Signatures of the ultrastrong light-matter coupling regime

TLDR

Light‑matter coupling in a microcavity is quantified by the vacuum Rabi frequency, and when this exceeds loss rates the eigenstates become polaritons; ultra‑strong coupling is predicted when the Rabi frequency approaches the transition frequency. Here we report unambiguous signatures of this regime in a quantum‑well intersubband microcavity. We measure the cavity‑polariton dispersion in a room‑temperature linear optical experiment. The measurements reveal anti‑resonant light‑matter coupling and photon‑energy renormalization of the vacuum field.

Abstract

In a microcavity, light-matter coupling is quantified by the vacuum Rabi frequency $\Omega_R$. When $\Omega_R$ is larger than radiative and non-radiative 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 (ultra-strong coupling) was predicted when $\Omega_R$ becomes comparable to the transition frequency. Here we report unambiguous 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 anti-resonant light-matter coupling and the photon-energy renormalization of the vacuum field.