Superradiant Ensembles of Terahertz Polaritonic Meta-Atoms

We experimentally demonstrate the robustness of strong light-matter interaction in an ensemble of terahertz cavities with variable radiation loss. Each cavity in the form of a planar metallic resonator is strongly coupled to intersubband transitions of semiconductor quantum wells. The polariton spectra, measured by time-domain spectroscopy, do not diminish when the cavity ensemble is brought to the superradiant regime with a radiative loss rate increased by a factor of three. In contrast, the splitting of the polariton frequencies gets larger, and indicates an increased vacuum Rabi frequency. We attribute the observed phenomenon to this well-known many-body effect in quantum electrodynamics, but here associated to an ensemble of otherwise independent polariton systems.