find that the retardation regime can be approached by reducing plasmon momentum in artificially stacked multilayer graphene systems with large Drude weight, and this can significantly increase the plasmon lifetime. In addition, an explicit theoretical model along with finite-element simulation was given, consistent with the experimental findings. Our work opens another avenue to manipulate terahertz plasmons in graphene.

Metasurfaces have exhibited unprecedented capabilities to manipulate electromagnetic (EM) waves, but many of them suffer from frequency dispersions, which significantly degrade their performance. Although extensive efforts have been devoted to the design of achromatic metasurfaces at optical frequencies, their microwave counterparts are less studied, with a few attempts suffering from large thicknesses. Here, based on high-efficiency meta-atoms exhibiting tailored reflection phases of both resonant and geometric origins, we propose a generic scheme to design achromatic metasurfaces for controlling EM far-field wavefronts within microwave broadband. We follow the proposed strategy to design and fabricate three achromatic metadevices, and experimentally demonstrate that they can realize anomalous reflection, focusing, and Bessel beam generations, respectively, at every frequency within 8-10 GHz. Our work paves the road for realizing high-performance achromatic metadevices in low-frequency domains, which may find many applications such as radar detection, microwave imaging, and wireless communications.

Metasurfaces composed by arrays of coupled plasmonic resonators have attracted tremendous attention due to their extraordinary abilities to manipulate electromagnetic (EM) waves. However, existing theories for such systems are either empirical with model parameters obtained by fitting with simulations, or can only be applied to high-frequency systems where metals exhibit finite permittivity. Here, we extend our recently established leaky-eigenmode (LEM) theory to the microwave regime where metals exhibit infinite permittivity, with all parameters directly computable without fitting procedures. After validating our theory with both simulations and experiments on a benchmark metasurface, we illustrate how to utilize the theory to guide designing microwave metasurfaces with freely tailored line-shapes, including particularly the generation of a bound state in the continuum. All theoretical predictions are verified by experiments and simulations. Our study provides a powerful tool to guide designing functional microwave meta-devices for various applications.

Transparent conductors (TCs) exhibiting simultaneously high electric conductance and high optical transparency are strongly desired in optoelectronic applications. However, TCs realized so far are either based on low-conductivity materials or nanostructured high-conductivity noble-metal films, thus sacrificing electric conductivity, which is unfavorable for applications. Herein, a new configuration of TC, consisting of a continuous metal film sandwiched between two purposely designed metasurfaces is proposed, which can yield high transparency in a predesigned visible regime with high electric conductance preserved by the unstructured metal film. Based on coupled-mode theory (CMT) analysis, a generic phase diagram to guide the design of such trilayer structures is presented, yielding different performances in terms of peak transmittance and bandwidth. Using a transmitting electron-beam lithography technology, a series of free-standing TCs are fabricated, whose optical properties are then characterized experimentally. The best fabricated sample exhibits an optical transmittance of 55.5% at 691 nm, which is five times higher than that of a bare metal film, yet keeping the high electric conductance of a continuous metal film. The findings present a universal methodology for realizing TCs, which should find broad applications in displays, solar cells, and tunable plasmonic filters.

Spatio-spectral selectivity, the capability to select a single mode with a specific wavevector (angle) and wavelength, is imperative for light emission and imaging. Continuous band dispersion of a conventional periodic structure, however, sets up an intrinsic locking between wavevectors and wavelengths of photonic modes, making it difficult to single out just one mode. Here, we show that the radiation asymmetry of a photonic mode can be explored to tailor the transmission/reflection properties of a photonic structure, based on Fano interferences between the mode and the background. In particular, we find that a photonic system supporting a band dispersion with certain angle-dependent radiation-directionality can exhibit Fano-like perfect reflection at a single frequency and a single incident angle, thus overcoming the dispersion locking and enabling the desired spatio-spectral selectivity. We present a phase diagram to guide designing angle-controlled radiation-directionality and experimentally demonstrate double narrow Fano-like reflection in angular (±5°) and wavelength (14 nm) bandwidths, along with high-contrast spatio-spectral selective imaging, using a misaligned bilayer metagrating with tens-of-nanometer-scale thin spacer. Our scheme promises new opportunities in applications in directional thermal emission, nonlocal beam shaping, augmented reality, precision bilayer nanofabrication, and biological spectroscopy.

