Lossless Metasurfaces Research Articles

A Quasi-Bound States in the Continuum Dielectric Metasurface-Based Antenna-Reactor Photocatalyst.

Metasurfaces are a class of two-dimensional artificial resonators, creating new opportunities for strong light-matter interactions. One type of nonradiative optical metasurface that enables substantial light concentration is based on quasi-Bound States in the Continuum (quasi-BIC). Here we report the design and fabrication of a quasi-BIC dielectric metasurface that serves as an optical frequency antenna for photocatalysis. By depositing Ni nanoparticle reactors onto the metasurface, we create an antenna-reactor photocatalyst, where the virtually lossless metasurface funnels light to drive a chemical reaction. This quasi-BIC-Ni antenna-reactor drives H2 dissociation under resonant illumination, showing strong polarization, wavelength, and optical power dependencies. Both E-field-induced electronic and photothermal heating effects drive the reaction, supported by load-dependent reactivity studies and our theoretical model. This study unlocks new opportunities for photocatalysis that employ dielectric metasurfaces for light harvesting in an antenna-reactor format.

A single-layered elastic metasurface for switching wide-angle asymmetric transmission of flexural waves

A limitation of present elastic metasurfaces remains in their modest flexibility to meet convertible functions on demand. Here, a feasible single-layered lossless metasurface is theoretically proposed and experimentally demonstrated for adjusting the asymmetric transmission of flexural waves. The easily reconstructed unit is derived from multiple pillared resonators; then, the number of units per period can be changed depending on the desired integer parity. In addition, the asymmetric transmission is physically realized by the uneven diffraction of the ±1st orders in opposite fields of the designed metasurface. Requiring neither active modules nor passively multilayer or loss-induced strategies, our design using only a layer of lossless metasurface allows the elastic-wave behavior to switch between efficient symmetric and asymmetric transmissions. Furthermore, a high contrast ratio of transmitted energy is verified in experiments and simulations within a wide-angle range. The present work is connected with the pragmatic applications of metasurfaces in timely directional vibration control and compactly elastodynamic rectifications.

Unlocking the out-of-plane dimension for photonic bound states in the continuum to achieve maximum optical chirality

The realization of lossless metasurfaces with true chirality crucially requires the fabrication of three-dimensional structures, constraining experimental feasibility and hampering practical implementations. Even though the three-dimensional assembly of metallic nanostructures has been demonstrated previously, the resulting plasmonic resonances suffer from high intrinsic and radiative losses. The concept of photonic bound states in the continuum (BICs) is instrumental for tailoring radiative losses in diverse geometries, especially when implemented using lossless dielectrics, but applications have so far been limited to planar structures. Here, we introduce a novel nanofabrication approach to unlock the height of individual resonators within all-dielectric metasurfaces as an accessible parameter for the efficient control of resonance features and nanophotonic functionalities. In particular, we realize out-of-plane symmetry breaking in quasi-BIC metasurfaces and leverage this design degree of freedom to demonstrate an optical all-dielectric quasi-BIC metasurface with maximum intrinsic chirality that responds selectively to light of a particular circular polarization depending on the structural handedness. Our experimental results not only open a new paradigm for all-dielectric BICs and chiral nanophotonics, but also promise advances in the realization of efficient generation of optical angular momentum, holographic metasurfaces, and parity-time symmetry-broken optical systems.

Multiple beams direction and power control using passive lossless metasurface by surface waves

In this work, a passive lossless metasurface can arbitrarily control multiple beams' direction and power utilizing surface waves is proposed. The incident wave is coupled into surface wave on the metasurface, then the surface wave propagates along the metasurface, and continuously decouples into multiple radiation beams in the one side of the metasurface. Moreover, these multiple radiation beams’ direction and power can be arbitrary controlled by the design of surface impedance of metasurface. In our work, a method for accurate estimation of coupled (decoupled) surface wave amplitude is established, so the surface impedance determined by the surface wave and the radiation beams can be precisely designed and passively lossless. The metasurface having two equal power radiation beams with −300 and 600 radiation angles, and the metasurface having two unequal power radiation beams also with −300 and 600 radiation angles are designed as examples, respectively, verifying our design method.

