High performance bianisotropic metasurfaces: asymmetric transmission of light

Nanoscale optical nonreciprocity with nonlinear metasurfaces

Optical nonreciprocity is manifested as a difference in the transmission of light for the opposite directions of excitation. Nonreciprocal optics is traditionally realized with relatively bulky components such as optical isolators based on the Faraday rotation, hindering the miniaturization and integration of optical systems. Here we demonstrate free-space nonreciprocal transmission through a metasurface comprised of a two-dimensional array of nanoresonators made of silicon hybridized with vanadium dioxide (VO2). This effect arises from the magneto-electric coupling between Mie modes supported by the resonator. Nonreciprocal response of the nanoresonators occurs without the need for external bias; instead, reciprocity is broken by the incident light triggering the VO2 phase transition for only one direction of incidence. Nonreciprocal transmission is broadband covering over 100 nm in the telecommunication range in the vicinity of λ = 1.5 µm. Each nanoresonator unit cell occupies only ~0.1 λ3 in volume, with the metasurface thickness measuring about half-a-micron. Our self-biased nanoresonators exhibit nonreciprocity down to very low levels of intensity on the order of 150 W/cm2 or a µW per nanoresonator. We estimate picosecond-scale transmission fall times and sub-microsecond scale transmission rise. Our demonstration brings low-power, broadband and bias-free optical nonreciprocity to the nanoscale.

Few-layer bifunctional metasurfaces enabling asymmetric and symmetric polarization-plane rotation at the subwavelength scale

We introduce and numerically validate the concept of few-layer bifunctional metasurfaces comprising two arrays of quasiplanar subwavelength resonators and a middle grid (array of rectangular holes) that offer both symmetric and asymmetric transmissions connected, respectively, with symmetric and asymmetric polarization-plane rotation functionalities. The proposed structures are thinner than λ / 7 and free of diffractions. Usually, the structure's symmetry or asymmetry, i.e. unbroken or broken spatial inversion symmetries, are considered for metasurfaces as prerequisites of the capability of symmetric or asymmetric conversion of linearly polarized waves, respectively. Due to the achieved adjustment of the resonances enabling the rotation of the polarization plane simultaneously for both orthogonal polarizations of the incident wave, the symmetric polarization-plane rotation functionality can be obtained within one subwavelength band, whereas the asymmetric polarization-plane rotation functionality associated with the asymmetric transmission is obtained within another subwavelength band. This combination of the functionalities in one subdiffraction structure is possible due to the optimal choice of the grid parameters, since they may strongly affect the coupling between the two resonator arrays. Although normal incidence is required for the targeted bifunctionality, the variations of the incidence angle can also be exploited for the enrichment of the overall functional capability. Variations of the polarization angle give another important degree of freedom. The connection between the polarization-angle dependence of cross-polarized transmission and capability of symmetric and asymmetric polarization-plane rotation functionalities is highlighted. The feasible designs of the bifunctional metasurfaces are discussed.

Creating pairs of exceptional points for arbitrary polarization control: asymmetric vectorial wavefront modulation

Exceptional points (EPs) can achieve intriguing asymmetric control in non-Hermitian systems due to the degeneracy of eigenstates. Here, we present a general method that extends this specific asymmetric response of EP photonic systems to address any arbitrary fully-polarized light. By rotating the meta-structures at EP, Pancharatnam-Berry (PB) phase can be exclusively encoded on one of the circular polarization-conversion channels. To address any arbitrary wavefront, we superpose the optical signals originating from two orthogonally polarized -yet degenerate- EP eigenmodes. The construction of such orthogonal EP eigenstates pairs is achieved by applying mirror-symmetry to the nanostructure geometry flipping thereby the EP eigenmode handedness from left to right circular polarization. Non-Hermitian reflective PB metasurfaces designed using such EP superposition enable arbitrary, yet unidirectional, vectorial wavefront shaping devices. Our results open new avenues for topological wave control and illustrate the capabilities of topological photonics to distinctively operate on arbitrary polarization-state with enhanced performances.

Light People: Professor Cheng Zhang

EDITORIAL

Nanophotonics has emerged as a cutting-edge interdisciplinary research field today. Its primary objective is to leverage the interaction between light and matter at the wavelength and sub-wavelength scales, with the purpose of designing and manufacturing miniaturized, multifunctional, and high-performance optical devices and systems. Professor Cheng Zhang from Huazhong University of Science and Technology has dedicated his career to nanophotonic device research. His work encompasses a wide range of areas, including plasmonic devices, optical metamaterials, and metasurfaces. Through the design of innovative artificial electromagnetic structures and the exploration of emerging nanofabrication techniques, Professor Cheng Zhang has effectively achieved versatile control over various properties of electromagnetic waves, including amplitude, phase, and polarization states. Furthermore, his research extends to the continuous exploration of novel optical phenomena, aimed at realizing high-performance engineering applications. In this edition of Light People, we will take you deep into the world of Professor Cheng Zhang, a young scientist exemplifying the spirit of innovation, relentless improvement, and unwavering pursuit of excellence. You will discover how he has overcome numerous challenges in the realm of nanophotonic research.

Multi-functional device: manipulating linear and circular-polarization conversion in a terahertz chiral metamaterial

We propose a terahertz chiral metamaterial as a multi-functional device to manipulate asymmetry transmission of linear polarized waves, linear-to-elliptical polarization conversion and circular dichroism in transmission mode while asymmetry reflection of circular polarized waves. For incidence of linear polarized waves, dual-band asymmetry transmission is shown around 0.42 THz and 1.04 THz where asymmetry transmission factors reach up to two peak values: ∼0.51 and ∼0.55, respectively. Intense linear-to-elliptical polarization conversion occurs at 0.81 THz and 0.97 THz, respectively. For incidence of circular polarized waves, a strong circular dichroism appears at 0.36 THz where circular dichroism parameter reaches to ∼0.64 and asymmetry reflection is displayed around 0.36 THz with the maximum of asymmetry reflection factors approaching to 0.55.

