Analytical solutions of the perfect absorber for terahertz sensing

We develop the analytical solutions (ANS) of the hybrid structure comprising the metallic grating (MG) with subwavelength width, the stacked-dielectric layers in front of the MG, and one dielectric layer behind the MG. The ANS are well consistent with the finite difference time domain simulation. We apply the ANS to optimize the flat-surface front-perfect absorber (FPA) with ultrahigh Q-factor up to 3×10⁴ based on the coupling between the quasi-waveguide resonance (QWR) and the Fabry-Perot (FP) resonance. The proposed FPA can detect the small variations in refractive indices of thin samples and the presence of thin films with deep-subwavelength thicknesses.