Higher-order topological insulators (HOTIs) can support boundary states at least two dimensions lower than the bulk, attracting intensive attention from both fundamental science and application sides. Lattice-based tight-binding models such as Benalcazar-Bernevig-Hughes model have driven significant advancements in realizing HOTIs across various physical systems. Here, beyond lattice model, we demonstrate that a cylinder with an arbitrary cross section, composed of a homogeneous electromagnetic medium featuring nontrivial second Chern numbers c_2=±1in a synthetic five-dimensional space, can exhibit topologically protected HOTI-type hinge states in three-dimensional laboratory space. Interestingly, this hinge state is essentially a chiral zero mode arising from the interaction between Weyl arc surface states, guaranteed by a nontrivial c_2, and an effective magnetic field induced by the curvature of the cylinder surface. Compared to conventional schemes to generate HOTIs, our approach is more robust, as it is an intrinsic topological phase and therefore does not rely on additional symmetry protections such as time-reversal, parity, or chiral symmetry. We experimentally realize such a cylinder using a photonic metamaterial and confirm the existence of hinge states via microwave near-field measurements. Our work introduces the concept of boundary gauge fields and establishes the link between synthetic-space c_2 and real-space HOTI states, thereby generalizing HOTIs to corner-less systems.

Holography plays a crucial role in optics, yet traditional methods require complex setups and bulky devices, being unfavourable for optical integration. Although metasurface-based holograms can be ultra-compact, holographic images generated by previously realized metadevices were mostly scalar ones, with a few vectorial holograms realized so far suffering from restrictions on efficiency, incident polarization, and resolution. We propose and experimentally demonstrate an efficient meta-platform to generate vectorial holographic images with high resolutions under arbitrary incident polarizations. Combining Gerchberg–Saxton algorithm and the wave-decomposition technique, we establish a generic strategy to retrieve the optical properties (e.g., reflection phases and polarization-conversion capabilities) of meta-atoms required to construct a metasurface for generating a predesigned vectorial holographic image under a predesigned incident polarization. We next design a series of high-efficiency and deep-subwavelength single-structure meta-atoms exhibiting tailored reflection phases and polarization-conversion capabilities governed by both structural resonances and the Pancharatnam–Berry effect, and experimentally characterize their optical scattering properties. We finally construct a series of ultra-thin metadevices with these meta-atoms and experimentally demonstrate that they can generate pre-designed vectorial holographic images under illuminations of circularly polarized light at 1064 nm. We provide a highly efficient and ultra-thin platform to generate predesigned vectorial holographic images under illuminations of light with arbitrary given polarization, which can inspire numerous future applications in on-chip photonics.

Robust edge states and corner states in photonic topological insulators provide effective ways to manipulate the propagation of electromagnetic waves. Bandgaps of the previously reported photonic topological insulators are independent of each other. In this paper, a higher-order valley photonic insulator composed of arrays of dendritic structure is designed. The band structure shows an overlapped bandgap is observed, and the overlapped bandgap divides the band structure into three bandgaps. The evolution of the overlapped bandgap is investigated by changing the geometric parameters and increasing the fractal to break the $$C_{3v}$$ symmetry drastically. Besides, it is notable that the band structure of the proposed valley photonic insulator is flat bands. It is demonstrated that undistorted transmission can be observed as a plane electromagnetic wave transmits through the proposed valley photonic insulator. The interface consisting of two valley photonic structures with distinct topological nontrivial phases is constructed. Edge states and corner states with strong energy localization are obtained in multi-band frequencies. Remarkably, two triangular structures with different Wannier center configurations both have corner states in the same bandgap, which does not obey the valley selectivity. The phenomenon is caused by the weak valley locking property due to the overlapped bandgap. The proposed valley photonic insulators are expected to benefit applications in optical devices such as topological lasers.