Direct Observation of Lateral Field Confinement in Symmetry‐Protected THz Bound States in the Continuum

AbstractElectromagnetic field confinement on sub‐wavelength scales is possible at the expense of increasing optical losses. Examples are surface plasmon polaritons and resonant structures, which suffer from absorption losses and radiative outcoupling, respectively. Here, the first experimental evidence of strong electromagnetic field confinement along the lateral (out‐of‐plane) direction in a near lossless metasurface that supports a symmetry‐protected Bound State in the Continuum is shown, which fully suppresses the radiative losses of resonant scatterers. This experimental demonstration, supported by coupled dipole calculations, is measured by a double near‐field probe technique to locally excite and detect the time evolution and confinement of the electric field onto the surface, achieving an amplitude decay length from the metasurface of λ/38 at 0.39 THz.

Design of Planar and Conformal, Passive, Lossless Metasurfaces That Beamform

A general technique for synthesizing both planar and conformal beamforming metasurfaces is presented that utilizes full-wave modeling techniques and rapid optimization methods. The synthesized metasurfaces consist of a patterned metallic cladding supported by a finite-size grounded dielectric substrate. The metasurfaces are modeled using integral equations which accurately account for mutual coupling and the metasurface's finite dimensions. The synthesis technique consists of three phases: a direct solve phase to obtain an initial metasurface design with complex-valued impedances satisfying the desired far-field beam specifications, a subsequent optimization phase that converts the complex-valued impedances to purely reactive ones, and a final patterning phase to realize the purely reactive impedances as a patterned metallic cladding. The optimization phase introduces surface waves which facilitate passivity. The metasurface is optimized using gradient descent with a semi-analytic gradient obtained using the adjoint variable method. Three examples are presented: a low-profile directly-fed metasurface antenna with near perfect aperture efficiency, a scanned-beam reflectarray design with controlled sidelobes, and a conformal metasurface reflectarray. The far-field and near-field performance of the metasurfaces are verified and the bandwidth and loss tolerance of the metasurfaces are investigated.

Extreme Beam-Forming With Impedance Metasurfaces Featuring Embedded Sources and Auxiliary Surface Wave Optimization

We present the end-to-end design of compact passive and lossless metasurface (MTS) antennas with integrated feeds. The complete low-profile system consists of a single-layered reactive impedance MTS printed on top of a grounded dielectric substrate, and is fed by sources which are embedded inside the substrate. An accurate and efficient volume-surface integral equation-based model of the device is developed, and used as the basis for the rapid optimization of its performance. The optimized designs leverage tailored auxiliary surface waves supported by the impedance MTS to distribute the localized source power across their apertures. This facilitates the realization of extreme field transformations such as wide-angle beam-forming or shared-aperture beam-forming, with nearly 100% aperture efficiencies. The procedure also allows for arbitrary beam-shaping with complete main beam and side lobe control. We also derive several feasibility-related constraints, which can significantly enhance the power efficiency as well as the bandwidth of the MTS antennas when they are implemented in practice. Full-wave numerical simulations confirm the effectiveness of the presented approach, as well as the extreme field transformation capabilities of the synthesized designs.

A hybrid coupler for directing quantum light emission with high radiative Purcell enhancement to a dielectric metasurface lens

Quantum photonic technologies such as quantum sensing, metrology, and simulation could be transformatively enabled by the availability of integrated single photon sources with high radiative rates and photon collection efficiencies. We address these challenges for quantum emitters formed from color center defect sites such as those in hexagonal boron nitride, which are promising candidates as single photon sources due to their bright, stable, polarized, and room temperature emission. We report design of a nanophotonic coupler from color center quantum emitters to a dielectric metasurface lens. The coupler is comprised of a hybrid plasmonic–dielectric resonator that achieves a large radiative Purcell enhancement and partial control of far-field radiation. We report radiative Purcell factors up to 285 and photon collection efficiencies up to 89% for a lossless metasurface, applying a continuous hyperboloidal phase-front. Our hybrid plasmonic–dielectric coupler interfacing two nanophotonic elements is a compound optical element, analogous to those found in microscope objective lenses, which combine multiple optical functions into a single component for improved performance.

