Anisotropic Metasurface Research Articles

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.

Hybrid anisotropic plasmonic metasurfaces with multiple resonances of focused light beams.

Plasmonic metasurfaces supporting collective lattice resonances have attracted increasing interest due to their exciting properties of strong spatial coherence and enhanced light-matter interaction. Although the focusing of light by high-numerical-aperture (NA) objectives provides an essential way to boost the field intensities, it remains challenging to excite high-quality resonances by using high-NA objectives due to strong angular dispersion. Here, we address this challenge by employing the physics of bound states in the continuum (BICs). We design a novel anisotropic plasmonic metasurface combining a two-dimensional lattice of high-aspect-ratio pillars with a one-dimensional plasmonic grating, fabricated by a two-photon polymerization technique and gold sputtering. We demonstrate experimentally multiple resonances with absorption amplitudes exceeding 80% at mid-IR using an NA = 0.4 reflective objective. This is enabled by the weak angular dispersion of quasi-BIC resonances in such hybrid plasmonic metasurfaces. Our results suggest novel strategies for designing photonic devices that manipulate focused light with a strong field concentration.

Polarization conversion in anisotropic dielectric metasurfaces originating from bound states in the continuum.

We use the semianalytical Cartesian multipole method to investigate the light transmission and reflection spectra of anisotropic dielectric metasurfaces, and extend the multipole decompositions method to account for cross-polarization conversion effects. We observe sharp high-Q resonances arising from a distortion of symmetry-protected bound states in the continuum in asymmetric dielectric metasurfaces, i.e., quasibound states in the continuum. In addition, by further introducing in-plane symmetric breaking perturbation, the polarization conversions of linearly polarized light can be achieved through quasibound states in the continuum. With the aid of temporal coupled-mode theory, we can obtain the limit of cross-conversion under single high-Q resonance, i.e., 0.25. Our work will help to design dielectric metasurfaces to control the polarization states of light.

Chiral sensing with achiral anisotropic metasurfaces

Recently, we proposed a metasurface design for chiral sensing that (i) results in enhanced chiroptical signals by more than two orders of magnitude for ultrathin, subwavelength, chiral samples over a uniform and accessible area, (ii) allows for complete measurements of the total chirality (magnitude and sign of both its real and imaginary part), and (iii) offers the possibility for a crucial signal reversal (excitation with reversed polarization) that enables chirality measurements in an absolute manner, i.e., without the need for sample removal. Our design is based on the anisotropic response of the metasurface, rather than the superchirality of the generated near-fields, as in most contemporary nanophotonic-based chiral sensing approaches. Here, we derive analytically, and verify numerically, simple formulas that provide insight to the sensing mechanism and explain how anisotropic metasurfaces, in general, offer additional degrees of freedom with respect to their isotropic counterparts. We provide a detailed discussion of the key functionalities and benefits of our proposed design and we demonstrate practical measurement schemes for the unambiguous determination of an unknown chirality. Last, we provide the design principles towards broadband operation - from near-infrared to near-ultraviolet frequencies - opening the way for highly sensitive nanoscale chiroptical spectroscopy.

Amplification and modulation effect of elliptical surface polaritons on a thermal diode

With the development of surface polaritons of two-dimensional metasurfaces with high mode density, considerable attention has been paid to their application in optimising the performance of the thermal diode; however, presently, the main focus is on isotropic metasurfaces. In this study, we investigated the amplification and modulation effect of elliptical surface polaritons on a near-field radiative thermal diode. Introducing black phosphorus (BP) in forward and reverse terminals of the thermal diode (comprising VO2 and Si) demonstrated prominent amplification of rectification performance (of ∼3.59 times) in the modified thermal diode than in the one without BP. Selecting different electron densities of BP and different doping densities of Si revealed the influence of the hybridisation effect of elliptical polaritons on thermal rectification and near-field radiative heat transfer. Finally, based on the in-plane intrinsic anisotropic lattice of BP, the modified thermal diode facilitated highly flexible manipulation of rectification performance through mechanical rotating, which to the best of our knowledge, has never been discussed in the field of near-field radiative diodes. Our findings provides significant insights to intensify asymmetric energy transfer and explore the radiative heat transfer mechanism of a coupled system comprising an anisotropic metasurface and an out-plane anisotropic material.