High-performance circular polarization modulation using a dielectric metasurface

We demonstrate a chiral metasurface that exhibits a giant chiroptical response as well as functions as an optical diode due to geometrical asymmetry for circularly polarized light (CPL). Engineering the Mie-type multipole radiation using geometrical features led to performance values in terms of near-unity transmission and circular dichroism (CD) efficiency (about 0.96) and an extinction ratio of  ∼3.8×104 for 1550 nm wavelength. A continuous stopband of 1538-1556 nm is achieved for an unchosen component of CPL while keeping the transmission efficiency of the chosen CPL component larger than 0.9. Because of the high extinction ratio and CD efficiency, the proposed metasurface has the potential for chiroptical applications including high-contrast polarization imaging, precise Stokes parameters measurement, optical diodes, and polarization detection for CPL.

Switchable Plasmonic Chirality for Light Modulation: From Near-Field to Far-Field Coupling

This paper describes a quasi-planar chiral metamaterial of metal-insulator-metal (MIM) tetramer arrays that support multiplasmon modes from a hybridization scheme to achieve significant chiroptical responses with the largest circular dichroism (CD) value of 42%. The chiroptical responses can be actively switched on and off by tuning the field coupling regime from near field to far field through the insulator (or spacer) thickness. Numerical calculations demonstrate that near-field coupling of the hybridized plasmons on the stacked metallic tetramers governs the chiroptical responses at small insulator thickness (t_SiO2 < 160 nm). In contrast, far-field coupling of the plasmon radiations dominates at large spacing (t_SiO2 > 160 nm) as phase retardation plays a crucial role. The quasi-planar chiral metamaterial with tunable plasmonic chirality enables efficient light modulation for polarization conversion: from circular to elliptical/linear polarization.

Optically tunable bianisotropy in a sphere made from an epsilon-near-zero material

Bianisotropic media can be used to engineer absorbance, scattering, polarization, and dispersion of electromagnetic waves. However, the demonstration of a tunable light-induced bianisotropy at optical frequencies is still lacking. Here, we propose an experimentally feasible concept for a light-induced tunable bianisotropic response in a homogeneous sphere made of an epsilon-near-zero (ENZ) material. By exploiting the large linear absorption and the large possible intensity-dependent changes in the permittivity of ENZ materials, the direction-dependent scattering and absorption cross sections could be obtained. Our findings pave the way for further studies and applications in the optical regime requiring full dynamic control of the bianisotropic behavior.

Asymmetric reflection based on asymmetric coupling in single-layer extrinsic chiral metasurfaces

We propose and experimentally demonstrate that giant asymmetric reflection of circularly polarized light based on asymmetric coupling can be achieved in single-layer extrinsic chiral metasurfaces at oblique incidence. The asymmetric coupling and asymmetric reflection in the extrinsic chiral metasurfaces are caused by extrinsic chirality, allowing them to have extremely high values. An asymmetric reflection of approximately 40% is measured. Furthermore, the asymmetric reflection of extrinsic chiral metasurfaces is demonstrated not only in intensity but also in phase retardation, which induces asymmetric polarization state conversion. An approximately 14° asymmetric reflected polarization offset from the symmetry axis is achieved. Our research provides an effective new method for constructing huge asymmetric coupled systems to manipulate electromagnetic waves.

Phase-to-pattern inverse design for a fast realization of a functional metasurface by combining a deep neural network and a genetic algorithm

Metasurface provides an unprecedented means to manipulate electromagnetic waves within a two-dimensional planar structure. Traditionally, the design of meta-atom follows the pattern-to-phase paradigm, which requires a time-consuming brute-forcing process. In this work, we present a fast inverse meta-atom design method for the phase-to-pattern mapping by combining the deep neural network (DNN) and genetic algorithm (GA). The trained classification DNN with an accuracy of 92% controls the population generated by the GA within an arbitrary preset small phase range, which could greatly enhance the optimization efficiency with less iterations and a higher accuracy. As proof-of-concept demonstrations, two reflective functional metasurfaces including an orbital angular momentum generator and a metalens have been numerically investigated. The simulated results agree very well with the design goals. In addition, the metalens is also experimentally validated. The proposed method could pave a new avenue for the fast design of the meta-atoms and functional meta-devices.

Theoretical Design of a Bionic Spatial 3D-Arrayed Multifocal Metalens

With the development of micro/nano-optics, metasurfaces are gaining increasing attention working as novel electromagnetic wave control devices. Among which, metalenses have been developed and applied as a typical application of metasurfaces owing to their unique optical properties. However, most of those previous metalenses can only produce one focal point, which severely limits their applications. Inspired by the fly compound eye, we propose a special kind of spatial multifocal metalens. Our metalenses can reverse the polarization state of the incident circularly polarized light, which is then focused. In addition, a horizontally aligned multifocal metalens can be achieved by designing reasonable phase and region distributions, which is similar to a vertically aligned one. Most significantly, a spatially 3D-arrayed multifocal metalens with low crosstalk is well achieved by combining these two distribution methods. The proposed bionic 3D-arrayed multifocal metalens with amazing focusing effect promises applications in imaging, nanoparticle manipulation, optical communication, and other fields.

Efficient generation of a dual-polarized vortex wave with an ultrathin Huygens' metasurface

In this paper, an ultrathin Huygens' metasurface is designed for generating an orbital angular momentum (OAM) beam. The Huygens' metasurface is a double-layered metallic structure on a single-layer PCB. Based on induced magnetism, the Huygens' metasurface achieves the abilities of available near-complete transmission phase shift around 28 GHz. According to the principle of vortex wave generation, a Huygens' metasurface is designed, implemented and measured. The simulated and measured results show that the dual-polarized OAM transmitted waves with the mode l = 1 can be efficiently generated on a double-layered Huygens' metasurface around 28 GHz. The measured peak gain is 23.4 dBi at 28 GHz, and the divergence angle is 3.5°. Compared with conventional configurations of OAM transmitted beam generation, this configuration has the advantages of high gain, narrow divergence angle, and low assembly cost. This investigation will provide a new perspective for engineering application of OAM beams.