Large and efficient unidirectional plane-wave–surface-wave metasurface couplers based on modulated reactance surfaces

Planar metasurfaces for efficient unidirectional conversion of a plane wave into a surface wave are presented. They are designed as a reciprocal device of a decoupler or leaky-wave antenna based on spatially modulated reactance surfaces. In a surface wave-to-leaky wave conversion, the surface reactance is determined from optimization of the complete aperture field over the propagating and evanecent spectra such that the conversion surface becomes locally and globally passive and lossless, while the desired wave conversion is achieved. The coupling efficiency is given by the aperture efficiency of the reciprocal antenna, which can closely approach unity for large apertures. The optimized surface reactance is spatially discretized and translated into physical metasurfaces, comprising an array of subwavelength inclusions. In TM polarization, three lossless metasurface design examples of 40 and 80 wavelengths are provided with performance validation using full-wave simulations: a metallic groove array, a thickness-modulated grounded dielectric substrate, and a width-modulated grounded dielectric ridge array. Deviations of the surface reactance from the optimized distribution reduce the conversion efficiency, but high coupling efficiencies in excess of 98% for the metallic structure and 60% for dielectric structures are predicted.

Backscattering cross-section from a metamaterial coated sphere covered with a metasurface

AbstractAn analysis about the backscattering characteristics of a metamaterial coated magnetodielectric sphere covered with a metasurface has been presented. The effects of various types of metamaterial coatings and surface reactances of lossless metasurface upon the backscattering cross-section of a metamaterial coated magnetodielectric sphere covered with a metasurface have been studied. It is shown that the negligible backscattering cross-section from a double near-zero metamaterial coated magnetodielectric sphere can be enhanced significantly by using specific types of lossless metasurfaces. These types of enhanced backscattering cross-section find applications in the radar detection problems. The proposed theory is also extended to the lossy double negative metamaterial coated magnetodielectric sphere covered with a lossless metasurface. During the study, it is found that for a specific part of the lossy double negative metamaterial bandwidth, two specific types of lossless metasurfaces can be used to reduce the backscattering cross-section as compared to the backscattering cross-section of a lossy double negative metamaterial coated magnetodielectric sphere without metasurface.

Efficient Asymmetric Transmission of Elastic Waves in Thin Plates with Lossless Metasurfaces

Requiring neither active components nor complex designs, we propose and experimentally demonstrate a generic framework for undistorted asymmetric elastic-wave transmission in a thin plate just using a layer of lossless metasurface. The asymmetric transmission stems from the uneven diffraction of +1 and -1 orders on opposite sides of the metasurface, respectively. Compared with previous loss-induced strategies, the present metasurface maintains a nearly total transmission for the transportation side, but a total reflection from the opposite side, exhibiting a higher contrast ratio of transmission. Moreover, we illustrate that this strong asymmetric behavior is robust to the frequency, the incident angle and the loss effect. The present work paves new avenues to compact rectification, high resolution ultrasonography, vibration and noise control in elastodynamics and acoustics.

