Quantum Optical Theories of Molecular Optomechanics

Abstract

AbstractWe present several quantum optics models to describe regimes of molecular optomechanics for single molecules coupled to plasmonic cavity systems and show how this relates to surface enhanced Raman spectroscopy (SERS). We first present a general-medium open system picture of single molecule SERS (or molecular optomechanics) in a system-bath approximation, and show how the detected Raman spectrum originates from an interplay of nonlinear light generation and propagation. We apply this theory to study several different resonator systems whose cavity modes are described through a quasinormal theory of open cavities, and identify important limits of commonly used electric-field rules for Raman signal enhancement. Second, in the good cavity limit, we present a quantum optics picture of off-resonant SERS to explore molecular optomechanics in the sideband-resolved regime, where the cavity-emitted spectrum results in anharmonic resonances. This latter regime exploits hybrid metal-dielectric resonators, which yield deep sub-wavelength plasmonic confinement and dielectric-like quality factors. Finally, we present a generalized master equation approach to describe resonant SERS in the strong coupling regime of cavity-QED, while also in the ultrastrong vibrational coupling regime, and show several applications of the theory, including signatures of phonon-dressed polaritons in the emission spectrum.