Terahertz Switchable Single-/ Dual-band Metamaterial Absorber based on nested circular-shaped structure

We design an optically-controlled single-/dual-band switchable terahertz absorber, whose unit cell incorporates a concentric gold disk and ring separated by a semiconductor photoconductive germanium (Ge) ring. When the Ge ring is not excited by external optical pump, the ring resonator and disk resonator will independently resonate at 0.89 THz and 2.37 THz with absorptivity of about 99.7% and 99.9%. With the increase of the optical pump power, the dual-band absorption peaks will dynamically switch to a single absorption peak at 1.10 THz with absorptivity of 99.7%. The absorption mechanism is demonstrated by the absorption properties of the decomposed structure and the surface current distributions at the resonant frequencies of the composite structure. The proposed simple and tunable absorber may have potential applications including modulators, switches and many other devices.