Delaying an electromagnetic (EM) wave pulse on a thin screen for a significant time before releasing it is highly desired in many applications, such as optical camouflage, information storage, and wave-matter interaction boosting. However, available approaches to achieve this goal either require thick and complex systems or suffer from low efficiencies and a short delay time. This paper proposes an ultra-thin meta-platform that can significantly delay an EM-wave pulse after reflection. Specifically, our meta-platform consists of three meta-surfaces integrated together, of which two are responsible for efficiently coupling incident EM-wave pulse into surface waves (SWs) and vice versa, and the third one supports SWs exhibiting significantly reduced group velocity. We employ theoretical model analyses, full-wave simulations, and microwave experiments to validate the proposed concept. Our experiments demonstrate a 13 ns delay of an EM pulse centered at 12.975 GHz, enabled by a lambda/8-thick and 38-lambda-long meta-device with an efficiency of 32% (or 70%) with (or without) material loss taken into account. A larger delay time can be enabled by devices with larger sizes considering that the SWs group velocity of our device can be further reduced via dispersion engineering. These findings establish a new road for delaying an EM-wave pulse with ultra-thin screens, which may lead to many promising applications in integration optics.

Propagating waves and surface waves are two distinct types of light-transporting modes, the free control of which are both highly desired in integration photonics. However, previously realized devices are bulky in sizes, inefficient, and/or can only achieve one type of light-manipulation functionality with a single device. Here, we propose a generic approach to design bi-channel meta-devices, constructed by carefully selected meta-atoms possessing reflection phases of both structural-resonance and geometric origins, which can exhibit two distinct light-manipulation functionalities in near-field (NF) and far-field (FF) channels, respectively. After characterizing the scattering properties of basic meta-atoms and briefly stating the theoretical strategy, we design/fabricate three different meta-devices and experimentally characterize their bi-channel wave-control functionalities in the telecom regime. Our experiments show that the first two devices can multiplex the generations of NF and FF optical vortices with different topological charges, while the third one exhibits anomalous surface plasmon polariton focusing in the NF and hologram formation in the FF simultaneously. Our results expand the wave-control functionalities of metasurfaces to all wave-transporting channels, which may inspire many exciting applications in integration optics.

Multiplexing multiple yet distinct functionalities in one single device is highly desired for modern integration optics, but conventional devices are usually of bulky sizes and/or low efficiencies. While recently proposed metasurfaces can be ultra-thin and highly efficient, functionalities multiplexed by metadevices so far are typically restricted to two, dictated by the number of independent polarization states of the incident light. Here, we propose a generic approach to design metadevices exhibiting wave-control functionalities far exceeding two, based on coherent wave interferences continuously tuned by varying the incident polarization. After designing a series of building-block metaatoms with optical properties experimentally characterized, we construct two metadevices based on the proposed strategy and experimentally demonstrate their polarization-tuned multifunctionalities at the wavelength of 1550 nm. Specifically, upon continuously modulating the incident polarization along different paths on the Poincare’s sphere, we show that the first device can generate two spatially non-overlapping vortex beams with strengths continuously tuned, while the second device can generate a vectorial vortex beam carrying continuously-tuned polarization distribution and/or orbital angular momentum. Our proposed strategy significantly expands the wave-control functionalities equipped with a single optical device, which may stimulate numerous applications in integration optics.

Owing to the chirality of Weyl nodes characterized by the first Chern number, a Weyl system supports one-way chiral zero modes under a magnetic field, which underlies the celebrated chiral anomaly. As a generalization of Weyl nodes from three-dimensional to five-dimensional physical systems, Yang monopoles are topological singularities carrying nonzero second-order Chern numbers $${c}_{2}=\pm 1$$. Here, we couple a Yang monopole with an external gauge field using an inhomogeneous Yang monopole metamaterial and experimentally demonstrate the existence of a gapless chiral zero mode, where the judiciously designed metallic helical structures and the corresponding effective antisymmetric bianisotropic terms provide the means for controlling gauge fields in a synthetic five-dimensional space. This zeroth mode is found to originate from the coupling between the second Chern singularity and a generalized 4-form gauge field—the wedge product of the magnetic field with itself. This generalization reveals intrinsic connections between physical systems of different dimensions, while a higher-dimensional system exhibits much richer supersymmetric structures in Landau level degeneracy due to the internal degrees of freedom. Our study offers the possibility of controlling electromagnetic waves by leveraging the concept of higher-order and higher-dimensional topological phenomena.

