  The deployment of plasmonic nanostructures to enhance nonlinear signal
generation requires effective far-to-near field coupling and phase matching for
frequency conversion. While the latter can be easily achieved at plasmonic
hotspots, the former is an antenna problem that requires dedicated structural
design and optimization. Plasmonic gratings are a simple but effective platform
for nonlinear signal generation since they provide a well-defined momentum for
photon-plasmon coupling and local hotspots for frequency conversion. In this
work, a plasmonic azimuthally chirped grating (ACG), which provides spatially
resolved broadband momentum for photon-plasmon coupling, was exploited to
investigate the plasmonic enhancement effect in two nonlinear optical
processes, namely two-photon photoluminescence (TPPL) and second-harmonic
generation (SHG). The spatial distributions of the nonlinear signals were
determined experimentally by hyperspectral mapping with ultrashort pulsed
excitation. The experimental spatial distributions of nonlinear signals agree
very well with the analytical prediction based solely on photon-plasmon
coupling with the momentum of the ACG, revealing the antenna function of the
grating in plasmonic nonlinear signal generation. This work highlights the
importance of the antenna effect of the gratings for nonlinear signal
generation and provides insight into the enhancement mechanism of plasmonic
gratings in addition to local hotspot engineering.
