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

Arbitrarily rotating polarization direction and manipulating phases in linear and nonlinear ways using programmable metasurface

Independent controls of various properties of electromagnetic (EM) waves are crucially required in a wide range of applications. Programmable metasurface is a promising candidate to provide an advanced platform for manipulating EM waves. Here, we propose an approach that can arbitrarily control the polarization direction and phases of reflected waves in linear and nonlinear ways using a stacked programmable metasurface. Further, we extend the space-time-coding theory to incorporate the dimension of polarization, which provides an extra degree of freedom for manipulating EM waves. As proof-of-principle application examples, we consider polarization rotation, phase manipulation, and beam steering at linear and nonlinear frequencies. For validation, we design, fabricate, and measure a metasurface sample. The experimental results show good agreement with theoretical predictions and simulations. The proposed approach has a wide range of applications in various areas, such as imaging, data storage, and wireless communication.

Mixing Rules for Left-Handed Disordered Metamaterials: Effective-Medium and Dispersion Properties

Left-handed materials are known to exhibit exotic properties in controlling electromagnetic fields, with direct applications in negative reflection and refraction, conformal optical mapping, and electromagnetic cloaking. While typical left-handed materials are constructed periodic metal-dielectric structures, the same effect can be obtained in composite guest-host systems with no periodicity or structural order. Such systems are typically described by the effective-medium approach, in which the components of the electric permittivity tensor are determined as a function of individual material properties and doping concentration. In this paper, we extend the discussion on the mixing rules to include left-handed composite systems and highlight the exotic properties arising from the effective-medium approach in this framework in terms of effective values and dispersion properties.

Twist-Angle-Controlled Nonlinear Circular Dichroism on Gold-Crystal Hybrid Metasurfaces

Optical chirality, which plays important roles in liquid crystal display and biological and chemical detection, has been attracting scientists' attention due to its potential applications in optical information processing. Usually, the chiral optical response of natural molecules is very weak. However, the emergence of metasurfaces offers a promising solution to solve this issue. By judiciously designing the geometry of meta-atoms, we have realized strong optical circular dichroism (CD) in both linear and nonlinear optical regimes. However, tuning of the CD with a metasurface remains challenging. Here, we propose the twist-angle-controlled nonlinear CD effect by using the second-harmonic generation process on a gold-crystal hybrid metasurface. The CD effect of the second-harmonic waves can be tuned well by controlling the twist angle between the two constituent materials. The proposed hybrid metasurface may open new avenues for developing ultracompact and multifunctional nonlinear optical devices.

Metasurface array for single-shot spectroscopic ellipsometry

Spectroscopic ellipsometry is a potent method that is widely adopted for the measurement of thin film thickness and refractive index. Most conventional ellipsometers utilize mechanically rotating polarizers and grating-based spectrometers for spectropolarimetric detection. Here, we demonstrated a compact metasurface array-based spectroscopic ellipsometry system that allows single-shot spectropolarimetric detection and accurate determination of thin film properties without any mechanical movement. The silicon-based metasurface array with a highly anisotropic and diverse spectral response is combined with iterative optimization to reconstruct the full Stokes polarization spectrum of the light reflected by the thin film with high fidelity. Subsequently, the film thickness and refractive index can be determined by fitting the measurement results to a proper material model with high accuracy. Our approach opens up a new pathway towards a compact and robust spectroscopic ellipsometry system for the high throughput measurement of thin film properties.

Reconfigurable transmissive metasurface with a combination of scissor and rotation actuators for independently controlling beam scanning and polarization conversion

Polarization conversion and beam scanning metasurfaces are commonly used to reduce polarization mismatch and direct electromagnetic waves in a specific direction to improve the strength of a wireless signal. However, identifying suitable active and mechanically reconfigurable metasurfaces for polarization conversion and beam scanning is a considerable challenge, and the reported metasurfaces have narrow scanning ranges, are expensive, and cannot be independently controlled. In this paper, we propose a reconfigurable transmissive metasurface combined with a scissor and rotation actuator for independently controlling beam scanning and polarization conversion functions. The metasurface is constructed with rotatable unit cells (UCs) that can switch the polarization state between right-handed (RHCP) and left-handed circular polarization (LHCP) by flipping the UCs to reverse their phase variation. Moreover, independent beam scanning is achieved using the scissor actuator to linearly change the distance between the UCs. Numerical and experimental results confirm that the proposed metasurface can perform beam scanning in the range of 28° for both the positive and negative regions of a radiation pattern (RHCP and LHCP beams) at an operational frequency of 10.5 GHz.