Plasmonic vortices have shown a wide range of applications in on-chip photonics due to their fascinating properties of the orbital angular momenta (OAM) and phase singularity. However, conventional devices to generate them suffer from issues of low efficiencies and limited functionalities. Here, we establish a systematic scheme to construct high-efficiency bifunctional metasurfaces that can generate two plasmonic vortices exhibiting distinct topological charges, based on a series of reflective meta-atoms exhibiting tailored reflection-phases dictated by both resonant and geometric origins. As a benchmark test, we first construct a meta-coupler with meta-atoms exhibiting geometric phases only, and experimentally demonstrate that it can generate a pre-designed plasmonic vortex at the wavelength of 1064 nm with an efficiency of 27% (56% in simulation). Next, we design/fabricate two bifunctional metasurfaces with meta-atoms integrated with both resonant and geometric phases, and experimentally demonstrate that they can generate divergent (or focused) or convergent (or defocused) plasmonic vortices with district OAM as shined by circularly polarized light with opposite helicity at 1064 nm wavelength. Our work provides an efficient platform to generate plasmonic vortices as desired, which can find many applications in on-chip photonics.

Freely tailoring the wavefronts of surface waves (SWs) plays a vital role in on-chip photonics applications, but diffraction-optics based devices usually exhibit limited tuning functionalities and low working efficiencies. Here, we propose a new strategy to manipulate the near-field wavefronts of SWs with Pancharatnam–Berry (PB) metasurfaces exhibiting predesigned phase profiles. As a SW beam flows across such a metasurface, waves scattered by different PB meta-atoms interfere with each other, forming a new SW wavefront under appropriate wavevector matching condition. As a proof of concept, we design and fabricate a series of PB metasurfaces working in the microwave regime and experimentally demonstrate that they can efficiently realize SW manipulations, including deflection, focusing, Bessel beam, and Airy beam generations. These findings may inspire the realization of highly miniaturized on-chip devices for integrated photonics applications.

Multifunctional terahertz (THz) devices in transmission mode are highly desired in integration-optics applications, but conventional devices are bulky in size and inefficient. While ultra-thin multifunctional THz devices are recently demonstrated based on reflective metasurfaces, their transmissive counterparts suffer from severe limitations in efficiency and functionality. Here, based on high aspect-ratio silicon micropillars exhibiting wide transmission-phase tuning ranges with high transmission-amplitudes, a set of dielectric metasurfaces is designed and fabricated to achieve efficient spin-multiplexed wavefront controls on THz waves. As a benchmark test, the photonic-spin-Hall-effect is experimentally demonstrated with a record high absolute efficiency of 92% using a dielectric metasurface encoded with geometric phases only. Next, spin-multiplexed controls on circularly polarized THz beams (e.g., anomalous refraction and focusing) are experimentally demonstrated with experimental efficiency reaching 88%, based on a dielectric meta-device encoded with both spin-independent resonant phases and spin-dependent geometric phases. Finally, high-efficiency spin-multiplexed dual holographic images are experimentally realized with the third meta-device encoded with both resonant and geometric phases. Both near-field and far-field measurements are performed to characterize these devices, yielding results in agreement with full-wave simulations. The study paves the way to realize multifunctional, high-performance, and ultra-compact THz devices for applications in biology sensing, communications, and so on.

Freely switching light transmission and absorption via an achromatic reflectionless screen is highly desired for many photonic applications (e.g., energy-harvesting, cloaking, etc.), but available meta-devices often exhibit reflections out of their narrow working bands. Here, we rigorously demonstrate that an optical metasurface formed by two resonator arrays coupled vertically can be perfectly reflectionless at all frequencies below the first diffraction mode, when the near-field (NF) and far-field (FF) couplings between two constitutional resonators satisfy certain conditions. Tuning intrinsic loss of the system can further modulate the ratio between light transmission and absorption, yet keeping reflection diminished strictly. Designing/fabricating a series of metasurfaces with different inter-resonator configurations, we experimentally illustrate how varying inter-resonator NF and FF couplings can drive the system to transit between different phase regions in a generic phase diagram. In particular, we experimentally demonstrate that a realistic metasurface satisfying the discovered criteria exhibits the desired achromatic reflectionless property within 160–220 THz (0–225 THz in simulation), yet behaving as a perfect absorber at ~ 203 THz. Our findings pave the road to realize meta-devices exhibiting designable transmission/absorption spectra immune from reflections, which may find many applications in practice.

