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Fingerprints of strong coupling between molecular vibrations and microcavities

When the coherent energy exchange between a light mode and quantum emitters is faster than the decay and decoherence of either constituent, the energy levels of both systems become inextricably linked and the system reaches the strong coupling regime. A novel system reaching this limit is formed when collectively coupling a photonic mode inside an optical microcavity to molecular bond vibrations in a material filling the cavity. In this case, the cavity modes couple to superpositions of the molecular vibrations, so-called bright states, forming hybrid light-matter quasiparticles known as (vibro-)polaritons. All other superpositions of vibrational excitations (dark states) remain uncoupled. In this talk, we will discuss various consequences and possible applications of achieving the strong coupling regime in such systems. We first present a fully quantum framework for the phenomenon of collective strong coupling with molecular vibrations. Within this setting, we show that elastic collisions with a background bath of phonons, which normally only introduce dephasing, may affect the dynamics and population of the system. In particular, for long-range phonons, the dynamics of the bright state is effectively decoupled from other system eigenstates. For the case of independent baths (or shortrange phonons), incoherent energy transfer occurs between the polaritons and the dark states. However, these processes are suppressed when the coupling is strong enough that the Rabi splitting ΩR between polaritons is larger than the frequency range of the bath modes. In both cases, the dynamics can thus be described through a single collective oscillator coupled to a photonic mode. We will then present theoretical results on Raman scattering from vibropolaritons, motivated by recent experiments that reported enhancements of up to three orders of magnitude in the Raman signal under strong coupling, with emission at energy shifts approximately corresponding to the polaritons. Using a series of increasingly complex models, we demonstrate how the emergence of vibrational strong coupling affects Raman scattering processes. We show that the total Raman cross section is maintained constant when the system evolves from weak to strong coupling, with a redistribution of the signal among the two polaritons as the main fingerprint. These theoretical results indicate that nonlinear effects might play a role in the experiments.