Realizing Minimally Perturbed, Nonlocal Chiral Metasurfaces for Direct Stokes Parameter Detection

Recent development in nonlocal resonance based chiral metasurfaces draws great attention due to their abilities to strongly interact with circularly polarized light at a relatively narrow spectral bandwidth. However, there still remain challenges in realizing effective nonlocal chiral metasurfaces in optical frequency due to demanding fabrications such as 3D-multilayered or nanoscaled chiral geometry, which, in particular, limit their applications to polarimetric detection with high-Q spectra. Here, we study the underlying working principles and reveal the important role of the interaction between high-Q nonlocal resonance and low-Q localized Mie resonance in realizing effective nonlocal chiral metasurfaces. Based on the working principles, we demonstrate one of the simplest types of nonlocal chiral metasurfaces which directly detects a set of Stokes parameters without the numerical combination of transmitted values presented from typical Stokes metasurfaces. This is achieved by minimally altering the geometry and filling ratio of every constituent nanostructure in a unit cell, facilitating consistent-sized nanolithography for all samples experimentally at a targeted wavelength with relatively high-Q spectra. This work provides an alternative design rule to realizing effective polarimetric metasurfaces and the potential applications of nonlocal Stokes parameters detection.

Metalens for Accelerated Optoelectronic Edge Detection under Ambient Illumination

Analog systems may allow image processing, such as edge detection, with low computational power. However, most demonstrated analog systems, based on either conventional 4-f imaging systems or nanophotonic structures, rely on coherent laser sources for illumination, which significantly restricts their use in routine imaging tasks with ambient, incoherent illumination. Here, we demonstrated a metalens-assisted imaging system that can allow optoelectronic edge detection under ambient illumination conditions. The metalens was designed to generate polarization-dependent optical transfer functions (OTFs), resulting in a synthetic OTF with an isotropic high-pass frequency response after digital subtraction. We integrated the polarization-multiplexed metalens with a polarization camera and experimentally demonstrated single-shot edge detection of indoor and outdoor scenes, including a flying airplane, under ambient sunlight illumination. The proposed system showcased the potential of using polarization multiplexing for the construction of complex optical convolution kernels toward accelerated machine vision tasks such as object detection and classification under ambient illumination.

Laser-written wave plates inside the silicon enabled by stress-induced birefringence

Laser writing enables optical functionality by altering the optical properties of materials. To achieve this goal, efforts generally focus on laser-written regions. It has also been shown that birefringence surrounding the modified regions can be exploited for achieving functionality. The effect has been used to fabricate wave plates in glass, with significant potential for other materials. Here, we establish analogous stress control and birefringence engineering inside silicon. We first develop a robust analytical model enabling the prediction of birefringence maps from arbitrary laser-written patterns. Then, we tailor three-dimensional laser lithography to create the first, to the best of our knowledge, polarization-control optics inside silicon.

Universal Polarization Transformations: Spatial Programming of Polarization Scattering Matrices Using a Deep Learning-Designed Diffractive Polarization Transformer

Controlled synthesis of optical fields having nonuniform polarization distributions presents a challenging task. Here, a universal polarization transformer is demonstrated that can synthesize a large set of arbitrarily-selected, complex-valued polarization scattering matrices between the polarization states at different positions within its input and output field-of-views (FOVs). This framework comprises 2D arrays of linear polarizers positioned between isotropic diffractive layers, each containing tens of thousands of diffractive features with optimizable transmission coefficients. After its deep learning-based training, this diffractive polarization transformer can successfully implement Ni No = 10 000 different spatially-encoded polarization scattering matrices with negligible error, where Ni and No represent the number of pixels in the input and output FOVs, respectively. This universal polarization transformation framework is experimentally validated in the terahertz spectrum by fabricating wire-grid polarizers and integrating them with 3D-printed diffractive layers to form a physical polarization transformer. Through this set-up, an all-optical polarization permutation operation of spatially-varying polarization fields is demonstrated, and distinct spatially-encoded polarization scattering matrices are simultaneously implemented between the input and output FOVs of a compact diffractive processor. This framework opens up new avenues for developing novel devices for universal polarization control and may find applications in, e.g., remote sensing, medical imaging, security, material inspection, and machine vision.

End-to-End Diverse Metasurface Design and Evaluation Using an Invertible Neural Network

