Polariton assisted down-conversion of photons via nonadiabatic molecular dynamics: a molecular dynamical Casimir effect*

Recently it has been possible to reach new regimes in which the light-matter interaction energy is comparable to electronic or vibrational energies using optical microcavities or nanostructures, giving rise to new hybrid light-matter states called polaritons. Most recent works focus on using strong light-matter coupling to change molecular processes such as photodissociation and charge and energy transfer.Here, we focus on a less addressed complementary question: can the emergent molecular dynamics be harnessed for photonic applications? We calculate the quantum dynamics of the photoisomerization of a single molecule embedded in an optical microcavity. We found that for specific cavity frequencies and sufficiently strong couplings, molecular photoexcitation into an electronic excited state can be followed by the spontaneous emission of two photons of a lower frequency via the cavity after isomerization, thus offering a new mechanism for photonic down-conversion using molecular polaritons. We show how this mechanism occurs when the light-matter coupling is strong enough so the nuclear dynamics cannot be considered to be adiabatic with respect to the electronic and photonic degrees of freedom.