  Recent developments in surface-enhanced Raman scattering (SERS) enable
observation of single-bond vibrations in real-time at room temperature. By
contrast, mid-infrared (MIR) vibrational spectroscopy is limited to inefficient
slow detection. Here we develop a new method for MIR sensing using SERS. This
method utilizes nanoparticle-on-foil (NPoF) nanocavities supporting both
visible and MIR plasmonic hotspots in the same nano-gap formed by a monolayer
of molecules. Molecular SERS signals from individual NPoF nanocavities are
modulated in the presence of MIR photons. The strength of this modulation
depends on the MIR wavelength, and is maximized at the 6-12$\mu$m absorption
bands of SiO$_2$ or polystyrene placed under the foil. Using a single-photon
lock-in detection scheme we time-resolve the rise and decay of the signal in a
few 100ns. Our observations reveal that the phonon resonances of SiO$_2$ can
trap intense MIR surface plasmons within the Reststrahlen band, tuning the
visible-wavelength localized plasmons by reversibly perturbing the
nanostructure crevices. This suggests new ways to couple nano-scale bond
vibrations for optomechanics, with potential to push detection limits down to
single-photon and single-molecule regimes.
