Raman Spectroscopy at Metal Interfaces: A Numerical Study of the Strong Coupling Regime.

We investigate how proximity to a metal nanostructure, particularly to a flat mirror or a cavity confined between two mirrors, affects the vibronic structure of the Raman scattering signals. We find that such proximity, particularly for the strong-coupling situation encountered in cavity environments, plays multiple roles in shaping Raman signals beyond the now familiar signal enhancement known as surface-enhanced Raman scattering (SERS). First, in analogy to the electromagnetic SERS mechanism, near or between mirrors, the local field experienced by a molecule differs from that in vacuum. In particular, between mirrors, the cavity enhances the effective excited-state population by trapping the electromagnetic (EM) field inside it. Second, the nearby metal surface provides a relaxation channel and a lineshape broadening mechanism, and inside a cavity, this lineshape is inherited by the cavity polaritons. This relaxation results in a loss of yield, but the associated broadening also leads to significant absorption over a larger frequency range. Third, near-metallic interface interference between incident and reflected light can lead to a richly structured Raman spectrum. For instance, we find that the Rabi contraction (which results from depopulating the ground state) can interfere with Raman signals (and the effect appears to be of the same order as Raman itself). These cavity effects are calculated by a full-scale finite-difference time-domain (FDTD) simulation and highlight the convoluted but fascinating roles of photonic materials on optical signals.