Signatures of the ultrastrong light-matter coupling regime

Abstract

In a microcavity, light-matter coupling is quantified by the vacuum-Rabi frequency ${\ensuremath{\Omega}}_{R}$. When ${\ensuremath{\Omega}}_{R}$ is larger than radiative and nonradiative loss rates, the system eigenstates (polaritons) are linear superposition of photonic and electronic excitations, a condition actively investigated in diverse physical implementations. Recently, a quantum electrodynamic regime (ultrastrong coupling) was predicted when ${\ensuremath{\Omega}}_{R}$ becomes comparable to the transition frequency. Here we report signatures of this regime in a quantum-well intersubband microcavity. Measuring the cavity-polariton dispersion in a room-temperature linear optical experiment, we directly observe the antiresonant light-matter coupling and the photon-energy renormalization of the vacuum field.