Heterogeneous Amplitude−Phase Metasurface for Distinct Wavefront Manipulation

Achieving simultaneously multiple distinct wavefront manipulations using a single flat plate is pivotal in increasing the integration level and information capacity of an optoelectronic system. As of today, the state‐of‐the‐art metadevices have been devoted to multiple functionalities by imparting two independent phase patterns triggered at two orthogonal polarization states. However, in fact, it is terribly challenging to realize individual amplitude−phase (A−P) control using an anisotropic metasurface. Herein, a heterogeneous strategy for achieving an A−P manipulation utilizing heterogeneous indium tin oxide (ITO) resistive films and metal patterns is demonstrated. By synergizing dual‐layer ITO films and a sandwiched dual‐mode metallic layer on a back metallic ground, the proposed meta‐atom exhibits near‐zero and near‐unity reflections for dual‐orthogonal linear polarizations. The above feature can be utilized to trigger polarization‐dependent functionalities by uniformly and inhomogeneously distributing the ITO and metallic pattern, respectively. Using this proposed architecture, two metasurfaces with integrated radiation−absorption and integrated diffusion−absorption function are implemented. Both numerical and experimental results demonstrate that two well‐designed metadevices enable to achieve a similar broadband absorptivity (above 90% within 7.8−18.5 GHz) but completely distinct wavefront manipulations triggered by two polarizations. The heterogeneous metasurface concept paves the way toward realizing high‐performance multifunctionalities with complex wavefront manipulation capabilities.

Realizing of cross-polarization conversion for linearly polarized waves in multiband by orthotropic metasurface

A multiband reflective linear-polarization (LP) converter based on an anisotropic metasurface is proposed. The unit cell of the metasurface is composed of a hollow oval sheet and a rectangle patch. Simulation results show that the converter can convert LP wave to its orthogonal state and the polarization conversion ratio (PCR) is above 90% in four frequency bands (4.60–12.27 GHz, 13.79–13.90 GHz, 14.61–14.69 GHz, 15.44–15.52 GHz). Interestingly, the relative bandwidth of first band reaches up to 90.9%, which is even broader than most single-band polarization conversion metasurfaces. Furthermore, we analyze the physical mechanism of polarization conversion of the metasurface, and simplify the derived formulas to make approximate calculations for co- and cross-polarization coefficients. The proposed formulas have better applicability than the equations in previous work. The laboratory sample is fabricated, and we have experimentally characterized our idea in microwave region. This design can advance applications in microwave sensors and mm-wave systems.

High-Efficiency, Dual-Band Beam Splitter Based on an All-Dielectric Quasi-Continuous Metasurface.

Metasurface-based beam splitters attracted huge interest for their superior properties compared with conventional ones made of bulk materials. The previously reported designs adopted discrete metasurfaces with the limitation of a discontinuous phase profile. In this paper, we propose a dual-band beam splitter, based on an anisotropic quasi-continuous metasurface, by exploring the optical responses under x-polarized (with an electric field parallel to the direction of the phase gradient) and y-polarized incidences. The adopted metasurface consists of two identical trapezoidal silicon antenna arrays with opposite spatial variations that lead to opposite phase gradients. The operational window of the proposed beam splitter falls in the infrared and visible region, respectively, for x- and y-polarized light, resulting from the different mechanisms. When x-polarized light is incident, the conversion efficiency and total transmission of the beam splitter remains higher than 90% and 0.74 within the wavelength range from 969 nm to 1054 nm, respectively. In this condition, each array can act as a beam splitter of unequal power. For y-polarized incidence, the maximum conversion efficiency and transmission reach approximately 100% and 0.85, while the values remain higher than 90% and 0.65 in the wavelength range from 687 nm to 710 nm, respectively. In this case, each array can be viewed as an effective beam deflector. We anticipate that it can play a key role in future integrated optical devices.

