  Plasmonic nanocavities have proven to be a powerful optical platform for
confining light at a length scale much smaller than the diffraction limit.
Enhanced electromagnetic fields within the cavity mode volume enable multiple
significant effects that lead to applications in both the linear and nonlinear
optical regimes. In this work, we demonstrate enhanced second harmonic
generation from individual plasmonic nanopatch antennas which are formed by
separating silver nanocubes from a smooth gold film using a sub-10 nm zinc
oxide spacer layer. When the nanopatch antennas are excited at their
fundamental plasmon frequency, a 10^4-fold increase in the intensity of the
second harmonic generation wave is observed. Moreover, by integrating quantum
emitters that have an absorption energy at the fundamental frequency, a second
order nonlinear exciton-polariton strong coupling response is observed with a
Rabi splitting energy of 19 meV. The nonlinear frequency conversion using
nanopatch antennas thus provides an excellent platform for nonlinear control of
the light-matter interactions in both weak and strong coupling regimes which
will have a great potential for applications in optical engineering and
information processing.
