Chiral Meta-atom Research Articles

Switchable Terahertz Metasurfaces for Spin-Selective Absorption and Anomalous Reflection Based on Vanadium Dioxide.

Conventional chiral metasurfaces are constrained by predetermined functionalities and have limited versatility. To address these constraints, we propose a novel chirality-switchable terahertz (THz) metasurface with integrated heating control circuits tailored for spin-selective anomalous reflection, leveraging the phase-change material vanadium dioxide (VO2). The reversible and abrupt insulator-to-metal phase transition feature of VO2 is exploited to facilitate a chiral meta-atom with spin-selectivity capabilities. By employing the Pancharatnam-Berry phase principle, complete 2π reflection phase coverage is achieved by adjusting the orientation of the chiral structure. At the resonant frequency of 0.137 THz, the designed metasurface achieves selective absorption of a circularly polarized wave corresponding to the state of the VO2 patches. Concurrently, it reflects the circularly polarized wave of the opposite chirality anomalously at an angle of 28.4° while maintaining its handedness. This chirality-switchable THz metasurface exhibits promising potential across various applications, including wireless communication data capacity enlargement, polarization modulation, and chirality detection.

2D-to-3D buckling transformability enabled reconfigurable metamaterials for tunable chirality and focusing effect

Recently, multifarious deformation approaches in nature have promoted dynamic manipulation for electromagnetic (EM) waves in metamaterials, and those representative strategies are mainly focused on the modulation of spectral parameters. Several works have also achieved tunable phase-gradient meta-devices. Here, to broaden the modulation freedom of mechanical deformation, we initially propose two reconfigurable metamaterials consisting of mirrored S-shaped meta-atoms selectively bonded on biaxially pre-stretched substrates. Planar meta-atoms with spin-insensitive transmittance are buckled into 3D morphologies to break residual symmetries by releasing the stress and to facilitate spin-dependent transmittance under circularly polarized incidence. Owing to the geometric anisotropy of S-shaped meta-atoms along the x and y axes, 3D chiral meta-atoms exhibit discriminate circularly cross-polarized transmittance under opposite spins. The underlying physical mechanism reveals that EM resonance originates from the excitation of electric dipoles and magnetic dipoles, and their cross coupling finally triggers the chiral effects of 3D meta-atoms. By introducing the gradient-phase design that keeps unchanged under various strains, two types of meta-atoms with specified orientations are interleaved to design a double-foci metalens, and its 2D-to-3D morphology transformation shortens the focusing length and facilitates the intensity change of two foci. Our approach in designing reconfigurable EM metamaterials with 2D-to-3D buckling transformability can be further extended toward terahertz even optical wavebands, and it may assist with deriving more applicable multi-functionalities in the aspects of imaging, sensing, and holograms.

Chiral metasurface zone plate for transmission-reflection focusing of circularly polarized terahertz waves.

The properties of traditional Fresnel zone plates have been greatly enhanced by metasurfaces, which allow the control of polarization, orbital angular momentum, or other parameters on the basis of focusing. In this Letter, a new, to the best of our knowledge, method for circularly polarized wave manipulation based on a zone plate is proposed. Chiral meta-atoms and binary geometric phase are used for the simultaneous focusing of reflected and transmitted terahertz waves. The silicon-based dielectric chiral units, which show great performance of spin-selective transmission near 0.54 THz, separate the orthogonal circularly polarized components. A binary Pancharatnam-Berry (P-B) phase gradient is obtained by rotating the unit 90 degrees, then the phase zone plate can be easily designed. The simulation results show that the proposed chiral metasurface zone plate has the function of reflection-transmission separation and focusing for the circularly polarized terahertz waves. In addition, we also demonstrate the possibility of using a 1064-nm continuous infrared laser to adjust the intensity of our devices, based on photo-generated carriers in silicon. The design principle of the chiral metasurface zone plates can be extended to other wavelengths, providing new ideas for the regulation of circularly polarized light.

Chirality-Assisted Phase Metasurface for Circular Polarization Preservation and Independent Hologram Imaging in Microwave Region

A chirality-assisted phase is introduced to innovatively impose separate phase profiles in two circular polarization (CP) preserving channels, namely, two co-polarized output components under left-handed and right-handed circularly polarized (LHCP and RHCP) illuminations. Such a scheme is physically implemented by a proposed multi-layered meta-atom that achieves chiral characteristics by inner twisting angles between adjacent functional elements. A proof-of-concept metasurface is constructed to realize separate holographic images in two polarization-preserved fields with different propagating distances, respectively. In order to achieve sufficient utilization of the two polarization-preserved components, a further approach that can suppress polarization conversion is proposed with a polyatomic unit composed of four separate chiral meta-atoms arranged in a 2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> 2 array. A dual-holographic metasurface is designed to impose characters “ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L$</tex-math> </inline-formula> ” and “ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R$</tex-math> </inline-formula> ” in two co-polarized fields with little energy distributed in corresponding cross-polarized fields. A prototype of the metasurface is fabricated and experimentally measured. The measurements agree well with theoretical predictions and full-wave simulations, showing in the co-polarized field a character “ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L$</tex-math> </inline-formula> ” under LHCP incidence and an “ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R$</tex-math> </inline-formula> ” under LHCP incidence. The proposed mechanism provides a theoretical foundation for further CP channel expansion and modulations and has great potential in multifunctional antenna design and wireless communications.

