Metasurfaces consist of 2-dimensional array of resonators that allow for controlling the phase of the optical wavefronts to realize lenses and deflectors.
Realizing such devices with dynamically reconfigurable optical characteristics would greatly extend the scope of their applications. Here, we experimentally demonstrate a dynamic metasurface cavity structure, operating in Terahertz (THz) frequency range. The dynamic metasurface cavity structure consists of modified split ring resonators realized on top of vanadium dioxide (VO2) thin films, shown in Fig. 1(a). By applying a bias current to the device, VO2 present at the split gaps can be made to transition from insulator to metallic state. This modulates the optical response of the device. The fabricated metasurface device is characterized using a THz time domain spectroscopy (THz-TDS) setup and the measured reflectance and phase shift are shown in Fig. 1(b) & (c). Varying the applied bias current from 0 mA to 170 mA, results in various interesting changes in both reflectance and phase response of the cavity structure. A maximum reflectance modulation of 97 % and phase modulation up to 180° is observed.
The above measured results are fully reproduced (not shown in the manuscript) by modelling the metasurface cavity structure using transmission line theory and the effective surface admittance approach . In the above approach, propagation of a plane wave in a dielectric is described by an equivalent transmission line. The metasurface at the interface of two dielectric media is considered as an effective surface admittance, attached to the junction between two transmission lines, as shown in Fig. 2. The above equivalent model fully explains the dynamic resonant behavior of the metasurface cavity structure as well as the accompanied phase modulation characteristics.
The reported results demonstrate the potential of such devices for realizing tunable holograms, high efficiency modulators and tunable optical filters. The analytical approach presented here, can be applied for design and analysis of metasurface cavity structures based on other material systems at frequencies ranging from THz to mid infrared.