Employing deep learning models to design high-performance metasurfaces has garnered significant attention due to its potential benefits in terms of accuracy and efficiency. A deep learning-based metasurface design framework typically comprises a forward prediction path for predicting optical responses and a backward retrieval path for generating geometrical configurations. In the forward design path, a specific geometrical configuration corresponds to a unique optical response. However, in the inverse design path, a single performance metric can correspond to multiple potential designs. This one-to-many mapping poses a significant challenge for deep learning models and can potentially impede their performance. Although representing the inverse path as a probabilistic distribution is a widely adopted method for tackling this problem, accurately capturing the posterior distribution to encompass all potential solutions remains an ongoing challenge. Furthermore, in most pioneering works, the forward and backward paths are captured using separate models. However, the knowledge acquired from the forward path does not contribute to the training of the backward model. This separation of models adds complexity to the system and can hinder the overall efficiency and effectiveness of the design framework. Here, we utilized an invertible neural network (INN) to simultaneously model both the forward and inverse process. Unlike other frameworks, INN focuses on the forward process and implicitly captures a probabilistic model for the inverse process. Given a specific optical response, the INN enables the recovery of the complete posterior over the parameter space. This capability allows for the generation of novel designs that are not present in the training data. Through the integration of the INN with the angular spectrum method, we have developed an efficient and automated end-to-end metasurface design and evaluation framework. This novel approach eliminates the need for human intervention and significantly speeds up the design process. Utilizing this advanced framework, we have effectively designed high-efficiency metalenses and dual-polarization metasurface holograms. This approach extends beyond dielectric metasurface design, serving as a general method for modeling optical inverse design problems in diverse optical fields.

Dual-Functional Tunable Metasurface for Meta-Axicon with a Variable Depth of Focus and Continuous-Zoom Metalens

Optical metasurfaces have been widely investigated for their versatile ability to manipulate wavefront and miniaturize traditional optical components into ultrathin planar devices. The integration of metasurfaces with multifunctionality and tunability has fundamentally transformed optics with unprecedented control over light propagation and manipulation. This study introduces a pioneering framework for the development of tunable metasurfaces with multifunctionality, and an example of a tunable metasurface of dual functionalities is proposed and numerically verified as one of the tunable meta-axicon for generating Bessel beams with a variable depth of focus (DOF) and a continuous-zoom metalens. Specifically, this design achieves dual-functional phase modulation by helicity-multiplexing from the combination of the geometric phase as well as the propagation phase and realizes tunability for both functionalities through rotational actuation between double metasurface layers. As a result, dual functionalities with continuous tunability of the proposed TiO2 metasurface are enabled independently for the left and right circularly polarized (LCP and RCP) incidences at 532 nm. Specifically, LCP light triggers the metasurface to function as a tunable axicon, generating non-diffracting Bessel beams with variable numerical apertures (NA) and DOFs. Conversely, the RCP incidence induces it to operate as a continuous-zoom metalens and generates variable spherical wavefront focusing on diverse focal lengths. This study not only initially implements the design of tunable meta-axicon, but also achieves the integration of such a tunable meta-axicon and continuous-zoom metalens within a single metasurface configuration. The proposed device could find potential applications in biological imaging, microscopic measurement, laser fabrication, optical manipulation, multi-plane imaging, depth estimation, optical data storage, etc.

Dielectric diatomic metasurface-assisted versatile bifunctional polarization conversions and incidence-polarization-secured meta-image

Dielectric metasurface empowering efficient light polarization control at the nanoscale, has recently garnered tremendous research interests in the field of high-resolution image encryption and display, particularly at low-loss wavelengths in the visible band. Nevertheless, due to the single fixed polarization conversion function, the image (either positive or negative image) can always be decrypted in a host-uncontrollable manner as long as the user applies an analyzer to select the polarization component of the output light. Here, we resort to half-waveplate- and quarter-waveplate-like silicon nanopillars to form a metamolecule of a dielectric diatomic metasurface, which can yield versatile linearly polarized (LP) and circularly polarized (CP) light upon orthogonally linear-polarized incidences, providing new degrees of freedom for image display and encryption. We show both theoretically and numerically that versatile different paired LP and CP combinations could be achieved by simply adjusting the orientation angles of the two nanopillars. The bifunctional polarization conversion functions make possible that a meta-image can only be seen when incident light is linearly polarized at a specific polarization angle, whereas no image can be discerned for the orthogonal polarization incidence case, indicating the realization of incidence-polarization secured meta-image. This salient feature holds for all individual metamolecules, reaching a remarkable image resolution of 52,916 dots per inch. By fully exploiting all polarization conversions of four designed metamolecules, three-level incidence polarization-secured meta-image can also be expected.

Dynamic dual-functional optical wave plate based on phase-change meta-molecules

Optical metasurfaces have shown great potential for revolutionizing wave plates by enabling compact footprints and diversified functionalities. However, most metasurface wave plates (meta-WPs) are typically passive, featuring defined responses after fabrication, whereas dynamic meta-WPs have so far often been limited to ON and OFF states. Here, we design a dynamic dual-functional meta-WP based on judiciously designed low-loss Sb₂Se₃ meta-molecules at the telecom wavelength of 1.55 µm which enables reconfigurable linear-to-circular and linear-to-linear polarization conversion for orthogonal linear polarizations when Sb₂Se₃ transits between amorphous and crystalline states. In addition, a comprehensive electro-thermal simulation is carried out to verify the phase change process for realistic implementation. The designed dynamic dual-functional wave plate may open new avenues for developing integrated adaptive photonics with dynamic and multiplexed functionalities.