Full Complex-Amplitude Modulation of Surface Waves Based on Spin-Decoupled Metasurface.

This work proposes a method for surface wave (SW) coupling along with flexible complex amplitude modulation of its wavefront. The linearly polarized incident plane wave is coupled into the surface mode with complex wavefront by exploiting the spin-decouple nature of a reflective chiral meta-atom. As verification, two kinds of metasurface couplers are designed. The first kind contains two examples for SW airy beam generation with and without deflection under linearly polarized illumination, respectively. The second kind is a bi-functional device capable of SW focusing under left-handed circularly polarized illumination, and propagating wave deflection under right-handed circularly polarized illumination, respectively, to verify the fundamental spin-decoupled character. Simulated and experimental results are in good agreement. We believe that this method provides a flexible approach for complex SW applications in integrated optics, optical sensing, and other related fields.

Reflective one-to-multi-polarization conversion via separate control of phase and magnitude.

Polarization manipulation is a key issue in electromagnetic (EM) research. Research on 90° polarization rotators and circularly-polarized wave generators has been widely conducted. In this study, a polarization conversion metasurface that can shift one linearly-polarized EM wave into multi-polarization outgoing waves at certain frequencies is demonstrated, including co-, cross-, left-hand, and right-hand circular-polarization components. The surface was made of periodically arranged chiral meta-atoms. The polarization manipulation method is based on the independent control of phase and magnitude, in which the phase control is based on the Berry-phase theory of linearly-polarized EM waves, while the magnitude control is based on the cavity mode theory of the microstrip structure. Both eigenmode analysis (EMA) and characteristic mode analysis (CMA) were utilized for magnitude control, which was further verified by the surface current distribution. Finally, the metasurface was fabricated and measured, showing good agreement between the measured and simulated results. This research proposed what we believe to be a novel polarization method, which can be potentially applied in polarization manipulation, EM radiation, filters, wireless sensors, etc., over a frequency range from optics to microwave bands.

Thermal Effects on Optical Chirality, Mechanics, and Associated Symmetry Properties

A review is provided here about the thermal effects on optical chirality. To this goal, chiral objects dispersed in an embedding fluid are examined for their magnetoelectric coupling. Thermal effects on several chiral meta-atoms and their ensembles are examined. To this goal, DNA-like helical structures are examined in detail. The mechanical aspect of thermo-elasticity is reviewed along with transverse deformations while drawing analogies from condensed-matter physics. In this respect, the chirality-induced spin selection is reviewed along with the temperature-mediated electron–phonon interactions. A wide range of materials, such as polymers and biological cells, are also examined for temperature effects. A transition temperature delineating a sign flip in the chirality parameter is identified as well. Chirality-associated functionalities such as ratchet motions, switching, and modulations are investigated for their respective thermal effects. Issues of fabricating chiral meta-atoms are also discussed.

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.

A Low-RCS and High-Gain Planar Circularly Polarized Cassegrain Meta-Antenna

Achieving highly directive radiation with broadband operation, low scattering, and thin profile for a circularly polarized (CP) antenna is particularly challenging and yet rarely reported. Here, we propose a strategy of a CP Cassegrain meta-antenna by combining a planar helical antenna, a metasurface main reflector, and a metamaterial subreflector. The main reflector is designed to achieve focusing for CP waves at 13 GHz. The subreflector is chessboard-configured chiral metamaterial slab composed of two different types of chiral meta-atoms, aiming to achieve spin- and direction-selective CP transmissions and reflections. The distance between two reflectors is half of focal length, which enables our antenna to be dubbed as a folded reflectarray. The low radar cross section (RCS) is achieved based on scattering cancellation technique by realizing near 180° reflection phase difference between two neighboring chessboard submeta-atoms. Thanks to the architecture of the two reflectors, the proposed antenna exhibits high gain and low profile simultaneously according to image theory. For verification, a planar CP Cassegrain antenna, excited by a left-handed CP (LCP) planar helical antenna, is numerically studied, fabricated, and experimentally measured. Numerical results are in good agreement with the experimental ones, showing a peak right-handed CP (RCP) gain of 26.6 dBi at 12.6 GHz. Furthermore, the backward monostatic RCS of the antenna is dramatically reduced over −10 dB in a broad bandwidth from 8.4 to 15.7 GHz when it is illuminated by an LCP planar wave. Our proposed Cassegrain antenna features simultaneously broadband, high gain, low profile, and low RCS, providing a new avenue to low-profile CP reflectarrays with invisibility.