Dynamic control of THz polarization modulation and multi-channel beam generation using a programmable metasurface.

Polarization modulation and multichannel beam generation are crucial in multichannel communication and high-resolution imaging at THz frequency. In this work, we present a polarization-reprogrammable coding metasurface composed of VO2/Au composite concentric rings (CCRs). Owing to the phase-change property of VO2, the CCR is designed as a digital coding element for the polarization conversion. When VO2 remains insulator state at room temperature, the y-polarized incident wave is transformed into x-polarized wave, which can be regarded as digital state 0. When VO2 converts into metal state at critical temperature (68°C), the polarization of reflected wave stays unchanged, corresponding to digital state 1. Any desired linear polarization state of reflected beam is achieved by taking advantage of different coding sequences in a programmable manner. Furthermore, by combining phase gradient with polarization coding states, we propose an anisotropic programmable metasurface to control the multi-channel reflected beams dynamically. By arranging distinct coding sequences, we show that the EM reflected beams can be manipulated flexibly. The proposed programmable metasurface paves new ways towards THz polarization manipulation, signal detection and information communication.

Compact quasi-optical mode converter based on anisotropic metasurfaces

In this paper, a novel compact quasi-optical mode converter based on anisotropic metasurfaces for high-order mode terahertz electronic devices is presented. To demonstrate the design model, a Ka-band metasurface quasi-optical mode converter that converts cylindrical waveguide TE01 mode to circularly polarized Gaussian beam is designed and fabricated. Both electromagnetic simulation and experiment results show that the Gaussian beam can be observed from 35 to 38 GHz, corresponding to over 8.5% of the bandwidth. The maximum scalar Gaussian mode content of 97.85% is observed in the experiment, and the output radiation from the metasurface quasi-optical mode converter is approximate circular polarization. This work unveils the potential of compact quasi-optical mode converter based on metasurfaces.

Polarization Multiplexing Hologram Realized by Anisotropic Digital Metasurface

AbstractMetasurface holograms have the potential to be applied in a variety of practical fields. Here an anisotropic digital metasurface is proposed that can project polarization multiplexing patterns at the microwave frequency, in which a modified Gerchberg–Saxton algorithm is presented to calculate the phase masks. A 3‐bit coding metamaterial unit is also proposed, which is capable of independently manipulating the transmission phases of orthogonal linearly polarized electromagnetic waves. The influence of wave‐front has been analyzed by comparing two holographic results with distinct source optimization methods. A prototype of the anisotropic digital coding metasurface optimized with horn wave‐front is designed, manufactured, and tested. The experimental measurements present the expected holographic fields and have a good match with the simulation results.

Anisotropic digital metasurfaces relying on silicon Mie resonance

We propose an anisotropic terahertz metasurface based on silicon Mie resonance for achieving terahertz wave beam deflection manipulation. The presented coding metasurface can achieve various functions for two orthogonal linear polarized terahertz waves incidence. By elaborately designing 1-bit and 2-bit anisotropic coding metasurface with different coding matrix, we verify the anisotropic coding metasurface having excellent performance on manipulation reflection beam numbers and beam deflection angles under different polarization waves incidence. The proposed anisotropic terahertz metasurface provides an efficient and flexible method for regulation terahertz wave and has the potential for the promising applications of terahertz MIMO communication systems.

