Design and Analysis of an Ultra-Broadband Polarization-Independent Wide-Angle Plasmonic THz Absorber

Abstract

Broadband metamaterial absorbers in the terahertz (THz) range are of great importance because of their applications, especially in the field of THz detection and imaging. Characteristics of an ideal absorber include wide bandwidth and insensitivity to polarization and incidence angle. Here we propose a new structure of an ultra-broadband THz absorber by adding an excess dielectric layer on a metamaterial structure with two stacked chromium square resonators. The excess layer besides protecting chromium resonator from oxidation creates an extra Fabry-Pérot cavity that extends the bandwidth of the structure. The relative bandwidth of this structure for 90% absorption exceeds 150% and covers frequencies ranging from 0.52 to 3.66 THz, which is an advantage over the works reported in the literature review. The number of resonating layers is decreased related to the earlier works too. The proposed structure is not sensitive to the polarization of the incident radiation, at the same time its absorption is almost insensitive to the incident angle too. For both TE and TM polarization the absorption stays more than 80% up to 60° of the incident angles. Furthermore, the insensitivity of the proposed device to misalignment between layers is evaluated for misalignment up to 28 μm. The results show that the structure is insensitive to misalignment which makes its fabrication easy. We have simulated the proposed structure using the numerical finite element method (FEM). Also, we have extracted a circuit model for the suggested absorber which is in accordance to the numerical model and can be used to reduce the designing process time. Correspondingly, we efficiently explain the absorption mechanism using the circuit model and magnetic field analysis in the structure. We have shown that the broad bandwidth of the structure is the result of coupling between various modes of Fabry-Perot (FP) cavity resonances and surface plasmons (SPs). The FP resonances are coupled through the gap resonators between the different layers. This means absorbers are more compact than the convectional nλ/4 wave plate absorbers.