Although the photonic spin-Hall effect (PSHE) at optical interfaces has been widely studied in past years, its physical origin remains obscure. Here, through studying the scatterings of circularly polarized beams obliquely incident on a series of junctions linking two homogenous optical media, how the physical origin of the PSHE evolves as the interface changes from a slowly varying junction to a step-like sharp one is explored. Beams transmitted through a generic interface consist of two modes, a spin-maintained normal mode carrying a spin-redirection Berry (SRB) phase and a spin-flipped abnormal mode exhibiting a Pancharatnam–Berry (PB) phase. Under linear-polarization incidence, a spin-polarized beam transmitted through each junction is generally an interference of normal and abnormal modes corresponding to two different incident circular polarizations, and thus the resulting PSHE is dictated by the interplay and competition between two effects dictated by SRB and PB phases, respectively. Shrinking the interfacial region can increase the strength of the abnormal mode, making the measured PSHE change from the SRB-dominated one to the PB-dominated one. The results establish a unified framework to understand the PSHE at generic interfaces, offering practical ways to control the PSHE by “designing” the abnormal scatterings on optical interfaces.

Designing perfect anomalous reflectors is crucial for achieving many metasurface-based applications, but available design approaches for the cases of extremely large bending angles either require unrealistic gain–loss materials or rely on brute-force optimizations lacking physical guidance. Here, we propose a deterministic approach to design passive metasurfaces that can reflect impinging light to arbitrary nonspecular directions with almost 100% efficiencies. With both incident and out-going far-field waves given, we can retrieve the surface-impedance profile of the target metadevice by matching boundary conditions with all allowed near-field modes added self-consistently and then construct the metadevices deterministically based on passive meta-atoms exhibiting local responses. We design/fabricate two proof-of-concept microwave metadevices and experimentally demonstrate that the first one achieves anomalous reflection to a 70° angle with efficiency ∼98%, and the second one can generate multiple reflected beams with desired bending angles and power allocations. Our findings pave the way for realizing high-efficiency wave-control metadevices with desired functionalities.

A device that can couple propagating light into surface plasmon polaritons (SPPs) focused into a small region is highly desired for on-chip photonics applications (e.g., energy-harvesting, sensing, etc.). However, current technologies suffer from large device footprint, low working efficiency, and insufficient light-manipulation freedom. Here, a generic approach for designing plasmonic lenses to generate predesigned vector SPP vortices with high efficiencies is established. Constructed with a set of meta-atoms exhibiting tailored reflection phases and polarization-conversion capabilities, the devices can convert normally incident circularly polarized light into predesigned vector SPP vortices with high efficiencies, due to both phase and polarization matching. As the illustrations, this study experimentally demonstrates directional SPP conversion (coupling efficiency: 35%; utilization efficiency: 98%) and SPP focusing effect at the wavelength of 1064 nm, with two meta-couplers in stripe and arc shapes, respectively. Finally, a ring-shaped meta-coupler is designed/fabricated, and the generation of a vector SPP vortex with significantly enhanced efficiency as compared to previous schemes is experimentally demonstrated. The results pave the way for realizing on-chip plasmonic devices to efficiently utilize SPPs with minimal footprints.

On-chip photonic systems play crucial roles in nanoscience and nanoapplications, but coupling external light to these subwavelength devices is challenging due to a large mode mismatch. Here, we establish a new scheme for realizing highly miniaturized couplers for efficiently exciting on-chip photonic devices in a controllable way. Relying on both resonant and Pancharatnam-Berry mechanisms, our meta-device can couple circularly polarized light to a surface plasmon, which is then focused into a spot placed with a target on-chip device. We experimentally demonstrate two metacouplers. The first can excite an on-chip waveguide (with a 0.1 & lambda; x 0.2 & lambda; cross section) with an absolute efficiency of 51%, while the second can achieve incident spin-selective excitation of a dual-waveguide system. Backgroundfree excitation of a gap-plasmon nanocavity with the local field enhanced by >1000 times is numerically demonstrated. Such a scheme connects efficiently propagating light in free space and localized fields in on-chip devices, being highly favored in many integration-optics applications.