A novel ultra-wideband reflective cross-polarization converter based on anisotropic metasurface

ABSTRACT An ultra-thin and ultra-wideband anisotropic metasurface, acting as a linear polarization converter through reflection is designed and simulated in this research. The operation frequency range covers almost completely the Ku-band and the entire K- and Ka-band. The unit cell of the proposed metasurface is composed of a arrangement of concentric square metallic loops on a grounded dielectric substrate, in which a diagonal split is made, providing properties of an anisotropic material. Simulation results show that an efficient cross-polarization conversion, both for normal as well as for oblique incidence, is achieved with dB fractional bandwidth of 98.7% from 13.8 GHz to 40.7 GHz and with an average polarization conversion ratio of 97.5%. The ultra-wideband of the proposed structure results from multiple plasmonic resonances occurring at three neighboring frequencies (15.2 GHz, 25.9 GHz, and 38 GHz) resulting in a significant expansion of the operating frequency range.

Broadband linear‐to‐circular polarization reflector using anisotropic metasurface

A high-performance polarization reflector is introduced based on anisotropic metasurface. This structure can convert a linearly polarized (LP) incident electromagnetic (EM) wave into a circularly polarized (CP) reflection wave. The unit cell of the proposed metasurface is made of metallic patch imprinted on the top surface of a metal-backed single-layer dielectric substrate. The shape of this metallic patch is a combination of a deformed Jerusalem cross and two deformed L. The simulation results show that the LP incident EM wave is converted to CP reflected wave from 7 to 19.4 GHz, which the bandwidth of axis ratio less than 3 dB is 12.4 GHz and the relative bandwidth is 94%. The current distributions and equivalent circuit model are given to analyze the generation of circularly polarized waves. To verify the metasurface structure performance, a sample consisting of 24 × 24 unit cells is fabricated and measured. The experimental and simulation 3-dB AR bandwidth are basically consistent with each other, which verify the properties of the design. The proposed metasurface structure has potential application in microwave sensors and antenna design to manipulate the polarization states of EM waves.

Plasmonic modes at inclined edges of anisotropic two-dimensional materials

Confined modes at the edge arbitrarily inclined with respect to optical axes of nonmagnetic anisotropic 2D materials are considered. By developing the exact Wiener-Hopf and approximated Fetter methods we studied edge modes dispersions, field and charge density distributions. The 2D layer is described by the Lorentz-type conductivities in one or both directions, which is realistic for natural anisotropic 2D materials and resonant hyperbolic metasurfaces. We demonstrate that, due to anisotropy, the edge mode exists only at wave vectors exceeding the nonzero threshold value if the edge is tilted with respect to the direction of the resonant conductivity. The dominating contribution to field and charge density spatial profiles is provided by evanescent 2D waves, which are confined both in space near the 2D layer and along the layer near its edge. The degree of field confinement along the layer is determined by wave vector or frequency mismatch between the edge mode and continuum of freely propagating 2D modes. Our analysis is suitable for various types of polaritons (plasmon, phonon, exciton polaritons, etc.) at large enough wave vectors. Thanks to superior field confinement in all directions perpendicular to the edge these modes look promising for modern plasmonics and sensorics.

High-Q perfect absorption induced by the coupling of LSP and SPP modes

At optical frequencies, we design an anisotropic metasurface to realize high-Q perfect absorption in numerical calculation. By establishing the Norton equivalent network model, we first reveal that the root cause of this phenomenon is the coupling excitation of the localized surface plasmon (LSP) mode and the surface plasmon polariton (SPP) mode. This absorber can realize bifunctional switching and is expected to realize the energy conversion of photons–SPP–photons. Meanwhile, we analyze the control of the structural design on high-Q perfect absorption. This research will provide theoretical guidance for the further development of applications related to high-Q perfect absorption such as absorption filters with specific wavelengths and narrow band photon emitters.