Spinning thermal radiation from twisted two different anisotropic materials

Thermal radiation has applications in numerous fields, such as radiation cooling, thermal imaging, and thermal camouflage. Micro/nanostructures such as chiral metamaterials with polarization-dependent or symmetry-breaking properties can selectively emit circularly (spin) polarized polarization waves. In this paper, we propose and demonstrate the spinning thermal radiation from two twisted different anisotropic materials. Taking industrial polymer and biaxial hyperbolic material α-MoO₃ as an example, it is found that broadband spinning thermal radiation can be obtained from 13 µm to 18 µm. The spin thermal radiation of the proposed twisted structure originates from the combined effect of polarization conversion of circularly polarized wave and selective absorption of linearly polarized wave by the top and bottom layers of anisotropic materials, respectively. Besides, the narrowband spinning thermal radiation with 0.9 circular dichroism is achieved at wavelength of 12.39 µm and 18.93 µm for finite thickness α-MoO₃ due to the epsilon-near-zero mode, and the magnetic field distribution can confirm the phenomenon. This work achieves broadband and narrowband spin thermal radiation and significantly enhances circular dichroism, which may have applications in biological sensing and thermal detection.

Single-Layer Achiral Metasurface with Independent Amplitude-Phase Control for Both Left-Handed and Right-Handed Circular Polarizations

Amplitude-phase control for circular polarized (CP) waves is experiencing a research upsurge in electromagnetics owing to the kaleidoscopic electromagnetic responses and promising application prospects of circular polarizations, and chiral metasurfaces are more facile to achieve a series of intriguing chiral phenomena than natural materials. However, it is difficult for most existing chiral metasurfaces to independently tailor the amplitude and phase of left-handed circular polarized and right-handed circular polarized waves at the same frequency as they suffer the drawbacks of large thickness, multiple layers, and complex structure. Herein, an innovative strategy of single-layer achiral metasurfaces of thickness 0.13λ0 is proposed to independently and simultaneously manipulate the amplitude and phase of orthogonal CP waves. As a proof of concept, an amplitude and phase controlled dual-channel meta-hologram is designed to reconstruct diverse images with high fidelity under orthogonal CP illumination, and the simulated and experimental results collectively validate the availability of our methodology. Significantly, the meta-hologram is also applicable to full polarization states according to the decomposition of electromagnetic waves. The inspiring design of single-layer achiral metasurfaces provides a simple and effective approach to explore chiral effects, and they possess enormous application potential in multitudinous microwave devices.

Asymmetric Generation of Acoustic Vortex Using Dual-Layer Metasurfaces

In this Letter, we introduce a new paradigm for achieving robust asymmetric generation of acoustic vortex field through dual-layer metasurfaces by controlling their intrinsic topologic charges and the parity of geometry design. The underlying physics is contributed to the one-way process of orbital angular momentum (OAM) transition ensured by the broken spatial symmetry and the external topologic charge from the vortex diffraction. We further experimentally demonstrate this novel phenomenon. Our findings could provide new routes to manipulate the asymmetric response of vortex fields, including one-way excitation and propagation, and promise potential applications in passive OAM-based diodes.

Giant 2D-chiroptical response in an achiral metasurface integrated with black phosphorus

In this work, we proposed a black phosphorus (BP) achiral metasurface and theoretically study the chiroptical response arising from extrinsic 2D-chirality in the mid-infrared regime. The achiral metasurface is composed of a monolayer BP sheet sandwiched by a silver ring array and dielectric spacer stacking on a silver substrate. The giant circular conversion dichroism (CCD) of the achiral metasurface is allowed at oblique incidence for the cooperative interaction of BP anisotropic surface plasmon modes and localized surface plasmons in metal rings, and the integrated BP can dynamically modulate the chiroptical response by controlling the doping concentration of BP. Furthermore, we found that a multiband phenomenon for CCD response occurs when tuning the thickness of the spacer. The proposed hybrid achiral metasurface provides more flexible opportunities to realize active polarization modulator, biosensor and chiral detection.

On-chip asymmetric beam-steering for broadband visible light

Artificial optical nanostructures including three-dimensional (3D) metamaterials and two-dimensional (2D) metasurfaces have shown overwhelming capability to control electromagnetic waves in desirable manners. However, the challenges of manufacturing a complex 3D bulk architecture or achieving nanoscale alignment between multilayers limit their practical applications, and they are unable to be used in on-chip integrated photonic devices. Therefore, the emerging dimensionality-reduction to on-chip metadevices would be of promising research value. Here, we propose a visible-frequency on-chip dual-layer design by cascading one-dimensional (1D) plasmonic metawires with metagratings, which can effectively manipulate surface plasmon polariton (SPP) wavefronts and exhibit on-chip asymmetric beam-steering functionality. Our 1D metawires consist of trapezoidal plasmonic nanoantennas and can enable broadband (460-700 nm) on-chip beam-deflection with a high conversion efficiency. The cascading plasmonic coupling between metawires/metagrating is further demonstrated with broadband asymmetric propagation performance, which is crucial for on-chip plasmonic device development. Finally, we study and theoretically verify a cascade system that integrates a dual-functional (convergent/divergent) lens for the forward/backward propagation, respectively. Compared with conventional free-space multilayer metasurfaces, on-chip 1D metawires enjoy single-time lithography processing and no alignment requirement for implementation in multifunctional devices. We believe that the proof-of-concept on-chip metawires study will pave a new, to the best of our knowledge, way for creating multifunctional photonic integrated devices and hold tremendous potential in realizing on-chip transformation optics, information processing, spectrometers, as well as optical sensors.