Metasurface with dynamic chiral meta-atoms for spin multiplexing hologram and low observable reflection

Spin light manipulation based on chiral metasurfaces is a striking hotspot that has intrigued huge attention. Circular dichroism, a unique phenomenon of chiral atoms/molecules, has been regarded as another auxiliary dimension for guiding electromagnetic waves, which has been explored in the field of artificial material sciences yet a challenging issue. Here, a generic strategy based on dynamic chiral meta-atom for revealing strong circular dichroism as well as applicable electromagnetic functionality is proposed in microwave regime. We demonstrate a dynamic metasurface that enables the fully independent holograms reconstruction for one circular polarization or the other at the active operating state. On the other hand, the electromagnetic scattering is realized for lowering observable backward reflection at the passive state. Numerical simulation and experimental verification are conducted to manifest the feasibility. It is expected that the proposed strategy can be applied to broaden the horizon for dynamic chiral meta-devices and may find applications in information encryption, anti-counterfeiting, and other dynamic systems.

All-metallic high-efficiency generalized Pancharatnam–Berry phase metasurface with chiral meta-atoms

Abstract Metasurfaces based on the Pancharatnam–Berry (PB) phase have attracted significant attention in the domains of subwavelength optics and electromagnetics. Conventional theory predicts that the PB phase is exactly twice the rotation angle of the anisotropic meta-atoms. Differently, a recent advance has demonstrated that the generalized PB phase representing multiple times of the rotation angle could be obtained with high-fold rotational symmetry meta-atoms, but it suffers from the low cross-polarization conversion efficiency (the theoretical upper limit of 25%) that impedes its further applications, especially for meta-atoms with rotational symmetry ≥3. Here, we verify that the chiral meta-atoms with high-fold rotational symmetries could produce the generalized PB phase. Besides, the all-metallic configuration is utilized to design C2, C3, and C5 chiral catenary meta-atoms to improve their efficiency and bandwidth. The equivalent air waveguide with low loss between two adjacent meta-atoms is formed to analyse the higher performances of the all-metallic scheme for the realization of the generalized PB phase compared with the metal–insulator–metal and all-dielectric C3 meta-atoms. As a proof of concept, four metadevices including two spin-Hall metadevices and two holograms are experimentally demonstrated and their maximum efficiency could exceed 83% in simulation. This work could provide a high-efficiency platform for the study of the generalized PB phase in linear and nonlinear optics.

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.

An infrared energy harvester based on radar cross-section reduction of chiral metasurfaces through phase cancellation approach

Conventional metasurface absorbers rely on high dissipation losses by incorporating lossy materials. In this paper, we propose a novel mechanism of absorption based on phase cancellation of polarization states of scattered fields emerging from adjacent L-shaped chiral meta-atoms (unit cells). A linearly polarized wave forms helicoidal currents in each meta-atom leading to diagonally polarized radiated waves. When phase cancellation is employed by reorienting four such meta-atoms in a supercell configuration, contra-directed chiral currents flow in adjacent cells to cancel all the radiated fields in far-field region leading to a minimal broadside radar cross-section. From the reciprocity, the currents that are induced in the meta-atoms produce a null towards the incident direction which can be utilized for infrared energy harvesting. Full wave electromagnetic simulation indicates near perfect resonant absorption around 52.2 THz frequency. Enhanced bandwidth is shown by adding smaller resonators inside the supercell in nested form leading to dual band absorption at 45.2 THz and 53.15 THz.

Metasurfaces with Planar Chiral Meta-Atoms for Spin Light Manipulation

Spin light (i.e., circularly polarized light) manipulation based on metasurfaces with a controlled geometric phase (i.e., Pancharatnam-Berry (PB) phase) has achieved great successes according to its convenient design and robust performances, by which the phase control is mainly determined by the rotation angle of each meta-atom. This PB phase can be regarded as a global effect for spin light; here, we propose a local phase manipulation for metasurfaces with planar chiral meta-atoms. Planar chiral meta-atoms break fundamental symmetry restrictions and do not need a rotation for these kinds of meta-atoms to manipulate the spin light, which significantly expands the functionality of metasurface as it is incorporated with other modulations (e.g., PB phase, propagation phase). As an example, spin-decoupled holographic imaging is demonstrated with robust and broadband properties. Our work definitely enriches the design of metasurfaces and may trigger more exciting chiral-optics applications.

A dual band spin-selective transmission metasurface and its wavefront manipulation.

