Reflectionless Metasurfaces Research Articles

Uniform solution for the diffraction by an impedance and reflectionless metasurface planar junction.

The scattering process of waves by a planar junction of two half-planes, one of which is impedance and the other a reflectionless metasurface, is studied. The transition boundary method is considered. The scattered geometric optics field is obtained by subtracting the initial fields from the total geometric optics field. The diffracted field is obtained by considering the relation between diffracted and scattered geometric optics field expressions in the transition boundary. The ultimate expressions of the fields are examined numerically for different parameter groups.

Metalens mounted on a resonant tunneling diode for collimated and directed terahertz waves.

Refraction in materials is a fundamental phenomenon in optics and is a factor in the manipulation of light, such as wavefront shaping and beam control. However, conventional optical lenses incorporated in numerous optical sources are made of naturally occurring materials, and material properties predetermine the lens performance. For the development of terahertz flat optics, we experimentally demonstrate a gradient-refractive-index (GRIN) collimating metalens made of our original reflectionless metasurface with an extremely high refractive index, above 10 at 0.312 THz. The planar collimating metalens converts wide-angle radiation from a resonant tunneling diode (RTD) to a collimated plane wave and enhances the directivity of a single RTD 4.2 times. We also demonstrate directional angle control of terahertz waves by moving the metalens in parallel with the incoming wave. The metalens can be simply integrated with a variety of terahertz continuous-wave (CW) sources for 6G (beyond 5G) wireless communications and imaging in future advanced applications. Flat optics based on high refractive index metasurfaces rather than naturally occurring materials can offer an accessible platform for optical devices with unprecedented functionalities.

Terahertz focusing metalens of reflectionless meta-atoms with negative refractive indices.

Terahertz continuous-wave (CW) sources oscillating around the 1.0 THz band at room temperature have rapidly been developed to bridge the terahertz gap. However, reflectionless metasurfaces suitable for integration with terahertz CW sources as optical components have yet to be developed in the terahertz gap. Here, we propose a terahertz-focusing metalens consisting of reflectionless meta-atoms with a discrete distribution of negative refractive indices from ${-}{1.1}$ to ${-}{2.8}$. The proposed 2D gradient-refractive-index metalens converts an incident terahertz Gaussian beam to a line focus. We also experimentally demonstrate a metasurface of reflectionless meta-atoms with a negative refractive index of ${-}{2.8}$ adopted in the periphery of the metalens. The reflectionless metasurface in the terahertz gap would be a welcome contribution to the rapid growth of terahertz industrial applications with terahertz CW sources. Further, the design approach based on reflectionless meta-atoms with negative refractive indices could be applied to various 2D planar optical components with attractive functionalities such as collimating, arbitrary wavefront shaping, and light vortices.

Reflectionless metasurface with high refractive index in the terahertz waveband.

Terahertz flat optics is a design concept for replacing conventional three-dimensional bulky optical components with two-dimensional ultra-thin optical components. However, high refractive index materials suitable for flat optics are frequently subject to high Fresnel reflections due to the cumbersome control of the relative permeability it requires. Here we experimentally demonstrate a reflectionless metasurface with a high refractive index of 5.88 + j1.57, extremely low reflectance of 1.3%, high relative permittivity of 6.73 + j0.85, and the high relative permeability of 5.03 + j2.11 at 2.97 THz. The super-fine ink-jet printer using silver paste ink fabricates the metasurface consisting of 80,036 pairs of cut metal wires on both the front and back of a 5 μm-thick polyimide film. The findings also demonstrate that weak conductors as well as good conductors can be used in the design of reflectionless metasurfaces with a high refractive index in the terahertz waveband. The presented metasurface can offer an accessible platform for terahertz flat optics in 6G (beyond 5G) wireless communications and imaging.

Generalized hybrid anapole modes in all-dielectric ellipsoid particles [Invited

Numerous exciting optical effects in all-dielectric high-refractive-index structures are associated with so-called toroidal electrodynamics. Among these effects are anapoles, nonradiated states caused by interference phenomena, e.g. between electric dipole and toroidal dipole modes. For a spherical particle it is possible to reach simultaneous destructive interference for electric, magnetic, and corresponding toroidal dipole modes (so-called hybrid anapole mode), by varying the refractive index and/or particle size. However, there are no sufficient degrees of freedom within spherical geometry to extend the hybrid anapole mode effect to higher multipoles. Due to the optical theorem, it is also impossible to create the ideal anapole with destructive interference for all multipoles under plane wave illumination. In principle, it is possible to suppress radiation losses for the finite number of multipoles only by constructing the nanoantenna with complex geometry. Our approach of the hybrid anapole state excitation, we demonstrate in ellipsoidal all-dielectric particle providing cancellation of both electric and magnetic scattering up to quadrupole modes. This effect is achieved due to the optimised geometry of the ellipsoidal particle. Moreover, we provide classification of novel anapoles arising due to interference between moments and their mean- square radii (MSR) of electric, magnetic and toroidal family and introduce generalized anapoles for high order interaction between moments. Our concept is useful for the design of light controlling devices, reflectionless metasurfaces, high Q-factor opened resonators and nonscattering particle development.