Tunable circular dichroism through absorption in coupled optical modes of twisted triskelia nanostructures

We present a system consisting of two stacked chiral plasmonic nanoelements, so-called triskelia, that exhibits a high degree of circular dichroism. The optical modes arising from the interactions between the two elements are the main responsible for the dichroic signal. Their excitation in the absorption cross section is favored when the circular polarization of the light is opposite to the helicity of the system, so that an intense near-field distribution with 3D character is excited between the two triskelia, which in turn causes the dichroic response. Therefore, the stacking, in itself, provides a simple way to tune both the value of the circular dichroism, up to 60%, and its spectral distribution in the visible and near infrared range. We show how these interaction-driven modes can be controlled by finely tuning the distance and the relative twist angle between the triskelia, yielding maximum values of the dichroism at 20° and 100° for left- and right-handed circularly polarized light, respectively. Despite the three-fold symmetry of the elements, these two situations are not completely equivalent since the interplay between the handedness of the stack and the chirality of each single element breaks the symmetry between clockwise and anticlockwise rotation angles around 0°. This reveals the occurrence of clear helicity-dependent resonances. The proposed structure can be thus finely tuned to tailor the dichroic signal for applications at will, such as highly efficient helicity-sensitive surface spectroscopies or single-photon polarization detectors, among others.

An Extended k-Surface Framework for Electromagnetic Fields in Artificial Media

The complete understanding of the electromagnetic field characteristics in artificially created bulk or thin media is essential to the efficient harnessing of the multitude of linear and nonlinear effects resulting from it. Due to the fact that recently developed artificial metastructures exhibit controllable electric and magnetic properties that are completely different from natural ones, the spectrum of behavior resulting from subjecting such media to electromagnetic fields has to be revisited. In this paper, we introduce a k-surface framework that offers complete information on the dispersion properties of media with designer electric and magnetic responses with positive and negative values, as well as for the coupling between the two. The extension from the classic k-surface case resides in the consideration of magnetic and bianisotropic materials with positive and negative permittivity and permeability values, as well as the introduction of the chirality coefficient.To illustrate the applicability of our framework, we have investigated the conditions to obtain collinear second harmonic generation in the case of artificial media with positively and negatively valued electric and magnetic responses. As expected, the phase matching tuning curves, defined as the intersections between the k-surfaces at both frequencies, are significantly modified with respect to the classic ones.

Circular dichroism assisted metadevice for efficient transmission and broadband absorption

Owing to the intriguing capability of manipulating electromagnetic (EM) properties, the metasurface has aroused great attention of researchers and promoted its applications in EM invisibility. However, there are strong demands to provide an efficient transparent window for signals transmitting in EM invisibility devices. Here, we propose a scheme of a circular dichroism assisted metadevice to provide efficient transmission and broadband absorption in microwave frequencies. By employing chiral meta-atoms to introduce a strong asymmetric response for circularly polarized waves, a chiral metadevice for spin-selective absorption with an efficient transmission is presented. Then, we couple four chiral atoms into a polarization-insensitive atom pair, thus the achiral metadevice presents an identical high-efficiency absorption for both the x- and y-polarized wave. Here, both the chiral and achiral metadevices are realized by loading the metasurface-based absorber on a bandpass frequency selective surface. A proof-of-prototype is fabricated to verify the achiral design. The simulated and experimental results have demonstrated wideband, high-efficiency, polarization-insensitive absorption and high in-band transmission. Interestingly, the proposed paradigm can not only provide the potential for chirality-enhanced absorber design but also may trigger applications in spin-dependent systems, stealth antenna systems, and EM camouflage devices.

Spherical Aberration-Corrected Metalens for Polarization Multiplexed Imaging

We present a terahertz spherical aberration-corrected metalens that uses the dynamic phase to achieve polarization multiplexed imaging. The designed metalens has polarization-dependent imaging efficiencies and polarization extinction ratios that exceed 50% and 10:1, respectively. Furthermore, opposite gradient phases can be applied to orthogonal polarizations to shift the imaging of the two polarized sources in the longitudinal and transverse directions. Indeed, we find that the metalens has a smaller depth-of-focus than a traditional metalens when imaging point sources with limited objective lengths. These results provide a new approach for achieving multifunctional beam steering, tomographic imaging and chiroptical detection.

Mutual circular polarization conversions in asymmetric transmission and reflection modes by three-layer metasurface with gold split-rings

Plasmonic metasurfaces can be used to replace traditional polarization devices for various integrated optical devices because of their novel polarization control ability on a subwavelength scale. In particular, the asymmetric transmission of circularly polarized light by anisotropic metasurface has attracted much attention recently. Here, a simple and effective circular polarization converter composed of three layers of rotated gold split-rings and a Si₃N₄ substrate is proposed, which is appropriate for both transmission and reflection modes. For transmission mode, it can convert left- and right- handed circularly polarized light into orthogonally polarized light in two adjacent bands with conversion efficiencies of 65% and 75%, respectively. For the reflection mode, the mutual conversion efficiencies can be up to 58% and 64%, respectively. Meanwhile, the structure has moderate asymmetric transmission and reflection efficiencies. The operating band of the metasurface can be adjusted continuously and linearly by changing the refractive index of the substrate. The dual-band asymmetric effects may contribute to information encoding and decoding for communication applications. As an ultra-thin planar optical element, the proposed metasurface can be used in integrated photonics, optical sensing, and other fields.

Active circular dichroism coding meta-mirror for flexible beam-forming and dynamic amplitude control

Equipped with the capability of simultaneous phase and amplitude modulation, the chiral metasurfaces have broken through the weak chiroptical responses of natural media, giving birth to a number of unprecedented phenomena. However, the performance of passive metasurface is inadequate to realize dynamic manipulation to fit the diverse and changeable operation requirements, which would damage their engineering applications. Here, a circular dichroism meta-mirror consisting of Archimedean spiral-based meta-atoms is proposed to achieve dynamic scattering modulation. Combining the strategy of loading active element, the chiroptical responses of a metasurface is smoothly controlled. By controlling bias voltages, continuous scattered reflection amplitude steering can be achieved for the designated spin state while the orthogonal spin state can hardly be influenced. With the assistance of Pancharatnam-Berry phase principle, the metasurface possesses the capability of tailoring waves in coded manner. We experimentally verify the proposed strategy by a 1-bit checkerboard meta-mirror for four-beam scattering pattern. Importantly, the proposed paradigm may find applications in spin-sensitive systems and adaptive camouflage.