Chiral metasurfaces which can achieve different optical responses for left-handed and right-handed circularly polarized (CP) light have been proposed. Most of the research studies on chiral metasurfaces focus on improving circular dichroism (CD) and realizing dynamic manipulation of the chiro-optical response. However, there have only been a few reports on the multi-band chiro-optical response. Here, we propose an all-silicon chiral meta-atom which can realize spin-selective transmission in a dual band. In addition, a terahertz metasurface with spin-selective transmission through phase arrangement is designed by using chiral meta-atoms satisfying a gradient geometric phase. Under left-hand circularly polarized (LCP) incidence, the metasurface generates a focused right-hand circularly polarized (RCP) beam which is focused at a distance of 4.8 mm from the exit surface of the metasurface. Our work broadens the concept of metasurface design and may attract more researchers' attention on the applications of chiral metasurfaces.

Multipole decomposition for interactions between structured optical fields and meta-atoms.

Interactions between structured optical fields (SOFs) and meta-atoms have been intensively studied, and stimulated by recent advancements on the generation of SOFs and on the synthesis of exotic meta-atoms. Multipole expansion is an efficient and accurate theoretical framework for studying such problems. In this work, explicit expressions of SOFs and their beam-shape coefficients are provided, and their properties are also briefly discussed; the considered SOFs include Laguerre-Gaussian (LG) beams, tightly-focused LG beams, Bessel beams, and cylindrical vector beams. Using the multipole expansion, selective excitations of multipolar resonances of a sphere is discussed. In addition, angular momentum dichroisms of a chiral sphere and an anisotropically chiral meta-atom are calculated to demonstrate selective excitation of multipoles with the desired order, parity, and orientation using engineered SOFs with angular momentum.

Diodelike Spin-Orbit Interactions of Light in Chiral Metasurfaces

Diodelike spin-orbit interactions (SOIs) of light at metasurfaces bring novel functionalities to optical devices but have never been achieved so far. In this paper, we experimentally demonstrate a diodelike asymmetric metadevice that exhibits asymmetric SOIs of light in chiral metasurfaces. The exploited physical mechanism is based on the maximization of the polarization conversion for a designated circularly polarized light in transmission. Chiral metaatoms are employed to enhance the interaction between spin angular momentum and matter, while phase discontinuities are introduced at the interface for helicity-dependent generation of orbital angular momentum. Three proof-of-concept metasurfaces that demonstrate asymmetric circular polarization conversion, asymmetric spin deflection, and asymmetric spin-to-vortex conversion are designed, fabricated, and experimentally characterized, respectively. Compared with the Faraday effect under the external magnetic field, the designer metasurfaces here are all reciprocal and offering asymmetric angular momentum division with high efficiency. Our findings provide the possibility of asymmetric multifunctional metadevices for high-performance on-chip parallel multiplexing.

Observation of asymmetric electromagnetic field profiles in chiral metamaterials

We experimentally observe asymmetric electromagnetic field profiles along two-dimensional chiral metamaterials. The asymmetric field profiles depending on the chirality and the operation frequency have been reproduced well by the numerical simulation. Around a chiral meta-atom, distribution of a Poynting vector is found to be shifted asymmetrically. These results are explained in terms of an analogy with the side-jump mechanism in the electronic anomalous Hall systems.

Microwave analog of Stern-Gerlach effects using nonuniform chiral metamaterials

This study observes microwave beam splitting dependent on circular polarizations through nonuniform chiral metamaterials. Nonuniform chiral metamaterials with a gradation in refractive index are constructed using chiral meta-atoms that exhibit optical activities at microwave frequencies. Microwave scattering far-field patterns by the nonuniform chiral metamaterials demonstrate a deflection of microwaves, which transmit in a direction perpendicular to the refractive index gradient. Furthermore, circularly polarized microwaves with opposite ``spins of light'' go their separate ways in the nonuniform chiral metamaterials. This phenomenon is an optical analog of the Stern-Gerlach effects for electrons.

A Mirror Shape Chiral Meta Atom for C-Band Communication

This paper reports that a miniaturized chiral meta-atom has 5.14-GHz bandwidth and applicable for $C$ -band applications. The meta-atom is developed by an electric inductive-capacitive resonator, where the inverse E-shape structure is combined with the outer ring resonator. The dimension of the proposed single unit cell structure is $6.0\times 6.0$ mm2 that is printed on a Rogers RT 5880 material. Finite integration technique-based CST Microwave Studio has been utilized to design, simulation, and investigation purposes. An Agilent N5227A vector network analyser is used to retrieval the effective parameters and the measured and simulated results are well arranged together. The negative refraction bandwidth of 5.14 GHz (from 4.0 to 9.14 GHz) and the effective medium ratio 6.96 make the designed meta-atom is suitable for practical applications.