  Plasmon resonances at the surface of plasmonic antennas allow for extremely
strong enhancement of Raman scattering. Intrinsic to plasmonics, however, is
that extreme field confinement lacks precise spectral control, which would hold
great promise in shaping the optomechanical interaction between light and
molecular vibrations at will. We demonstrate an experimental platform composed
of a plasmonic nanocube-on-mirror antenna coupled to an open, tunable
Fabry-Perot microcavity for selective addressing of individual vibrational
lines of molecules with strong Raman scattering enhancement. Multiple narrow
and intense optical resonances arising from the hybridization of the cavity
modes and the plasmonic broad resonance are used to simultaneously enhance the
laser pump and the local density of optical states (LDOS) and are characterized
using rigorous modal analysis. The versatile bottom-up fabrication approach
permits quantitative comparison with the bare nanocube-on-mirror system, both
theoretically and experimentally. This shows that the hybrid system allows for
similar SERS enhancement ratios with narrow optical modes, paving the way for
dynamical back action effects in molecular optomechanics.
