Enhancing the selectivity of frequency selective surfaces for terahertz sensing applications

This paper introduces a new technique for enhancing the selectivity (or the quality factor, Q-factor) of frequency selective surfaces (FSS) for sensing applications. The proposed FSS functions as a free-space bandpass resonator, designed to sense the changing dielectric properties of minute amount of materials loaded on the FSS. The Q-enhancement technique is mainly based on two concepts; enhancing the field concentration in a given area and introducing transmission zeros in the FSS response. Two designs based on a modified complementary split-ring resonator (CSRR) at 300 GHz have been proposed. The first one is composed of complementary triple-split ring resonators. The splits divided the structure into arcs of different lengths. As a result, the transmission zero is obtained in the passband due to a destructive coupling. This produces a resonance Q-factor of 41. By controlling the orientation of the three splits, higher Q-factor of 84 is attainable. The second structure is designed using concentric triple-split rings. The added electromagnetic coupling between the concentric rings makes the transmission response steeper as compared with the single triple-split ring, and the quality factor increases from 41 to 90. By reducing the interspacing distance by three times, the Q-factor can be further increased to 256. The parameter studies of the FSS structures based on full-wave simulations have been presented.