  Plasmonic enhancement of nonlinear light-matter interaction can be achieved
via dedicated optimization of resonant plasmonic modes that are spectrally
matched to the different wavelengths involved in the particular nonlinear
optical process. In this work, we investigate the generation and enhancement of
broadband four-wave mixing (FWM) in a plasmonic azimuthally chirped grating
(ACG). The azimuthally varying grating periodicity in an ACG offers a
well-defined channel to mediate the near field and the far field over a broad
range of wavelengths. However, the particular mechanism responsible for field
enhancement in such a platform depends on the interplay between the effects
manifested by both the groove geometry and the grating's periodicity. This work
delineates the collective contribution of groove geometry-dependent localized
surface plasmon resonance (LSPR) and periodicity-dependent plasmonic surface
lattice resonance (PSLR) over a broad range of wavelengths to bring into effect
the enhancement of broadband FWM in an ACG.
