Free‐Standing Metasurfaces for High‐Efficiency Transmitarrays for Controlling Terahertz Waves

Abstract

subwavelength scatterers. With this emerging technique, many conventional devices such as beam defl ector, [ 18,21 ] Bessel beam lens, [ 22 ] vortex beam lens, [ 19,23,24 ] background-free circular polarizer, [ 25 ] and focusing lens [ 26 ] have been redesigned as 2D lightweight devices. Therefore, this new concept provides us with another way to design thinner transmitarrays with better performance, especially at THz frequencies, where the miniaturization is very desirable.Based on the generalized Snell's law, an ultrathin transmitarray at THz frequencies is proposed and demonstrated experimentally here. By carefully designing the geometry of unit cell, which is composed of three metallic layers separated by polyimide (PI) spacers, the fi ve basic constitutive elements offer a discretized 2π phase coverage, and can be utilized to manipulate the EM waves. Although there have been some designs previously reported on bending and rotating the THz waves by using single-layer metasurfaces, [27][28][29][30] all measured results for transmissions were severely suffered by strong Fabry-Perot effects, which are caused by the large thickness of substrates (over one free-space wavelength) and the high permitivities. In this work, owning to the fully free-standing design, all measured effi ciencies are obtained from the fabricated sample itself, which do not need to be normalized to the bare substrate, and hence can be directly applied to the current THz systems. Two examples are given to demonstrate the performance of the proposed transmitarrays in bending and focusing the terahertz waves.There are several reports on metasurfaces to bend the light to anomalous directions using single-layered metallic patterns at the optical frequency. [ 27,30,31 ] However, the maximum transmission effi ciency (copolarization) for a single nonmagnetic metasurface has a theoretical limit of 0.5, which was derived in ref. [ 32 ] by applying the reciprocity, symmetry, and passivity conditions to a two-port network. [ 32 ] To overcome the limitation, we proposed a triple-layered metasurface, which could provide a unity transmission and the entire 2π phase coverage simultaneously. Here, an electric-fi elddriven inductor-capacitor (ELC) resonant structure [ 33 ] is adopted to design the basic unit cell of transmitarrays because it can be switched between inductive and capacitive modes in the THz bandwidth of interest. The designed multilayered unit cell is formed by stacking three ELC resonators with two identical metasurfaces at outer layers, as illustrated schematically in Figure 1 a. Each metallic layer is separated by a polyimide spacer with 30 µm thickness. Considering the inevitable scratches, and any unpredictable contact that may damage the outer metallic layers in real applications, the top

Tutorial: Terahertz beamforming, from concepts to realizations

“…For example, a three-layer device consisting of "I"-shaped resonator unit cells has been employed for a frequency-scanning beam steerer operating around 0.9 THz, with a peak efficiency of 44%. 188 Another example of a three-layer device performs focusing, in which the individual resonator element is a complementary metallic annulus, and the phase response is tuned by its radius. 189 One drawback of employing complementary structures of this type is that the transmittance is only acceptable close to the resonance frequency, and hence by detuning the resonance frequency away from the operating frequency in order to manipulate phase response, the efficiency is severely impacted.…”

Polarization-independent all-silicon dielectric metasurfaces in the terahertz regime

“…However, the efficiency of single-layer plasmonic metasurfaces is limited to 25% [18,19]. Multilayer plasmonic structures can increase the efficiency [20][21][22][23][24][25][26][27], but they also increase fabrication difficulties and are not easy to integrate into an on-chip system.…”

Broadband Circular Polarizer Based on Plasmon Hybridizations

“…Metamaterials are artificial materials with properties which do not exist in natural material. They have been broadly used to control properties of the electromagnetic wave [1][2][3], especially the polarization [4][5][6][7]. Numerous metamaterialinspired polarizers have been designed to change the polarization properties of antennas [8][9][10][11][12].…”