Tunable Coherent Perfect Absorption Based on Varactor-Embedded Metamaterial Absorber

In this article, an amplitude- and frequency-tunable coherent perfect absorption (CPA) is presented using a varactor-embedded frequency selective surface (FSS)-based metamaterial absorber. The proposed structure mainly incorporates two dielectric layers, two metal-patterned layers equipped with varactors and resistors, and an intermediate metal cross layer for electrical connections. This coherent absorber is evaluated based on full-wave simulations. When a single electromagnetic wave is incident vertically from one side of the structure, two adjacent peaks are produced with absorption efficiencies of about 50%. The structure is then positioned in the standing wave field created by two counter-propagating coherent waves, where the two peak amplitudes can be selectively enhanced or weakened by adjusting the phase difference between the coherent waves. Moreover, by modulating the reverse voltage across the varactors, the capacitance of the varactors is altered, thereby enabling the tunability of the absorption frequency. Two coherent absorption peaks with phase differences of $\phi $ = 0° and $\phi $ = 180° are deliberately designed to be significantly enhanced within distinct frequency ranges. As a result, a CPA with a wide tunable frequency range within 2.3–6.6 GHz is synthesized, where the absorption rates of 2.3–3.8 and 4.25–6.59 GHz exceed 90%. To substantiate the soundness of our theory and design, an equivalent circuit model is constructed, and the experimental measurements are also carried out. All the results agree well with each other.