Terahertz Metamaterial Absorber and Equivalent Circuit Model for Refractive Index Sensing

As a kind of important functional device, terahertz metamaterial absorbers (TMA) have been focused on by many researchers for their capacity to absorb electromagnetic waves and wide application fields. In this work, we designed a terahertz metamaterial absorber with narrow-band absorption for refractive index sensing, which consisted of a circular metal ring resonator and a square metal ring resonator. The simulation results show that the absorptivity of the proposed TMA reached over 68.8% and 93.27% at 1.926 and 4.413 THz, respectively. Moreover, the absorption mechanism was studied through the electromagnetic field energy distribution, and the influence of structural parameters on absorption performance was exhibited. In refractive index sensing, a high sensitivity (S) of 2.537 THz/RIU (refractive index unit, RIU) was achieved by utilizing the coupling of ring resonators. The maximal quality factor (Q-factor) and figure of merit (FOM) of the TMA were 234.73 and 147.67 RIU^-1, respectively. Additionally, we established an RLC equivalent circuit model (ECM) for the TMA, and we further illustrated the performance of the TMA in refractive index sensing through fitting the sensitivity based on the ECM to the sensitivity of the TMA. Our study exhibits the considerable potential application for the field of terahertz sensing, and the ECM for refractive index sensing will be helpful for continual investigation.