Fundamentals of Lossless, Reciprocal Bianisotropic Metasurface Design

Lossless, reciprocal bianisotropic metasurfaces have the ability to control the amplitude, phase, and polarization of electromagnetic wavefronts. However, producing the responses that are necessary for achieving this control with physically realizable surfaces is a challenging task. Here, several design approaches for bianisotropic metasurfaces are reviewed that produce physically realizable metasurfaces using cascaded impedance sheets. In practice, three or four impedance sheets are often used to realize bianisotropic responses, which can result in narrowband designs that require the unit cells to be optimized in order to improve the performance of the metasurface. The notion of a metasurface quality factor is introduced for three-sheet metasurfaces to address these issues in a systematic manner. It is shown that the quality factor can be used to predict the bandwidth of a homogeneous metasurface, and it can also be used to locate problematic unit cells when designing inhomogeneous metasurfaces. Several design examples are provided to demonstrate the utility of the quality factor, including an impedance matching layer with maximal bandwidth and a gradient metasurface for plane wave refraction. In addition to these examples, several metasurfaces for polarization control are also reported, including an isotropic polarization rotator and an asymmetric circular polarizer.

Finite element modeling of electromagnetic properties in photonic bianisotropic structures

Given a constitutive relation of the bianisotropic medium, it is not trivial to study how light interacts with the photonic bianisotropic structure due to the limited available means of studying electromagnetic properties in bianisotropic media. In this paper, we study the electromagnetic properties of photonic bianisotropic structures using the finite element method. We prove that the vector wave equation with the presence of bianisotropic is self-adjoint under scalar inner product. we propose a balanced formulation of weak form in the practical implementation, which outperforms the standard formulation in finite element modeling. Furthermore, we benchmark our numerical results obtained from finite element simulation in three different scenarios. These are bianisotropy-dependent reflection and transmission of plane waves incident onto a bianisotropic slab, band structure of bianisotropic photonic crystals with valley-dependent phenomena, and the modal properties of bianisotropic ring resonators. The first two simulated results obtained from our modified weak form yield excellent agreements either with theoretical predictions or available data from the literature, and the modal properties in the last example, i.e., bianisotropic ring resonators as a polarization-dependent optical insulator, are also consistent with the theoretical analyses.

Fundamentals and applications of spin-decoupled Pancharatnam-Berry metasurfaces

Manipulating circularly polarized (CP) electromagnetic (EM) waves at will is significantly important for a wide range of applications ranging from chiral-molecule manipulations to optical communication. However, conventional EM devices based on natural materials suffer from limited functionalities, bulky configurations, and low efficiencies. Recently, Pancharatnam-Berry (PB) phase metasurfaces have shown excellent capabilities in controlling CP waves in different frequency domains, thereby allowing for multi-functional PB meta-devices that integrate distinct functionalities into single and flat devices. Nevertheless, the PB phase has intrinsically opposite signs for two spins, resulting in locked and mirrored functionalities for right CP and left CP beams. Here we review the fundamentals and applications of spin-decoupled metasurfaces that release the spin-locked limitation of PB metasurfaces by combining the orientation-dependent PB phase and the dimension-dependent propagation phase. This provides a general and practical guideline toward realizing spin-decoupled functionalities with a single metasurface for orthogonal circular polarizations. Finally, we conclude this review with a short conclusion and personal outlook on the future directions of this rapidly growing research area, hoping to stimulate new research outputs that can be useful in future applications.

Chiral Quasi-Bound States in the Continuum

Quasi-bound states in the continuum (QBICs) are Fano resonant states with long optical lifetimes controlled by symmetry-breaking perturbations. While conventional Fano responses are limited to linear polarizations and do not support tailored phase control, here we introduce QBICs born of chiral perturbations that encode arbitrary elliptical polarization states and enable geometric phase engineering. We thereby design metasurfaces with ultrasharp spectral features that shape the impinging wave front with near-unity efficiency. Our findings extend Fano resonances beyond their conventional limits, opening opportunities for nanophotonics, classical and quantum optics, and acoustics.

Arbitrary polarization conversion dichroism metasurfaces for all-in-one full Poincaré sphere polarizers

The control of polarization, an essential property of light, is of broad scientific and technological interest. Polarizers are indispensable optical elements for direct polarization generation. However, arbitrary polarization generation, except that of common linear and circular polarization, relies heavily on bulky optical components such as cascading linear polarizers and waveplates. Here, we present an effective strategy for designing all-in-one full Poincaré sphere polarizers based on perfect arbitrary polarization conversion dichroism and implement it in a monolayer all-dielectric metasurface. This strategy allows preferential transmission and conversion of one polarization state located at an arbitrary position on the Poincaré sphere to its handedness-flipped state while completely blocking its orthogonal state. In contrast to previous methods that were limited to only linear or circular polarization, our method manifests perfect dichroism of nearly 100% in theory and greater than 90% experimentally for arbitrary polarization states. By leveraging this attractive dichroism, our demonstration of the generation of polarization beams located at an arbitrary position on a Poincaré sphere directly from unpolarized light can substantially extend the scope of meta-optics and dramatically promote state-of-the-art nanophotonic devices.

Bandwidth-unlimited polarization-maintaining metasurfaces

Any arbitrary state of polarization of light beam can be decomposed into a linear superposition of two orthogonal oscillations, each of which has a specific amplitude of the electric field. The dispersive nature of diffractive and refractive optical components generally affects these amplitude responses over a small wavelength range, tumbling the light polarization properties. Although recent works suggest the realization of broadband nanophotonic interfaces that can mitigate frequency dispersion, their usage for arbitrary polarization control remains elusively chromatic. Here, we present a general method to address broadband full-polarization properties of diffracted fields using an original superposition of circular polarization beams transmitted through metasurfaces. The polarization-maintaining metasurfaces are applied for complex broadband wavefront shaping, including beam deflectors and white-light holograms. Eliminating chromatic dispersion and dispersive polarization response of conventional diffractive elements lead to broadband polarization-maintaining devices of interest for applications in polarization imaging, broadband-polarimetry, augmented/virtual reality imaging, full color display, etc.

