Hidden nonlinear optical susceptibilities in linear polaritonic spectra

Linear spectra of molecular polaritons, quasiparticles formed by the strong coupling of N molecules to a confined photonic mode, are typically well described successfully by classical linear optics. This raises the fundamental question of where genuinely quantum and nonlinear optical effects arise in such systems. Here, we derive a general analytical expression for the polaritonic linear response and elucidate previously overlooked quantum corrections arising from cavity vacuum-induced molecular transitions. Using a systematic 1/ N expansion, we show that finite-size quantum processes imprint nonlinear spectroscopic features, including Raman-like sidebands, onto the linear spectrum. We further demonstrate that while such effects are suppressed in conventional lossy cavities, they become manifest in regimes where the photon dissipation rate is commensurate with the single-molecule light–matter coupling strength. These findings establish clear theoretical criteria for identifying cavity-induced quantum electrodynamical effects in polaritonic spectra and delineate experimental regimes where they may be observed.