Direction-Duplex Janus Metasurface for Full-Space Electromagnetic Wave Manipulation and Holography

Janus metasurfaces, a category of two-faced two-dimensional (2D) materials, are emerging as a promising platform for designing multifunctional metasurfaces by exploring the intrinsic propagation direction (k-direction) of electromagnetic waves. Their out-of-plane asymmetry is utilized for achieving distinct functions selectively excited by choosing the propagation directions, providing an effective strategy to meet the growing demand for the integration of more functionalities into a single optoelectronic device. Here, we propose the concept of direction-duplex Janus metasurface for full-space wave control yielding drastically different transmission and reflection wavefronts for the same polarized incidence with opposite k-directions. A series of Janus metasurface devices that enable asymmetric full-space wave manipulations, such as integrated metalens, beam generators, and fully direction-duplex meta-holography, are experimentally demonstrated. We envision the Janus metasurface platform proposed here to open new possibilities toward a broader exploration of creating sophisticated multifunctional meta-devices ranging from microwaves to optical systems.

Multifunctional Metasurfaces Enabled by Multifold Geometric Phase Interference

Geometric phase is frequently used in artificially designed metasurfaces; it is typically used only once in reported works, leading to conjugate responses of two spins. Supercells containing multiple nanoantennas can break this limitation by introducing more degrees of freedom to generate new modulation capabilities. Here, we provide a method for constructing supercells for geometric phases using triple rotations, each of which achieves a specific modulation function. The physical meaning of each rotation is revealed by stepwise superposition. Based on this idea, spin-selective holography, nanoprinting, and their hybrid displays are demonstrated. As a typical application, we have designed a metalens that enables spin-selective transmission, allowing for high-quality imaging with only one spin state, which can serve as a plug-and-play chiral detection device. Finally, we analyzed how the size of supercells and the phase distribution inside it can affect the higher order diffraction, which may help in designing supercells for different scenarios.

Metasurface designed with quantitative field distributions

A new method for designing metasurfaces has been proposed and demonstrated, which allows for the generation of precise quantitative field distributions. This unique approach involves combining a tandem neural network with an iterative algorithm to optimize the metasurface design, enabling accurate control over the intensity and polarization of the resulting field. This strategy is both efficient and robust and has the potential to accelerate the development of metasurface devices with complex functionalities.

Metasurface-Based Solid Poincaré Sphere Polarizer

The combination of conventional polarization optical elements, such as linear polarizers and waveplates, is widely adopted to tailor light's state of polarization (SOP). Meanwhile, less attention has been given to the manipulation of light's degree of polarization (DOP). Here, we propose metasurface-based polarizers that can filter unpolarized incident light to light with any prescribed SOP and DOP, corresponding to arbitrary points located both at the surface and within the solid Poincaré sphere. The Jones matrix elements of the metasurface are inverse-designed via the adjoint method. As prototypes, we experimentally demonstrated metasurface-based polarizers in near-infrared frequencies that can convert unpolarized light into linear, elliptical, or circular polarizations with varying DOPs of 1, 0.7, and 0.4, respectively. Our Letter unlocks a new degree of freedom for metasurface polarization optics and may break new ground for a variety of DOP-related applications, such as polarization calibration and quantum state tomography.

Phase-assisted Bessel-metasurface: a single-sized approach for simultaneous printing and holography

Due to the unprecedented wavefront shaping capability, the metasurface has demonstrated state-of-the-art performances in various applications, especially in printing and holography. Recently, these two functions have been combined into a single metasurface chip to achieve a capability expansion. Despite the progress, current dual-mode metasurfaces are realized at the expense of an increase in the difficulty of the fabrication, reduction of the pixel resolution, or strict limitation in the illumination conditions. Inspired by the Jacobi-Anger expansion, a phase-assisted paradigm, called Bessel metasurface, has been proposed for simultaneous printing and holography. By elaborately arranging the orientations of the single-sized nanostructures with geometric phase modulation, the Bessel metasurface can not only encode a greyscale printing image in real space but can reconstruct a holographic image in k-space. With the merits of compactness, easy fabrication, convenient observation, and liberation of the illumination conditions, the design paradigm of the Bessel metasurface would have promising prospects in practical applications, including optical information storage, 3D stereoscopic displays, multifunctional optical devices, etc.

Full-space wavefront manipulation enabled by asymmetric photonic spin-orbit interactions

Optical metasurfaces empower complete wavefront manipulation of electromagnetic waves and have been found in extensive applications, whereas most of them work in either transmission or reflection space. Here, we demonstrate that two independent and arbitrary phase profiles in transmission and reflection spaces could be produced by a monolayer all-dielectric metasurface based on the asymmetric photonic spin-orbit interactions, realizing full-space wavefront independent manipulation. Furthermore, the supercell-based non-local approach is employed to suppress crosstalk between adjacent nanopillars in one supercell for broadband and high-efficiency wavefront manipulation in full space. Compared with the conventional unit cell-based local approach, such a method could improve efficiency about 10%. As a proof of concept, two metadevices are designed, in which the maximum diffraction efficiencies are ∼95.53%/∼74.07% within the wavelength range of 1500-1600 nm in reflection/transmission space under circularly polarized light incidence. This configuration may offer an efficient way for 2π-space holographic imaging, augmented reality, virtual reality technologies, three-dimensional imaging, and so forth.