Elastic Wave Energy Entrapment for Reflectionless Metasurface

This paper presents an elastic metasurface that can entrap the elastic wave energy of both longitudinal and shear wave modes in a lossy region, resulting in almost no reflection environment, which we call a reflectionless metasurface. Unlike previous attempts in acoustics, elastic waves are governed by different physics, such as mode conversion, and different behavior of shear waves. Accordingly, achieving a reflectionless elastic metasurface requires a different approach to that of previous studies on acoustic metasurfaces for sound absorption. Here, physics for entrapping the elastic wave energy is proposed by combining two phenomena: the local resonance of metasurface unit cells and the surface wave conversion. As a result, the elastic wave energy originating from an incident longitudinal wave can be highly concentrated in the metasurface, and energy dissipation can be significantly increased if any damping is implemented in the metasurface region. To realize the idea, a reflectionless metasurface, consisting of rod-shaped unit cells with various lengths, is designed. Numerical and experimental investigations show that the proposed reflectionless metasurface, with a small amount of loss, can effectively reduce both the reflected longitudinal and shear waves under longitudinal wave incidence with broad angles. We expect that the proposed idea, based on a thin artificial elastic layer, will provide an alternative route to design ultrasonic absorbers.

Designing a reflectionless and bianisotropic metasurface antenna using dihedral corner reflector excitation

In this paper, the design process of a bianisotropic metasurface for acquiring a highly directive beam is presented. We propose a dihedral corner reflector-excited metasurface antenna through which the aperture of a single-fed reflector is covered by a metasurface such that optimal aperture illumination is accomplished. Fields within the reflector are derived based on the modal technique. The general susceptibility synthesis method is used to design a passive, lossless, and reflectionless metasurface by introducing bianisotropy into it. To achieve the optimal radiation characteristics, a rigorous semianalytical method is proposed and utilized to predict directivity, side-lobe level, and source position. Owing to the thoroughness of our approach, the designed metasurface has a plethora of applications at microwave, terahertz, and optical frequencies, especially for designing highly directive antennas. The semianalytical results are verified using the finite-element method by commercial COMSOL Multiphysics electromagnetic solver.

Compound Metaoptics for Amplitude and Phase Control of Wave Fronts.

Metasurfaces allow tailored control of electromagnetic wave fronts. However, due to local conservation of power flow, passive, lossless, and reflectionless metasurfaces have been limited to imparting phase discontinuities-and not power density discontinuities-onto a wave front. Here, we show how the phase and amplitude profiles of a wave front can be independently controlled using two closely spaced phase-discontinuous metasurfaces. The two metasurfaces, each designed to exhibit spatially varying refractive properties, are separated by a wavelength-scale distance and together form a compound metaoptic. A method of designing the compound metaoptic is presented, which enables transformation between arbitrary complex-valued field distributions without reflection, absorption, polarization loss, or active components. Such compound metaoptics may find applications in the optical trapping of particles, displaying three-dimensional holographic images, shrinking the size of optical systems, or producing custom (shaped and steered) far-field radiation patterns.

Generalized spatial Talbot effect based on all-dielectric metasurfaces

The Talbot effect is among the most fundamental wave propagation effect universal across the frequency spectrum and has been used extensively in both spatial and temporal domain for wide variety of applications. In this work, a generalized spatial Talbot effect is presented based on all-dielectric Huygens' metasurface, which when combined with the conventional Fractional Talbot effect, enables the scaling of aperture periodicities by any arbitrary real number. This is achieved by allowing and engineering the phase discontinuity distributions in space on the aperture using phase engineered all-dielectric reflectionless metasurfaces, in conjunction with free-space propagation. Specific implementations using Huygens' metasurfaces are proposed and their operation to achieve such non-integer scalings of the input aperture period is demonstrated using numerical results based on Fourier propagation method.

New Design for Transmitted Phase of Reflectionless Metasurfaces With 2 <inline-formula> <tex-math notation="TeX">$\pi$</tex-math></inline-formula> Coverage

Huygens' equivalent principle suggests that surface electric currents can be combined with magnetic currents to tailor electromagnetic (EM) wavefronts. Usually, the passive strategy is evolved into the concepts of sheet admittance and impedance. This paper attempts to resume the concepts of passive electric and magnetic currents and propose a novel design strategy for the transmitted phase of reflectionless meta surfaces with complete 2π phase coverage. Split meander lines, plus spiral rings, are carefully adjusted to accommodate complex equivalent currents for generating the required transmitted phase. Both experimental and theoretical results verify that meta cells proposed in this paper can be integrated to control the EM wave flexibly and efficiently. Furthermore, the concepts of combined passive electric and magnetic currents can be easily extended to design optical antennas and other plasmonic devices.