  Metallic nanoparticle-on-a-mirror (NPoM) cavities enable extreme field
confinement in sub-nm gaps, leading to unrivaled performance for nonlinear
processes such as surface-enhanced Raman scattering (SERS). So far, prevailing
experimental approaches based on NPoMs have been performed by means of
free-space light excitation and collection under oblique incidence, since the
fundamental radiatively-coupled NPoM mode does not scatter in the normal
direction. Retaining this working principle, here we numerically show that
plasmonic cavity modes in NPoM configurations can be efficiently excited in an
integrated SERS approach through TM guided modes of silicon nitride (SiN)
waveguides. Intensity enhancements beyond 10$^{5}$ can be achieved for gap
spacings around 1 nm. So as to reduce unwanted SiN Raman background, the output
Stokes signals are transferred to transversely placed waveguides, reaching
coupling efficiencies of up to 10%. Geometrical parameters such as the gap
thickness as well as the radius and position of the nanoparticle provide full
control over the main spectral features, thereby enabling us to engineer and
drive the optical response of NPoMs for high-performance SERS in Si-based
photonic integrated platforms.
