A configurable two-layer four-bias graphene-based THz absorber

A novel structure for a THz absorber covering the THz band (0.1–10 THz) is presented. Exploiting nanographene disks and ribbons beside the dual-bias method, three modes of operation are introduced with the graphene gate biasing as the control parameter. The structure includes two layers consisting of graphene patterns on TOPAS dielectric and a thick gold plate at the bottom. The superior performance of the structure mainly relies on the use of feasible geometric patterns and the characteristics of graphene, while an evolutionary genetic algorithm is used to optimize a cost function defined based on four chemical potential values. In comparison with conventional structures, the device proposed herein offers an increased number of gate biases and thereby more degrees of freedom to achieve greater tunability. To model the proposed device, a recently developed circuit model approach is modified to include the dual-bias scheme introduced herein, enabling a very simple calculation of the referred input impedance of the device that lies at the heart of the design procedure. The input impedance required for impedance matching theory is matched with the free space incident medium (120π Ω) to maximize the absorption. Finally, the results from the MATLAB algorithm are verified against finite element method simulations using the CST simulator, confirming the validity and accuracy of the proposed design. According to both the circuit model representation and the full-wave numerical modeling, the presented device absorbs THz waves with an absorption ratio of more than 90% in three operational modes, viz. mode A (0.7–2.2 THz), mode B (5.3–6.6 THz), and mode C (7.4–8.4 THz). This increases its potential for use in numerous applications in the THz band such as sensors, detectors, modulators, and even optical processors.