Characterization of Nematic Liquid Crystal Dielectric Properties using Complementary FSS featuring Electrically Small Cell Gaps across a wide Sub-THz Range

Abstract

Liquid crystal (LC) is a valuable material for constructing tunable devices in the sub-Terahertz (sub-THz) spectrum, and to utilize it effectively, it is imperative to accurately measure the permittivity of the LC. Frequency Selective Surfaces (FSS) serve as an effective tool for assessing LC permittivity in the sub-THz frequencies. However, existing research lacks measurements of the loss tangent and exhibits certain limitations and sources of error. In this paper, we propose a multi-loop complementary FSS (CFSS) structure to extract the permittivity of LC at sub-THz. The designed multi-loop CFSS exhibits high sensitivity despite featuring an extremely thin LC cell gap of less than 10 μm, the smallest cell gap to be reported in literature for such applications. The structure can estimate LC permittivity across a broadband range by utilizing multi-resonant characteristics. Moreover, we propose a procedure using an averaged inhomogeneous model, thereby enabling accurate calculation of LC permittivity. The proposed method estimates the parallel and perpendicular permittivity and loss tangent of the LC using the measurement results and comparison with the finite element simulation results. The permittivity extraction procedure is experimentally verified in the D-band (110 - 170 GHz).