Asymmetry in polariton dispersion as function of light and matter frequencies in the ultrastrong coupling regime

Abstract

In the ultrastrong light–matter coupling regime, non-perturbative effects are observed even for large detuning of the light and matter frequencies. In this regime, the rotating wave approximation breaks down and the contributions of counter-rotating and diamagnetic terms lead to significant spectral modifications. Using split ring resonators coupled to the cyclotron transition in a two dimensional electron gas, we show that the diamagnetic terms lead to an asymmetry between the light and matter branches. Our system allows tuning both, the cavity and the matter frequency over more than two octaves. We find, that the assumption of constant coupling rate Ω as function of detuning is not generally valid for the large frequency range relevant to ultrastrong coupling phenomena.