Recent Advances in Ultrathin Chiral Metasurfaces by Twisted Stacking

Artificial chiral nanostructures have been subjected to extensive research for their unique chiroptical activities. Planarized chiral films of ultrathin thicknesses are in particular demand for easy on-chip integration and improved energy efficiency as polarization-sensitive metadevices. Recently, controlled twisted stacking of two or more layers of nanomaterials, such as 2D van der Waals materials, ultrathin films, or traditional metasurfaces, at an angle has emerged as a general strategy to introduce optical chirality into achiral solid-state systems. This method endows new degrees of freedom, e.g., the interlayer twist angle, to flexibly engineer and tune the chiroptical responses without having to change the material or the design, thus greatly facilitating the development of multifunctional metamaterials. In this review, recent exciting progress in planar chiral metasurfaces are summarized and discussed from the viewpoints of building blocks, fabrication methods, as well as circular dichroism and modulation thereof in twisted stacked nanostructures. The review further highlights the ever-growing portfolio of applications of these chiral metasurfaces, including polarization conversion, information encryption, chiral sensing, and as an engineering platform for hybrid metadevices. Finally, forward-looking prospects are provided.

Observation of full-parameter Jones matrix in bilayer metasurface

Metasurfaces, artificial 2D structures, have been widely used for the design of various functionalities in optics. Jones matrix, a 2×2 matrix with eight parameters, provides the most complete characterization of the metasurface structures in linear optics, and the number of free parameters (i.e., degrees of freedom, DOFs) in the Jones matrix determines the limit to what functionalities we can realize. Great efforts have been made to continuously expand the number of DOFs, and a maximal number of six has been achieved recently. However, the realization of the ultimate goal with eight DOFs (full free parameters) has been proven as a great challenge so far. Here, we show that by cascading two layer metasurfaces and utilizing the gradient descent optimization algorithm, a spatially varying Jones matrix with eight DOFs is constructed and verified numerically and experimentally in optical frequencies. Such ultimate control unlocks opportunities to design optical functionalities that are unattainable with previously known methodologies and may find wide potential applications in optical fields.

Arbitrary Jones matrix on-demand design in metasurfaces using multiple meta-atoms

Super cells or multi-layer metasurfaces are used to realize various multi-functional and exotic functional devices. In such methods, the design space expands exponentially as more variable parameters are introduced; however, this will necessitate huge computational effort without special treatment. The function of a metasurface can be described mathematically by using a Jones matrix. When the gap between adjacent atoms is sufficiently large, the overall Jones matrix of a 3D lattice which is composed of multiple meta-atoms can be obtained by adding or multiplying each meta-atom's Jones matrix for a parallel or cascaded arrangement, respectively. Reversely, an arbitrary Jones matrix can be decomposed to achieve a combination of diagonal and rotation matrices. This means that the devices with various functions can be constructed by combining, cascading, and rotating a kind of atom, and thus the computation requirements will be reduced significantly. In this work, the feasibility of this approach is demonstrated with two cases, circular polarization selective transmission and resemble optical activity. Both the simulation and experiment are consistent with the hypothesis. This method can manipulate all degrees of freedom in a Jones matrix and reduce design complexity and may find applications to extend the scope of meta-optics.

Diatomic terahertz metasurfaces for arbitrary-to-circular polarization conversion

Polarization control is crucial for tailoring light-matter interactions. Direct manipulation of arbitrarily incident polarized waves could provide more degrees of freedom in the design of integrated and miniaturized terahertz (THz) devices. Metasurfaces with unprecedented wave manipulation capabilities could serve as candidates for fulfilling this requirement. Here, a kind of all-silicon metasurface is demonstrated to realize the conversion of arbitrary incident polarization states to circular polarization states in the THz band through the mutual interference of monolayer achiral meta-atoms. Also, we confirmed that the conversion intensities are controllable using the evolution behavior of arbitrary polarization states defined on the Poincaré sphere. Meta-platforms with circularly polarized incidence experience spin-selective destructive or constructive interference, exhibiting broadband circular dichroism (BCD) in the target frequency range. Based on the versatility of the proposed design, the feasibility of the theoretical derivation has been verified in the experiment process. By introducing the geometric phase principle, the proposed design is demonstrated to be an attractive alternative to achieve chiral wavefront manipulation. This work may provide a promising avenue to replace the cumbersome cascaded optical building blocks with an ultrathin meta-platform, which can be used in chiral spectroscopy, imaging, optical communication, and so on.