Efficient Wide-Band Large-Angle Refraction and Splitting of a Terahertz Beam by Low-Index 3D-Printed Bilayer Metagratings

Near-perfect anomalous reflection and refraction have been demonstrated using passive lossless metasurfaces and metagratings operating at microwave, infrared, and visible frequencies, while related studies at terahertz frequencies are lacking. Here we propose low-index (with a refractive index of 1.57) 3D-printed dielectric metagratings for efficient wide-band diffraction engineering at low terahertz frequencies. A simplified analytical model reveals that the number of propagating waveguide modes inside the grating is a key factor in diffraction engineering, and is insufficient in a low-index design regardless of the detailed dimensions in a period. Additional waveguide modes are introduced in asymmetric bilayer and trilayer metagratings, providing sufficient degrees of freedom for efficient large-angle anomalous refraction and beam splitting. These metagratings are inherently wide-band, benefiting from low dispersion of the waveguide modes. Three metagratings are designed, 3D printed, and tested experimentally at 0.14 THz for ${70}^{\ensuremath{\circ}}$ refraction, ${80}^{\ensuremath{\circ}}$ refraction, and $\ifmmode\pm\else\textpm\fi{}{70}^{\ensuremath{\circ}}$ beam splitting. The measured efficiency shows good agreement with the design. The proposed metagratings, with simple structures and large feature sizes, can be easily scaled to applications at higher terahertz frequencies.

Scattering characteristics of a metasurface covered chiral sphere.

An analysis of the scattering characteristics of a chiral sphere covered with a metasurface is presented. The electromagnetic fields inside and outside of a metasurface covered chiral sphere are expanded in terms of spherical vector wave functions. The influences of chirality and surface impedance of a metasurface upon the scattering cross section are investigated. It is seen that a specific type of metasurface can be used to significantly enhance and diminish the back scattering cross section from a chiral sphere having a fixed value of chirality but different radii. It is also observed that a chiral sphere having chirality close to unity apparently cannot be cloaked using a lossless metasurface as compared to a dielectric sphere, both having the same electromagnetic and geometrical parameters. A comparative study of the scattering characteristics of metasurface covered realistic chiral and low-loss Teflon dielectric spheres is developed. It is found that a metasurface covered realistic chiral sphere can be used to significantly reduce the scattering as compared to scattering from a metasurface covered Teflon sphere at a specific angle, provided that both spheres have the same radii and surface reactance.

Illusion electromagnetics for free-standing objects using passive lossless metasurfaces

An electromagnetic illusion technique for free-standing objects for far-field observation using passive lossless metasurfaces is presented. Over a closed surface, the electric and magnetic surface currents needed for generating the scattered fields associated with a given physical scatterer are obtained using the surface equivalence principle. The closed surface defines an impenetrable metasurface characterized by surface impedance. A set of auxiliary evanescent surface waves are introduced and optimized over the entire surface to make the metasurface pointwise passive and lossless. Example 2D designs in TE polarization show that desired scattered fields are established immediately away from the illusion device surface. The illusion metasurfaces may be realized as tensor gradient metasurfaces.

A review of exact solutions for conversion of a surface wave into a propagating wave

In this paper, we summarize our recent theoretical and numerical studies of conversion of a surface wave to a propagating wave (and vice versa) with maximized conversion efficiency. We show that the exact solution for the homogeneous plane wave to surface wave conversion in a homogeneous medium by a point-wise lossless metasurface is restricted by Maxwell’s equations. Such performance is possible only with approximations and implies some limitations, including impossibility to fully avoid unwanted scatterings. Nevertheless, an inhomogeneous plane wave can be converted into a surface wave perfectly, and it is described by an exact solution formed by a superposition of only two modes of the Floquet expansion. The results of the studies are expected to be of fundamental importance for leaky-wave antennas efficiency improvement. Besides, they can lead to novel applications in various research areas.

Extremely Asymmetrical Acoustic Metasurface Mirror at the Exceptional Point.

Previous research has attempted to minimize the influence of loss in reflection- and transmission-type acoustic metasurfaces. This Letter shows that, by treating the acoustic metasurface as a non-Hermitian system and by harnessing loss, unconventional wave behaviors that do not exist in lossless metasurfaces can be uncovered. Specifically, we theoretically and experimentally demonstrate a non-Hermitian acoustic metasurface mirror featuring extremely asymmetrical reflection at the exception point. As an example, the metasurface mirror is designed to have high-efficiency retroreflection when the wave comes from one side and near-perfect absorption when the wave comes from the opposite side. This work marries conventional gradient index metasurfaces with the exceptional point from non-Hermitian systems, and it paves the way for identifying new mechanisms and functionalities for wave manipulation.