Compact Double-Layer FR4-Based Focusing Lens Using High-Efficiency Huygens' Metasurface Unit Cells

High transmission efficiency metasurface unit cells have been designed based on surface electric and magnetic impedances derived from Huygens' principle. However, unit cells for low transmission loss (<1 dB) over a wide transmission phase range require at least three metallic layers, which complicates the unit cell design process. In this paper, we introduce high-efficiency Huygens' metasurface unit cell topologies in double-layer FR4 printed circuit board (PCB) by implementing surface electric and magnetic current using the top and bottom metallic patterns and via drills. Eleven unit cells were optimized for wide phase coverage (-150° to 150°) with a low average transmission loss of -0.82 dB at 10 GHz. To demonstrate the high-efficiency of the designed unit cells, we designed and fabricated two focusing lenses with dimensions of near 150 × 150 mm (5λ × 5λ) to focus a spherical beam radiated from short focal distances (f = 100 and 60 mm). The fabricated focusing lens showed 12.87 and 13.58 dB focusing gain for f = 100 and 60 mm at 10 GHz, respectively, with a 1 dB fractional gain bandwidth of near 10%. We expect that the proposed focusing lens based on high-efficiency double-layer metasurface unit cells can help realize compact and high-gain focusing lens-integrated antenna systems.

Narrow-band asymmetric transmission based on the dark mode of Fano resonance on symmetric trimeric metasurfaces

Asymmetric transmission (AT) is useful for polarization manipulation. We report narrowband AT that utilizes a triple-layered symmetric trimeric metasurface with near-field coupling of the dark mode of the Fano resonance. The coupling strength of the dark mode was tuned by using a mid-layer to break the dim AT between two slit layers. The peak transmission of linearly polarized waves and percentage bandwidth reached 0.7719 and 1.26% (numerical simulations) and 0.49 and 1.9% (experiments), respectively. Coupled-mode theory and field patterns are utilized to explain the underlying physical mechanisms of the mid-layer assisted field coupling. These results are useful for Fano-resonance-based devices.

Asymmetric transmission of linearly polarized waves based on Mie resonance in all-dielectric terahertz metamaterials

Interest in asymmetric transmission (AT) at terahertz frequencies has increased dramatically in recent years. We present an all-silicon metamaterial to achieve the AT effect for linearly polarized electromagnetic waves in the terahertz regime. The metamaterial is constructed by rectangular silicon pillars and a thick silicon substrate. The magnetic Mie resonance excited by the incident polarized terahertz wave contributes to the AT effect, which is verified by the field distributions. In addition, the rotation angle and dimensions of the silicon pillars are shown to have a great influence on the AT efficiency. The proposed metamaterial with straightforward design has promising applications in polarization control scenarios.

Characterisation and Manipulation of Polarisation Response in Plasmonic and Magneto-Plasmonic Nanostructures and Metamaterials

Optical properties of metal nanostructures, governed by the so-called localised surface plasmon resonance (LSPR) effects, have invoked intensive investigations in recent times owing to their fundamental nature and potential applications. LSPR scattering from metal nanostructures is expected to show the symmetry of the oscillation mode and the particle shape. Therefore, information on the polarisation properties of the LSPR scattering is crucial for identifying different oscillation modes within one particle and to distinguish differently shaped particles within one sample. On the contrary, the polarisation state of light itself can be arbitrarily manipulated by the inverse designed sample, known as metamaterials. Apart from polarisation state, external stimulus, e.g., magnetic field also controls the LSPR scattering from plasmonic nanostructures, giving rise to a new field of magneto-plasmonics. In this review, we pay special attention to polarisation and its effect in three contrasting aspects. First, tailoring between LSPR scattering and symmetry of plasmonic nanostructures, secondly, manipulating polarisation state through metamaterials and lastly, polarisation modulation in magneto-plasmonics. Finally, we will review recent progress in applications of plasmonic and magneto-plasmonic nanostructures and metamaterials in various fields.

Controlling angular dispersions in optical metasurfaces

Although metasurfaces have shown great potential for manipulating light, most previously realized meta-devices suffer from uncontrolled angular dispersions, making them unfavorable for many applications. Here, we propose a general strategy to realize optical metasurfaces with desired angular dispersions based on carefully controlling both the near-field couplings between meta-atoms and the radiation pattern of a single meta-atom. Utilizing such a strategy, we experimentally demonstrate a series of optical meta-devices with predesigned angular dispersions, including two incident-angle-insensitive absorbers, one incident-angle-selective absorber, and one multifunctional meta-polarizer whose functionality changes from a perfect mirror to a half-waveplate as the excitation angle varies. Finally, we design a gradient meta-device using meta-atom arrays with purposely controlled angular dispersions and numerically demonstrate that it can exhibit distinct wavefront-control functionalities when illuminated at different incident angles. Our findings establish a new platform for achieving angle-multiplexed functional meta-devices, significantly expanding the wave-manipulation capabilities of optical metasurfaces.

Anisotropic asymmetric transmission of circularly polarized terahertz waves in a three-dimensional spline assembly

Asymmetric transmission (AT) for circularly polarized (CP) electromagnetic (e-m) waves in chiral metamaterial (CMM) is a well-known phenomenon. However, most of the CMMs exhibit AT along only one direction. In this work, AT for CP waves with a magnitude of more than 0.5 along three principal directions of a newly made three-dimensional (3D) spline assembly is reported at terahertz frequencies. Surface current analysis is presented to explain the mechanism of AT for CP waves in the proposed 3D assembly.

Asymmetric chiroptical effect from chiral medium filled golden slit grating on substrate

In this Letter, we report a giant and robust asymmetric chiroptical effect (ACOE) in the chiral medium filled golden slit grating on glass substrate (CMGSG-GS). This ACOE comes from the influence of interface asymmetry on the electromagnetic cross-coupling in the CMGSG-GS, and it is inherently different than that reported in the Faraday medium and the planar anisotropic chiral metamaterials. Both the polarization eigenstate and the transmission matrix are highly dependent on the metal structure used in the CMGSG-GS. The polarization eigenstates of the CMGSG-GS are two co-rotating elliptical states with ellipticity of nearly 0, and they remain mostly unchanged for opposite directions. The transmission matrices of opposite directions are normal matrices, which do not show any symmetric law although the geometry of the CMGSG-GS owns a high rotational symmetry. The reported ACOE gives a measurable physical parameter to reveal the events happening at interface.

