Optimization of an extremely high Q-factor terahertz perfect absorber for environmental refractive index sensing using quasi-analytical solutions

Terahertz (THz) radiation can be used to monitor the environment by sensing its variation of refractive index. Previous studies have shown that a THz perfect absorber with several hundred Q-factors can function as sensitive THz refractive-index sensors (THz-RISs). However, the electromagnetic field simulation of THz-RIS is currently based on the numerical methods, which are inefficient when THz-RIS has extremely high Q-factors in several orders of magnitude. In this study, we present the quasi-analytical solutions of THz-RIS made by a 2D metallic hole array coated by polymer thin-film (2D-HACP) for the efficient optimization of extremely sensitive THz-RIS. It is found that the optimized 2D-HACP shows the absorption peak about 96.7% at the resonant frequency close to 1 THz. The absorption peak has full width at half-maximum about 39.1 MHz, yielding the Q-factor equal to 25,555. It is shown that the figure of merit associated with the variations of the real part and imaginary part of environment’s complex refractive index are equal to 2.56 × 10 4 1/RIU and 4.12 × 10 8 1/(RIU·THz), respectively. Therefore, the optimized 2D-HACP may work as an extremely sensitive environmental sensor by monitoring the tiny variation of the environment’s complex refractive index.