Design Method of Passive Lossless Metasurfaces With Auxiliary Waves for Beam Control

The use of cross-polarized auxiliary waves in the design procedure of the passive lossless metasurface has been reported as an effective approach to improve its performance. However, the design with only one auxiliary wave leads to low efficiency of an impenetrable metasurface in general when the incident and reflected waves interact with each other. This communication presents two auxiliary waves to design high-efficiency metasurfaces for arbitrary beam control. Besides the cross-polarized auxiliary wave, the copolarized auxiliary wave is proposed to design metasurfaces with a scalar impedance for beam control. Moreover, auxiliary waves are expanded into Fourier series in the procedure and their coefficients can be effectively determined with a numerical optimization method. Some instances are provided and the numerical results show that the designed metasurfaces effectively can control beams. The comparison between the designed metasurface and the conventional phase-gradient one is also provided.

Time-Varying Reactive Elements for Extreme Accumulation of Electromagnetic Energy

Accumulation of energy by reactive elements is limited by the amplitude of time-harmonic external sources. In the steady-state regime, all incident power is fully reflected back to the source, and the stored energy does not increase in time, although the external source continuously supplies energy. Here, we show that this claim is not true if the reactive element is time-varying, and time-varying lossless loads of a transmission line or lossless metasurfaces can accumulate electromagnetic energy supplied by a time-harmonic source continuously in time without any theoretical limit. We analytically derive the required time dependence of the load reactance and show that it can be in principle realized as a series connection of mixers and filters. Furthermore, we prove that properly designing time-varying LC circuits one can arbitrarily engineer the time dependence of the current in the circuit fed by a given time-harmonic source. As an example, we theoretically demonstrate a circuit with a linearly increasing current through the inductor. Such LC circuits can accumulate huge energy from both the time-harmonic external source and the pump which works on varying the circuit elements in time. Finally, we discuss how this stored energy can be released in form of a time-compressed pulse.

Lossless Scalar Metasurfaces for Anomalous Reflection Based on Efficient Surface Field Optimization

For polarization-preserving anomalous plane-wave reflections by impenetrable surfaces, an efficient numerical field optimization technique is presented for designing lossless metasurfaces. Auxiliary surface waves copolarized with the incident and reflected plane waves are introduced and optimized such that the scalar surface impedance characterizing the reflecting surface approach a purely reactive profile. The resulting inhomogeneous scalar reactance can be discretized for realization as a periodic metasurface. Full-wave simulation of a physical design that is directly translated from the optimized reactance profile shows near-perfect anomalous reflection.

Perfect Anomalous Reflection with a Bipartite Huygens’ Metasurface

In this paper we propose a new metasurface that is able to reflect a known incoming electromagnetic wave into an arbitrary direction, with perfect power efficiency. This seemingly simple task, which we hereafter call perfect anomalous reflection, is actually highly non-trivial due to the differing wave impedances and complex interference between the incident and reflected waves. Heretofore, proposed metasurfaces which achieve perfect anomalous reflection require complicated, deeply subwavelength and/or multilayer element structures which allow them to couple to and from leaky and/or evanescent waves. In contrast, we demonstrate that using a Binary Huygens' Metasurface (BHM) --- a passive and lossless metasurface with only two cells per period --- perfect anomalous reflection can be achieved over a wide angular and frequency range. Through simulations and experiments at 24 GHz, we show that a properly designed BHM can anomalously reflect an incident electromagnetic wave from $\theta_i = 50^\circ$ to $\theta_r = -22.5^\circ$, with perfect power efficiency to within experimental precision.