Light-matter interaction inside an optical cavity: A perspective

Light-matter interaction inside an optical cavity and formation of polaritonic states have gained interest in the past decades as it has direct applications in many research fields. Different regimes of light-matter coupling have been studied using different approximations, one of which is rotating wave approximation(RWA), which is said to be valid for weak coupling and near resonant regimes. In this study, we have categorized the light-matter coupling into four regimes depending on the validity of the RWA as moderate, $\lambda/\omega_c\leq 0.1$, strong, $0.1 \leq \lambda/\omega_c\leq 0.5$, ultra-strong, $0.5 \leq\lambda/\omega_c\leq 1.0$ and deep-strong, $\lambda/\omega_c\geq 1.0$ coupling, where $\lambda$ is the coupling strength and $\omega_c$ is the cavity frequency. In experiments, vacuum Rabi-splitting has been observed which is a clear indication of formation of polaritonic states. It is a common misunderstanding that the cavity remains in vacuum state when the coupled system is in the ground state. Here we show that upon coupling, the cavity has non-zero excitation even in the ground state, which is not captured by RWA. In fact, RWA breaks down completely to predict the ground state properties as it fails to capture the interaction between the matter and the cavity field.