Dynamically controlling terahertz (THz) waves with an ultracompact device is highly desired, but previously realized tunable devices are bulky in size and/or exhibit limited light-tuning functionalities. Here, we experimentally demonstrate dynamic modulation on THz waves with a dielectric metasurface in mode-selective or mode-unselective manners through pumping the system at different optical wavelengths. Quasi-normal-mode theory reveals that the physics is governed by the spatial overlap between wave functions of resonant modes and regions inside resonators perturbed by pump laser excitation at different wavelengths. We further design/fabricate a dielectric metasurface and experimentally demonstrate that it can dynamically control the polarization state of incident THz waves, dictated by the strength and wavelength of the pumping light. We finally numerically demonstrate pump wavelength-controlled optical information encryption based on a carefully designed dielectric metasurface. Our studies reveal that pump light wavelength can be a new external knob to dynamically control THz waves, which may inspire many tunable metadevices with diversified functionalities.

As topological charge constitutes an infinite-dimensional Hilbert space, vortex beam has numerous applications in optical communications and other fields where signal capacity is a vital requirement. Multifunctional vortex beams, showing up to different controllable responses subjected to separate combinations of polarization states, have significantly exhibited improved capacity of signal transport. Relying on prior physical knowledge, complex requirement brings tremendous challenge to the design of multifunctional vortex beams. Here, a deep-learning-based platform for designing metasurfaces is proposed, which can intelligently generate predesigned multifunctional vortex beams. Employing the proposed strategy, the demonstrations of bifunctional and trifunctional vortex beams are consistent with the design targets. Three samples are fabricated and measured by a Michelson interferometer. Clear observed interference patterns revealed the topological nature of the generated vortex beams, unambiguously justifying the design platform. This intelligent design strategy, which may inspire new ideas in other scientific fields, lays a solid foundation for the high-performance application of multifunctional vortex beams. This work fully exploits the potential of vortex beams for large-scale dense data communication and quantum optics with high quantum numbers, which may further promote the development of the integrated photonic chip.

Littrow diffraction, the ability to reflect light back along incident direction, is a key functionality of retroreflectors, exhibiting wide applications in nanophotonics. However, retroreflectors have hitherto low working efficiencies and narrow bandwidths, and work only for a specific polarization, being unfavorable for integration-optics applications. Here, we propose a type of metagrating consisting of an all-dielectric Bragg reflector and a periodic metasurface with freeform-shaped dielectric resonators, which enables broadband depolarized perfect Littrow diffraction at optical frequencies. The physics is governed by exact cancellations of specular reflections contributed by two Bragg modes in metagratings, enabled by careful structural optimization to yield the desired reflection-phase difference of Bragg modes within a wide frequency band and for two polarizations. As a proof of concept, we experimentally demonstrate retroreflections with unpolarized absolute efficiency higher than 98% (99% in design) at 1030–1090 nm using multilayer freeform metagratings. Our results pave the way for numerous applications based on high-efficiency Littrow diffraction (e.g., spectral laser beam combining), which is not bonded to a specific polarization or frequency.

Raman-fluorescence dual-mode enhanced nanoparticles have enormous potential for bioimaging with combined advantages of sensitivity and speed. This is primarily achieved through a trade-off between fluorescence quenching and electromagnetic (EM) enhancement on the plasmonic metal surface, as demonstrated in previous research. A strategy that can minimize EM-field attenuation and temporal photobleaching would be highly desirable. In this study, a novel approach using Raman-fluorescence enhanced dual-mode nanoparticles with the near-infrared fluorescence reporter IR780 directly embedded in the ultra-high EM fields between gold (Au) nanopetals of various morphology and a silver (Ag) coating without a spacer is presented. The results show these nanoparticles to be single-nanoparticle Raman sensitive and that they can generate a fluorescence enhancement factor as high as 1113 experimentally and 2000 by numerical simulation. The random morphology of the nanopetals supports broadband resonances for both fluorescence excitation and emission, resulting in nanowatt detectability, the dual-mode photostability of more than 30 min under continuous laser irradiation, and a long shelf life, making them promising for wide applications in bioimaging with ultra-brightness, low laser power, and long-duration monitoring. In summary, they represent a novel strategy for high-performance Raman-fluorescence enhancement dual-mode nanotags.

