All-dielectric metasurface for wavefront control at terahertz frequencies

Abstract

Recently, metasurfaces have gained popularity due to their ability to offer a spatially varying phase response, low intrinsic losses and high transmittance. Here, we demonstrate numerically and experimentally a silicon metasurface at THz frequencies that converts a Gaussian beam into a Vortex beam independent of the polarization of the incident beam. The metasurface consists of an array of sub-wavelength silicon cross resonators made of a high refractive index material on substrates such as sapphire and CaF 2 that are transparent at IR-THz spectral range. With these substrates, it is possible to create phase elements for a specific spectral range including at the molecular finger printing around 10 µm as well as at longer THz wavelengths where secondary molecular structures can be revealed. This device offers high transmittance and a phase coverage of 0 to 2π. The transmittance phase is tuned by varying the dimensions of the meta-atoms. To demonstrate wavefront engineering, we used a discretized spiraling phase profile to convert the incident Gaussian beam to vortex beam. To realize this, we divided the metasurface surface into eight angular sectors and chose eight different dimensions for the crosses providing successive phase shifts spaced by π/4 radians for each of these sectors. Photolithography and reactive ion etching (RIE) were used to fabricate these silicon crosses as the dimensions of these cylinders range up to few hundreds of micrometers. Large 1-cm-diameter optical elements were successfully fabricated and characterised by optical profilometry.

Diffractive optics for axial intensity shaping of Bessel beams

“…The second case was calculated using the GS algorithm, resulting in a random phase which is challenging to fabricate with EBL as it involves random and analog phase variations over small areas. Fabrication of spatially varying random phase patterns using metasurfaces can be achieved [27]. Capasso et al have recently demonstrated metasurface axicons [28] operating at visible wavelengths.…”

Metallic and dielectric metasurfaces in photoconductive terahertz devices: a review

“…137,138 However, the emitted THz power of the standalone metal metasurface is relatively low compared with the plasmonic PCA 69 and even to the surface photo-Dember THz emitter 136 and amounts, on average, to 0.15 mW, corresponding to an optical-to-THz conversion efficiency of up to 0.01% at 5.8 THz. 135 Furthermore, both metallic and dielectric metasurface configurations can be designed to effectively interact with THz radiation and thus can be used in ultrasensitive THz sensors, THz absorbers, highly selective THz detectors, tunable THz field modulators, [139][140][141][142] and THz mirrors, 143 as well as to control a wavefront at THz frequency, 144 etc.…”

Dielectric cross-shaped-resonator-based metasurface for vortex beam generation at mid-IR and THz wavelengths

“…In this study, we develop a vortex beam generator for the mid-IR spectral range (∼ 8.8 𝜇m), using cross-shaped resonators [31], which can support both Mie-type electric and magnetic dipole resonances to realize phase manipulation and to completely suppress reflection losses. The idea behind choosing this wavelength is to use the generated vortex beam to measure the absorbance of secondary protein structures such as beta sheets in silk which also lie at in the same wavelength region [32,33,34,35].…”