Theory and algorithm for tunable mantle cloaking through bulk Dirac semimetal metasurfaces around elliptical cylinders

In recent years, invisibility has become a research area of increasing interest due to the advances in material engineering. It may be possible to achieve invisibility through cloaking structures based on one or more layers of materials with proper electromagnetic properties. In this regards, we propose and analyze a bulk Dirac semimetal (BDS) cloaking metasurface to achieve scattering cancelation in the terahertz (THz) spectrum. A monolayer of the BDS as a mantle cloak is applied around a dielectric elliptical cylinder to achieve a tunable scattering width. The effectiveness of the designed cover is checked with a full-wave simulation based on the finite element method (FEM) as well as an analytical approach. A good agreement is observed between the results of the analytic method and those of the full-wave simulation. To achieve a mantle cloaking for TE and TM polarizations, an anisotropic metasurface should be applied. Therefore, we introduce the BDS strips as a mantle cloaking around an SiO2-coated PEC elliptical cylinder and propose an algorithm to determine the features of the anisotropic cover.

Simple design of efficient broadband multifunctional polarization converter for X-band applications

A simple design of a broadband multifunctional polarization converter using an anisotropic metasurface for X-band application is proposed. The proposed polarization converter consists of a periodic array of the two-corner-cut square patch resonators based on the FR-4 substrate that achieves both cross-polarization and linear-to-circular polarization conversions. The simulated results show that the polarization converter displays the linear cross-polarization conversion in the frequency range from 8 to 12 GHz with the polarization conversion efficiency above 90%. The efficiency is kept higher than 80% with wide incident angle up to 45°. Moreover, the proposed design achieves the linear-to-circular polarization conversion at two frequency bands of 7.42–7.6 GHz and 13–13.56 GHz. A prototype of the proposed polarization converter is fabricated and measured, showing a good agreement between the measured and simulated results. The proposed polarization converter exhibits excellent performances such as simple structure, multifunctional property, and large cost-efficient bandwidth and wide incident angle insensitivity in the linear cross polarization conversion, which can be useful for X-band applications. Furthermore, this structure can be extended to design broadband polarization converters in other frequency bands.

Half-wave plate based on a birefringent metamaterial in the visible range

In this paper, a half-wave plate (HWP) based on a birefringent metamaterial is numerically designed to operate in the visible range. The proposed structure consists of an array of double-pattern perpendicular rectangular apertures (RAA) engraved into opaque silver film deposited on a glass substrate. One of the apertures is glass filled. The operating principle is based on the excitation and the propagation of one guided mode inside each aperture but with different effective indices. After transmission, a phase difference is obtained between the two transverse components, which value depends mainly on the metal thickness. We have investigated the simplest configuration using a homemade code based on the finite difference time domain method to design a half-wave plate with optimized efficiency resulting in transmission coefficient of more than 60% together with a birefringence of 2.1 at an operation wavelength of 737nm. This kind of anisotropic metasurfaces promises a wide range of applications in integrated photonics.

Polarization-dependent and tunable absorption of terahertz waves based on anisotropic metasurfaces

Metamaterial absorbers can achieve high-efficiency electromagnetic absorption in a specific band, which have been used in biochemical sensing, photoelectric detection, imaging and other fields. Tunable metamaterial absorbers provide more possibilities for the development of multifunctional electromagnetic absorption devices. Here we propose a tunable and polarization-dependent terahertz metamaterial absorber which can work for both linearly and circularly polarized waves. By introducing single layer graphene and vanadium dioxide (VO2), switching between the two working states and wide-range tuning of the absorption efficiency are realized. When VO2 is in insulating state, the absorber shows different tunable absorption performance for the x- or y-polarized terahertz waves, in which the maximum absorption rate is close to 100%. When VO2 is in metallic state, the metasurface behaves as a chiral absorber, and the maximum absorption difference between the two circular polarizations is about 0.45, while the tuning efficiency reaches 86.3%. Under the two working conditions, the absorber can maintain efficient absorption with a large incident angle. In addition, as an application exploration of the absorber, we demonstrated its application in tunable and polarization multiplexed near-field image display.