Inhomogeneous Broadening of a Polaritonic Mode in the Ultrastrong Coupling Regime

Ultrastrong coupling (USC) regime has been an intriguing research topic in cavity quantum electrodynamics (QED) experiments due to its capability to modify the ground and the excited states of a coupled system[1-3]. We investigate the USC of Landau level (LL) transitions in a two dimensional electron gas to teraherz (THz) metasurfaces using THz time domain spectroscopy. Complementary THz split ring resonator (cSRR) arrays are used as metasurfaces]. A higher coupling rate can be achieved by reducing the capacitive gap of the resonator due to the increased vacuum field fluctuations. However, our results indicate that this reduction introduces an inhomogenous broadening of the upper polariton branch below a threshold gap width.