Full-space dual-helicity decoupled metasurface for a high-efficiency multi-folded reflective antenna

The independent tailoring of electromagnetic waves with different circular-polarized (CP) wavefront in both reflection and transmission channels is of broad scientific and technical interest, offering ultimate degrees of freedom in designing advanced devices with the merits of functionality integration and spatial exploitation. However, most metasurfaces only provide dependent wavefront control of dual-helicity in a single channel, restricting their applications to limited practical scenarios. Herein, we propose a full-space dual-helicity decoupled metasurface and apply it to assemble a multi-folded reflective antenna (MFRA) in the microwave regime. A multilayered chiral meta-atom is designed and optimized to reflect a particular helical wave while allowing the orthogonal helical wave to penetrate through, with simultaneous full span of phase modulations in both channels. When a uniform reflection and a hyperbolic transmission phase profile is imposed simultaneously on the metasurface in a polarization-selective manner, it can be engineered to conduct specular reflection for one helical wave and convergent transmission of the other helical wave. Combining the proposed metasurface with a metallic plate as a bottom reflector and an integrated microstrip patch antenna in the center of metasurface as a feed, a MFRA is realized with a low profile, high efficiency, and high polarization purity in a broad frequency band. The proposed design method of the dual-helicity decoupled metasurface and its antenna application provide opportunities for high-performance functional devices, promising more potential in future communication and detection systems.

Metasurface-based broadband polarization-insensitive polarization rotator

The polarizations of electromagnetic (EM) waves are very important for transceivers. We propose a broadband polarization-insensitive polarization rotator (PIPR), which can realize 90° polarization rotation for incident waves with an arbitrary azimuth angle. A unit of the PIPR is composed of two types of substrate integrated waveguide (SIW) units in a checkerboard pattern, which provides more than -0.2 dB transmission from 9.5 to 10.9 GHz. The electric field inside the cavity is analyzed to explain the working mechanism of the proposed rotator. A prototype is fabricated and measured to verify the proposed design, and satisfactory agreement between simulated and measured results is achieved, indicating that the converter has potential applications in imaging and communication systems.

Planar chiral metasurfaces with maximal and tunable chiroptical response driven by bound states in the continuum

Optical metasurfaces with high quality factors (Q-factors) of chiral resonances can boost substantially light-matter interaction for various applications of chiral response in ultrathin, active, and nonlinear metadevices. However, current approaches lack the flexibility to enhance and tune the chirality and Q-factor simultaneously. Here, we suggest a design of chiral metasurface supporting bound state in the continuum (BIC) and demonstrate experimentally chiroptical responses with ultra-high Q-factors and near-perfect circular dichroism (CD = 0.93) at optical frequencies. We employ the symmetry-reduced meta-atoms with high birefringence supporting winding elliptical eigenstate polarizations with opposite helicity. It provides a convenient way for achieving the maximal planar chirality tuned by either breaking in-plane structure symmetry or changing illumination angle. Beyond linear CD, we also achieved strong near-field enhancement CD and near-unitary nonlinear CD in the same planar chiral metasurface design with circular eigen-polarization. Sharply resonant chirality realized in planar metasurfaces promises various practical applications including chiral lasers and chiral nonlinear filters.

Here, the authors employ the physics of chiral bound states in the continuum and suggest planar chiral metasurfaces with simultaneous ultrahigh quality factor and near-perfect circular dichroism in both linear regime and nonlinear regime.

Polarization multiplexing metasurface for dual-band achromatic focusing

We propose a dual-band achromatic focusing metasurface based on polarization multiplexing and dispersion engineering. An anisotropic resonant phase meta-atom is designed to realize independent nonlinear phase manipulation along the orthogonal directions. Achromatic focusing metasurface and broadband reflectarray antenna are further constructed in the microwave region with a computer-assisted particle swarm optimization algorithm. The standard deviation of focus offset at 11-16 GHz (for x-polarization) and 18-24 GHz (for y-polarization) are compressed to 19.83% and 16.60% of the dispersive metasurface, respectively. The radiation gains of the reflectarray antenna increase by an average of 19.49 dB and 15.08 dB in the broadband region compared with the bare standard rectangle waveguides. Furthermore, such an achromatic metasurface can be utilized to realize different functions with polarization selectivity and applied to other frequency ranges, which holds great promise in integrated optics.

