Design and theoretical analysis of a tunable bifunctional metasurface absorber based on vanadium dioxide and photoconductive silicon

A tunable bifunctional metasurface absorber based on vanadium dioxide (VO₂) and photoconductive silicon (PSi) is proposed in a terahertz (THz) band. When the conductivities of VO₂ (<i>σ</i>_vo₂) and PSi (<i>σ</i>_PSi) are 10 S m^-1 and 1 × 10⁵ S m^-1, the designed absorber has a function of dual-broadband absorption. The absorptivity rate of over 90% is in the dual-broadband of 2.47-3.71 THz and 8.90-10.62 THz, corresponding to relative bandwidths (RBs) of 40.13% and 17.62%, respectively. When <i>σ</i>_vo₂ and <i>σ</i>_PSi are equal to 2 × 10⁵ S m^-1 and 1 × 10⁵ S m^-1, the proposed design has a function of single-broadband absorption. More than 90% absorptivity is achieved in 4.69-7.72 THz (RB = 48.83%). Furthermore, the absorptivity under the dual- and single-broadbands is manipulated by changing <i>σ</i>_PSi. An impedance matching theory, equivalent transmission-line (TL) model and electric field distribution are used to reveal the tunable bifunctional absorption mechanism. The influences of structure parameters, polarization mode and incidence angle on the dual- and single-broadband absorption are investigated. The dual- and single-broadband absorption performances are maintained within the incident angles of 55° and 60°, which also possess polarization insensitivity. The proposed absorber has a potential application value in multifunctional devices such as modulation, sensing and electromagnetic (EM) stealth.