Metasurface-based optical cavity structures, featuring a metallic metasurface on a dielectric slab backed by a metal plane, are pivotal in designing optical devices like flat lenses, wave plates, and holograms, spanning frequencies from microwave to mid-infrared. Recently, these structures have been explored for dynamically reconfigurable optical characteristics, enabling electrically tunable optical absorption and reflection phase modulation. Traditional approaches have faced significant insertion loss, but this work introduces an analytical method based on transmission line theory, representing the metasurface as a surface admittance. By examining under- and overcoupled resonance regimes, we design cavity thickness and individual metasurface components for substantial amplitude or phase modulation. Experimentally, a dynamic metasurface cavity at terahertz frequencies was demonstrated using metallic resonators embedded with vanadium dioxide patches. The device modulates its terahertz optical response through the insulator-to-metal transition in vanadium dioxide, achieving perfect absorption modulation and 180° reflection phase modulation. These findings highlight the potential for novel devices like tunable holograms, high-efficiency modulators, and frequency-tunable filters. This analytical approach can be extended to other material systems at terahertz to mid-infrared frequencies. -Scientific Journal cover design by scapiens

https://pubs.acs.org/doi/10.1021/acsphotonics.8b01014