Metal-Dielectric Hybrid Cavities for Studying Ultrastrong Interactions

Metasurfaces offer a metallic platform for achieving high electromagnetic field enhancement due to subwavelength light confinment. However, the quality (Q) factors are low due to high ohmic losses in metals. In contrast, dielectric cavities, such as one-dimensional photonic-crystal cavities (1D-PCCs), support electromagnetic modes with high Q factors but typically offer limited electromagnetic field enhancement. In this work, we propose a novel platform to study mode coupling between light modes and matter excitation: hybrid cavities formed by coupling metasurfaces with dielectric cavities. First, we simulated complementary metasurfaces with diverse geometries (e.g., nanoslot, square, and split rings), obtaining broad resonances in the terahertz frequency range. Then, we integrated these metasurfaces with dielectric cavities, which enabled ultrastrong intermode light hybridization, resulting in a Rabi splitting whose magnitude is on the order of the bare cavity resonance frequency. Furthermore, we embedded an active material supporting matter excitations (i.e., cyclotron resonance in a two-dimensional electron gas) into the hybrid structure, such as quantum wells. This type of structure will allow us to probe the dynamics of ultrastrongly coupled systems directly in the time domain under long photon lifetime conditions enabled by the high Q factors. Under this coupling regime, we aim to uncover novel physics beyond the Hopfield model such as observations of quantum electrodynamical effects.