Photonic Encryption Platform via Dual-Band Vectorial Metaholograms in the Ultraviolet and Visible

Metasurface-driven optical encryption devices have attracted much attention. Here, we propose a dual-band vectorial metahologram in the visible and ultraviolet (UV) regimes for optical encryption. Nine polarization-encoded vectorial holograms are observed under UV laser illumination, while another independent hologram appears under visible laser illumination. The proposed engineered silicon nitride, which is transparent in UV, is employed to demonstrate the UV hologram. Nine holographic images for different polarization states are encoded using a pixelated metasurface. The dual-band metahologram is experimentally implemented by stacking the individual metasurfaces that operate in the UV and visible. The visible hologram can be decrypted to provide the first key, a polarization state, which is used to decode the password hidden in the UV vectorial hologram through the use of an analyzer. Considering the property of UV to be invisible to the naked eye, the multiple polarization channels of the vectorial hologram, and the dual-band decoupling, the demonstrated dual-band vectorial hologram device could be applied in various high-security and anticounterfeiting applications.

Perfect diffractive circular metagrating for Bessel beam transformation

Bessel beams, with their non-diffractive property, have attracted great interest in recent years. Optical needle shaping of Bessel beams is highly desired in many applications, however, this typically requires low numerical aperture (NA) bulky 4f confocal systems incorporated with spatial light modulators or round filters. Here, we employ a circular dielectric metagrating for perfect Bessel beam transformation at a desired wavelength. The dielectric metagrating exhibits a high transmissive diffraction efficiency (up to 75%) for a broadband (460 nm to 560 nm), wide-angle range, and dual-polarization response, which is capable of a high-performance transformation of Bessel beams with arbitrary NAs. Our results show potential for special-beam-required applications such as light storage, imaging, and optical manipulation.

Generating ultraviolet perfect vortex beams using a high-efficiency broadband dielectric metasurface

Due to the topological charge-independent doughnut spatial structure as well as the association of orbital angular momentums, perfect vortex beams promise significant advances in fiber communication, optical manipulation and quantum optics. Inspired by the development of planar photonics, several plasmonic and dielectric metasurfaces have been constructed to generate perfect vortex beams, instead of conventional bulky configuration. However, owing to the intrinsic Ohmic losses and interband electron transitions in materials, these metasurface-based vortex beam generators only work at optical frequencies up to the visible range. Herein, using silicon nitride nanopillars as high-efficiency half-wave plates, broadband and high-performance metasurfaces are designed and demonstrated numerically to directly produce perfect vortex beams in the ultraviolet region, by combining the phase profiles of spiral phase plate, axicon and Fourier transformation lens based on geometric phase. The conversion efficiency of the metasurface is up to 86.6% at the design wavelength. Moreover, the influence of several control parameters on perfect vortex beam structures is discussed. We believe that this ultraviolet dielectric generator of perfect vortex beams will find many significant applications, such as high-resolution spectroscopy, optical tweezer and on-chip communication.

Bioinspired Quasi-3D Multiplexed Anti-Counterfeit Imaging via Self-Assembled and Nanoimprinted Photonic Architectures

Innovative multiplexing technologies based on nano-optics for anti-counterfeiting have been proposed as overt and covert technologies to secure products and make them difficult to counterfeit. However, most of these nano-optical anti-counterfeiting materials are metasurfaces and metamaterials with complex and expensive fabrication process, often resulting in materials that are not damage tolerant. Highly efficient anti-counterfeiting technologies with easy fabrication process are targeted for intuitive and effective authentication of banknotes, secure documents, and goods packing. Here, a simple strategy exploiting self-assembling and nanoimprinting technique to fabricate a composite lattice photonic crystal architecture featuring full spatial control of light, multiplexed full-pixel imaging, and multichannel cryptography combined with customized algorithms is reported. In particular, the real-time encryption/recognition of mobile quick response codes and anti-counterfeiting labels on a postage stamp, encoded by the proposed photonic architecture, are both demonstrated. The wave optics of scattering, diffraction, and polarization process involved are also described, validated with numerical simulations and experiments. By introducing a new degree of freedom in the 3D space, the multichannel image switching exhibits unprecedented variability of encryption, providing a promising roadmap to achieve larger information capacity, better security, and higher definition for the benefit of modern anti-counterfeiting security.

Observation of Ultrafast Self-Action Effects in Quasi-BIC Resonant Metasurfaces

High-index dielectric metasurfaces can support sharp optical resonances enabled by the physics of bound states in the continuum (BICs) often manifested in experiments as quasi-BIC resonances. They provide a way to enhance light-matter interaction at the subwavelength scale bringing novel opportunities for nonlinear nanophotonics. Strong narrow-band field enhancement in quasi-BIC metasurfaces leads to an extreme sensitivity to a change of the refractive index that may limit nonlinear functionalities for the pump intensities beyond the perturbative regime. Here we study ultrafast self-action effects observed in quasi-BIC silicon metasurfaces and demonstrate how they alter the power dependence of the third-harmonic generation efficiency. We study experimentally a transition from the subcubic to supercubic regimes for the generated third-harmonic power driven by a blue-shift of the quasi-BIC in the multiphoton absorption regime. Our results suggest a way to implement ultrafast nonlinear dynamics in high-index resonant dielectric metasurfaces for nonlinear meta-optics beyond the perturbative regime.

