Design, optimization, and characterization of deep sub-wavelength evanescent orders in terahertz metagratings

Abstract

Resonant evanescent orders, being an exclusive deep sub-wavelength phenomenon, are well-known for confining strong EM energy at the air-grating interface when excited utilizing 1-dimensional gratings. However, stimulating prominent evanescent orders demands thoughtful design variations in grating geometry. In this pretext, we have successfully designed and optimized THz gratings that can sustain strong evanescent orders while operating in the subwavelength frequency domain. We have performed a fast Fourier transform (FFT) on the position-dependent electric field distribution of the grating to study the evanescent orders for both of the incidence polarizations (TE and TM). In order to optimize the grating performance, we have systematically increased the grating ridge height at a fixed fill factor (FF = 0.5). In such a way, excited evanescent orders are turned out to be anisotropic in nature at relatively larger grating height. We attribute such anisotropic behaviour to the effective refractive index experienced by the orthogonal THz probe.