The interaction between light and cold atoms is a complex phenomenon potentially featuring many-body resonant dipole interactions. A major obstacle toward exploring these quantum resources of the system is macroscopic light propagation effects, which not only limit the available time for the microscopic correlations to locally build up, but also create a directional, superradiant emission background whose variations can overwhelm the microscopic effects. In this Letter, we demonstrate a method to perform “background-free” detection of the microscopic optical dynamics in a laser-cooled atomic ensemble. This is made possible by transiently suppressing the macroscopic optical propagation over a substantial time, before a recall of superradiance that imprints the effect of the accumulated microscopic dynamics onto an efficiently detectable outgoing field. We apply this technique to unveil and precisely characterize a density-dependent, microscopic dipolar dephasing effect that generally limits the lifetime of optical spin-wave order in ensemble-based atom-light interfaces.

Theoretical calculations on transition metal materials: from ab initio to tight-binding results

We investigate the possibility of preparing left-handed materials in metallic magnetic granular composites. Based on the effective medium approximation, we show that by incorporating metallic magnetic nanoparticles into an appropriate insulating matrix and controlling the directions of magnetization of metallic magnetic components and their volume fraction, it may be possible to prepare a composite medium which is left handed for electromagnetic waves propagating in some special direction and polarization in a frequency region near the ferromagnetic resonance frequency. This composite may be easier to make on an industrial scale. In addition, its physical properties may be easily tuned by rotating the magnetization locally. The anisotropic characteristics of this material is discussed. The exactly solvable example of the multilayer system is used to illustrate the results of the effective medium calculation.

We review our effort in understanding the rich electric, magnetic, and plasmonic properties of photonic metamaterials based on a planar fractal geometry. We employed both experiments and finite-difference time-domain simulations to study the electromagnetic properties of such systems and established effective medium models to characterize these complex structures. These fractal photonic metamaterials are shown to exhibit subwavelength and multiband functionalities with many interesting potential applications.

Despite the recent development and interest in the photonics of metallic wire structures, the relatively simple concepts and physics often remain obscured or poorly explained to those who do not specialize in the field. Electromagnetic Behaviour of Metallic Wire Structures provides a clear and coherent guide to understanding these phenomena without excessive numerical calculations. Including both background material and detailed derivations of the various different formulae applied, Electromagnetic Behaviour of Metallic Wire Structures describes how to extend basic circuit theory relating to voltages, currents, and resistances of metallic wire networks to include situations where the currents are no longer spatially uniform along the wire. This lays a foundation for a deeper understanding of the many new phenomena observed in meta-electromagnetic materials. Examples of applications are included to support this new approach making Electromagnetic Behaviour of Metallic Wire Structures a comprehensive and self-contained volume suitable for use by specialists, non-specialist, researchers and professionals in other relevant fields and even students.

In recent years, we have witnessed a rapid expansion of using super-thin metasurfaces to manipulate light or electromagnetic wave in a subwavelength scale. However, most designs are confined to a passive scheme and monofunctional operation, which hinders considerably the promising applications of the metasurfaces. Specifically, the tunable and multifunctional metasurfaces enable to facilitate switchable functionalities and multiple functionalities which are extremely essential and useful for integrated optics and microwaves, well alleviating aforementioned issues. In this book, we introduce our efforts in exploring the physics principles, design approaches, and numerical and experimental demonstrations on the fascinating functionalities realized. We start by introducing in Chapter 2 the "merging" scheme in constructing multi-functional metadevices, paying particular attention to its shortcomings issues. Having understood the merits and disadvantages of the "merging" scheme, we then introduce in Chapter 3 another approach to realize bifunctional metadevices under linearly polarized excitations, working in both reflection and transmission geometries or even in the full space. As a step further, we summarizes our efforts in Chapter 4 on making multifunctional devices under circularly polarized excitations, again including designing principles and devices fabrications/characterizations. Starting from Chapter 5, we turn to introduce our efforts on using the "active" scheme to construct multifunctional metadevices under linearly polarized wave operation. Chapter 6 further concentrates on how to employ the tunable strategy to achieve helicity/frequency controls of the circularly polarized waves in reflection geometry. We finally conclude this book in Chapter 7 by presenting our perspectives on future directions of metasurfaces and metadevices.