Polarization-insensitive achromatic metalens based on computational wavefront coding

Broadband achromatic metalens imaging is of great interest in various applications, such as integrated imaging and augmented/virtual reality display. Current methods of achromatic metalenses mainly rely on the compensation of a linear phase dispersion implemented with complex nanostructures. Here, we propose and experimentally demonstrate a polarization-insensitive achromatic metalens (PIA-ML) based on computational wavefront coding. In this method, simple circular or square nanopillars are individually coded such that the focal depths at wavelengths at both ends of the achromatic bandwidth overlap at the designed focal plane, which removes the limitation of requiring a linear phase dispersion. An optimized PIA-ML that works in the full optical communication band from 1300 to 1700nm was obtained using a particle swarm optimization algorithm. Experimental results show that both focusing and imaging of the fabricated metalens are consistent with theoretical predictions within the broadband wavelength range, which provides a new methodology for ultra-broadband achromatic imaging with simple-shaped nanostructures.

Dichroic Circular Polarizers Based on Plasmonics for Polarization Imaging Applications

Dichroic circular polarizers (DCP) represent an important group of optical filters that transfer only that part of the incident light with the desired polarization state and absorb the remainder. However, DCPs are usually bulky and exhibit significant optical loss. Moreover, the integration of these kinds of DCP devices can be difficult and costly as different compositions of chemicals are needed to achieve the desired polarization status. Circular polarizers based on metasurfaces require only thin films in the order of hundreds of nanometers but are limited by their sensitivity to angle of incidence. Furthermore, few existing solutions offer broadband operation in the visible range. By using computational simulations, this paper proposes and analyses a plasmonic DCP structure operating in the visible, from 400 nm to 700 nm which overcomes these drawbacks. The resulting circular dichroism transmission (CDT) is more than 0.9, and the maximum transmission efficiency is greater than 78% at visible wavelengths. These CDT characteristics are largely independent of angle of incidence up to angles of 80 degrees.

Pixelated bifunctional metasurface-driven dynamic vectorial holographic color prints for photonic security platform

Vectorial holography has gained a lot of attention due to the promise of versatile polarization control of structured light for enhanced optical security and multi-channel optical communication. Here, we propose a bifunctional metasurface which combines both structural color printing and vectorial holography with eight polarization channels towards advanced encryption applications. The structural colour prints are observed under white light while the polarization encoded holograms are reconstructed under laser illumination. To encode multiple holographic images for different polarization states, a pixelated metasurface is adopted. As a proof-of-concept, we devise an electrically tunable optical security platform incorporated with liquid crystals. The optical security platform is doubly encrypted: an image under white light is decrypted to provide the first key and the corresponding information is used to fully unlock the encrypted information via projected vectorial holographic images. Such an electrically tunable optical security platform may enable smart labels for security and anticounterfeiting applications.

The authors present a bi-functional metasurface, combining structural color printing observed under white light and polarization encoded It is appropriate. vectorial holography. A pixelated design is used encode multiple holographic images, and they demonstrate an electrically tunable optical security platform.

Efficient All-Dielectric Diatomic Metasurface for Linear Polarization Generation and 1-Bit Phase Control

Optical metasurface has exhibited unprecedented capabilities in the regulation of light properties at a subwavelength scale. In particular, a multifunctional polarization metasurface making use of light polarization to integrate distinct functionalities on a single platform can be greatly helpful in the miniaturization of photonic systems and has become a hot research topic in recent years. Here, we propose and demonstrate an efficient all-dielectric diatomic metasurface, the unit cell of which is composed of a pair of a-Si:H-based nanodisks and nanopillars that play the roles as polarization-maintaining and polarization-converting meta-atoms, respectively. Through rigorous theoretical analyses and numerical simulations, we show that a properly designed diatomic metasurface can work as a nanoscale linear polarizer for generating linearly polarized light with a controllable polarization angle and superior performances including a maximum transmission efficiency of 96.2% and an extinction ratio of 32.8 dB at an operation wavelength of 690 nm. Three metasurface samples are fabricated and experimentally characterized to verify our claims and their potential applications. Furthermore, unlike previously reported dielectric diatomic metasurfaces which merely manipulate the polarization state, the proposed diatomic metasurface can be easily modified to empower 1-bit phase modulation without altering the polarization angle and sacrificing the transmission efficiency. This salient feature further leads to the demonstration of a metasurface beam splitter that can be equivalently seen as the integration of a nonpolarizing beam splitter and a linear polarizer, which has never been reported before. We envision that various metadevices equipping with distinct wavefront shaping functionalities can be realized by further optimizing the diatomic metasurface to achieve an entire 2π phase control.