Compounding Meta-Atoms into Metamolecules with Hybrid Artificial Intelligence Techniques

Molecules composed of atoms exhibit properties not inherent to their constituent atoms. Similarly, metamolecules consisting of multiple meta-atoms possess emerging features that the meta-atoms themselves do not possess. Metasurfaces composed of metamolecules with spatially variant building blocks, such as gradient metasurfaces, are drawing substantial attention due to their unconventional controllability of the amplitude, phase, and frequency of light. However, the intricate mechanisms and the large degrees of freedom of the multielement systems impede an effective strategy for the design and optimization of metamolecules. Here, a hybrid artificial-intelligence-based framework consolidating compositional pattern-producing networks and cooperative coevolution to resolve the inverse design of metamolecules in metasurfaces is proposed. The framework breaks the design of the metamolecules into separate designs of meta-atoms, and independently solves the smaller design tasks of the meta-atoms through deep learning and evolutionary algorithms. The proposed framework is leveraged to design metallic metamolecules for arbitrary manipulation of the polarization and wavefront of light. Moreover, the efficacy and reliability of the design strategy are confirmed through experimental validations. This framework reveals a promising candidate approach to expedite the design of large-scale metasurfaces in a labor-saving, systematic manner.

Experimental verification of a broadband asymmetric transmission metamaterial in the terahertz region

In this work, a broadband terahertz asymmetric transmission metamaterial is experimentally demonstrated for a linearly polarized wave. The measured transmission coefficient T_yx is larger than 0.6 from 0.55 to 0.82 THz, and reaches a peak value of 0.714 at 0.62 THz, while the transmission coefficient T_xy is lower than 0.2 from 0.4 to 0.9 THz. The calculated asymmetric transmission parameter of the measurement ranges from 0.53 to 0.84 THz for magnitudes over 0.4. The peak value reached 0.65 at the frequency of 0.78 THz. The physical mechanism of the polarization conversion was also analyzed from the distributions of the surface currents and electric fields.

In this work, a broadband terahertz asymmetric transmission metamaterial is experimentally demonstrated for a linearly polarized wave.

Giant Tunable Circular Dichroism of Large-Area Extrinsic Chiral Metal Nanocrescent Arrays

Circular dichroism (CD) is an interesting phenomenon originating from the interaction of light with chiral molecules or other nanostructures lacking mirror symmetries in three-dimensional (3D) or two-dimensional (2D) space. While the observable effects of optical chirality are very weak in most of the natural materials, they can be designed and significantly enhanced in synthetic chiral structures, where the spatial symmetry of their component are broken on a nanoscale. Therefore, fabrication of composites capable of cheap, time-saving, and giant CD is desirable for the advanced optical technologies. Here, the giant CD of large-area metal nanocrescent array structures was investigated theoretically and experimentally. The largest value of the CD spectrum measured was larger than 0.5, and the CD spectrum was tuned effectively and extensively while maintaining a large peak intensity, which can be attributed to the selective excitation of the lattice surface modes (LSMs) by circularly polarized light. The analysis of the extrinsic chiral structure shows potential applications in chiral molecule sensing and polarizing imaging.

Controlling the spin-selective absorption with two-dimensional chiral plasmonic gratings

Spin-selective absorption in a two-dimensional (2D) chiral plasmonic grating is observed by excitation of chiral-dependent plasmonic cavity resonance. For the proposed structures, the incident right-handed circularly polarized light is absorbed with nearly 100% efficiency, whereas the incident left-handed circularly polarized light is reflected with same handedness. Moreover, we show that the location of spin-selective absorption can be controlled flexibly by tuning the plasmonic cavity dimension. The intensity of spin-selective absorption can be enhanced as well as suppressed based on Fabry-Perot interference phase relation. Such 2D chiral plasmonic gratings could find many potential applications in novel photon-spin selective devices, such as circularly polarized light detectors/emitters, chiral sensors, chiral cavities, and spin lasers.

Dual-band asymmetric optical transmission of both linearly and circularly polarized waves using bilayer coupled complementary chiral metasurface

It is highly desirable to develop asymmetric transmission (AT) devices for both linearly and circularly polarized light. However, currently existing metamaterial-based AT devices require multi-step micro-nano fabrication processes and usually realize AT responses only for linearly or circularly polarized waves, not simultaneously for both. We here propose a dual-band AT device for both linearly and circularly polarized waves in the near-infrared region by using a bilayer coupled complementary chiral metasurface, which includes a half-gammadion-shape gold (Au) structural layer and its Babinet's complimentary copy. Unlike other multilayer AT devices working at optical frequencies, it takes less micro-nano fabrication steps. Besides, with the help of chirality and the inherent near-field coupling effect between the two complementary Au layers, the maximal AT parameters for linearly and circularly polarized waves can reach up to 0.45 and 0.56, respectively. The underlying mechanisms of dual-band AT responses are also investigated in depth from the perspectives of chirality and coupling effect. Our work offers a new and simple approach to high-performance AT devices, helps to better understand near-filed coupling effect in coupled complementary metasurfaces, and also expands their application fields.

Multifunctional metaoptics based on bilayer metasurfaces

Optical metasurfaces have become versatile platforms for manipulating the phase, amplitude, and polarization of light. A platform for achieving independent control over each of these properties, however, remains elusive due to the limited engineering space available when using a single-layer metasurface. For instance, multiwavelength metasurfaces suffer from performance limitations due to space filling constraints, while control over phase and amplitude can be achieved, but only for a single polarization. Here, we explore bilayer dielectric metasurfaces to expand the design space for metaoptics. The ability to independently control the geometry and function of each layer enables the development of multifunctional metaoptics in which two or more optical properties are independently designed. As a proof of concept, we demonstrate multiwavelength holograms, multiwavelength waveplates, and polarization-insensitive 3D holograms based on phase and amplitude masks. The proposed architecture opens a new avenue for designing complex flat optics with a wide variety of functionalities.

High-efficiency Huygens' metasurface for terahertz wave manipulation

A fair amount of theoretical work has shown that Huygens' metasurfaces well modulate electromagnetic waves by properly designing electrical impedance Z_es and magnetic admittance Y_ms; however, the transmissive Huygens' metasurface is still challenging in the terahertz band. In this work, a transmission-type Huygens' metasurface with bilayer metallic patches has been proposed and theoretically demonstrated to show a reflectionless phase modulation for a linearly polarized terahertz wave. The simulation results show that the metasurface can achieve 2π phase coverage, and importantly the phase change can be simply achieved by changing a single geometric parameter of the metamolecule, along with a similar transmission effect. We design a high-efficiency beam deflector to realize an anomalous refraction with an angle of 19.8°. The proposed metasurface will provide a simple and direct way to realize efficient terahertz devices for wavefront manipulation.

Nonreciprocal Asymmetric Polarization Encryption by Layered Plasmonic Metasurfaces

As flexible optical devices that can manipulate the phase and amplitude of light, metasurfaces would clearly benefit from directional optical properties. However, single layer metasurface systems consisting of two-dimensional nanoparticle arrays exhibit only a weak spatial asymmetry perpendicular to the surface and therefore have mostly symmetric transmission features. Here, we present a metasurface design principle for nonreciprocal polarization encryption of holographic images. Our approach is based on a two-layer plasmonic metasurface design that introduces a local asymmetry and generates a bidirectional functionality with full phase and amplitude control of the transmitted light. The encoded hologram is designed to appear in a particular linear cross-polarization channel, while it is disappearing in the reverse propagation direction. Hence, layered metasurface systems can feature asymmetric transmission with full phase and amplitude control and therefore expand the design freedom in nanoscale optical devices toward asymmetric information processing and security features for anticounterfeiting applications.

Liquid crystal-based order electrically controlled q-plate system

This paper proposes a liquid crystal-based order adjustable q-plate system. The system, which is solid-state and electrically controlled without any mechanical components, consists of several bit cells and one symbol cell. The bit cells can be electrically selected whether to modulate the beam. The magnitude of the order of the q-plate system can be controlled by activating specific bit cells. And the sign of the order can be changed by controlling the voltages in the symbol cell. The whole system can realize the function of the order adjustable q-plate with the order ranging from -2n + 1 to 2n-1 with n bit cells. In our experiment, the system with 4 bits is verified. Based on the q-plate system, the vector beams and optical vortexes with the orders ranging from -15 to 15 can be generated.

Asymmetric Transmission in a Mie-Based Dielectric Metamaterial with Fano Resonance

Chiral metamaterials with asymmetric transmission can be applied as polarization-controlled devices. Here, a Mie-based dielectric metamaterial with a spacer exhibiting asymmetric transmission of linearly polarized waves at microwave frequencies was designed and demonstrated numerically. The unidirectional characteristic is attributed to the chirality of the metamolecule and the mutual excitation of the Mie resonances. Field distributions are simulated to investigate the underlying physical mechanism. Fano-type resonances emerge near the Mie resonances of the constituents and come from the destructive interference inside the structure. The near-field coupling further contributes to the asymmetric transmission. The influences of the lattice constant and the spacer thickness on the asymmetric characteristics were also analyzed by parameter sweeps. The proposed Mie-based metamaterial is of a simple structure, and it has the potential for applications in dielectric metadevices, such as high-performance polarization rotators.

Broadband circular polarizer for randomly polarized light in few-layer metasurface

Controlling the polarization state of light has been a significant issue for various integrated optical devices such as optical imaging, sensors, and communications. Recent advances in metamaterials enable the optical elements for controlling light to be miniaturized and to have various multi-functions in subwavelength scale. However, a conventional approach of a circular polarizer has an inherent limitation to eliminate the unwanted circular polarization, which means that the efficiency varies significantly depending on the polarization state of incident light. Here, we propose a novel concept of a circular polarizer by combining two functions of transmission and conversion for orthogonal circular polarizations with a total thickness of 440 nm. The proposed three-layer metasurface composed of rotating silver nanorods transmits the left-handed circularly polarized (LCP) light with maintaining its own polarization state, whereas the right-handed circularly polarized (RCP) light is converted into LCP light. Regardless of the polarization state of incoming light, the polarization of light in the last medium is LCP state in the broadband operating wavelength range from 800 nm to 1100 nm. The converted RCP and the transmitted LCP have efficiencies of up to 48.5% and 42.3%, respectively. Thus the proposed metasurface serves as a stable circular polarizer for a randomly polarized light. In addition, high-efficiency asymmetric transmission of about 0.47 is achieved at the same time due to the conversion characteristic of RCP component. The proposed metasurface has the significance as an ultra-thin optical element applicable to optical switching, sensors, and communications in unidirectional channel as well as a broadband circular polarizer for randomly polarized light.

Analyzing the polarization response of a chiral metasurface stack by semi-analytic modeling

We investigate a class of stacked metasurfaces where the interaction between layers is dominated by their respective far-field response. Using a semi-analytic scattering matrix approach, we exploit the Fabry-Perot-type response for different layer distances to show the spectral tunability of the resonant effect. This method presents a faster and more intuitive route to modeling Fabry-Perot-type effects than rigorous numerical simulations. The results are illustrated for a chiral metasurface stack that exhibits asymmetric transmission. Here, the effect of asymmetric transmission is highly sensitive to the layer distance, which is used as a free parameter in our model. To prove our theoretical findings we fabricate two variants of the stack with different layer distances and show that far-field interaction between layers is sufficient to generate the effect while being accessible by semi-analytic modeling. The analyticity of the approach is promising for designing sophisticated layered media containing stacks of arbitrary metasurfaces.