Nonlinear pulsed excitation of high-Q optical modes of plasmonic nanocavities.

We present a comprehensive theoretical and numerical analysis of the physical mechanisms pertaining to the nonlinear pulsed excitation of optical modes in plasmonic cavities made of metallic nanowires. Our analysis is based on extensive numerical simulations carried out both in the frequency and time domains. The numerical algorithm used in our study is based on the multiple scattering method and allows us to include in our analysis the effects of both the surface and bulk nonlinear polarizations generated at the second harmonic (SH). In particular, we investigate the physical properties of plasmonic modes excited at the SH as the result of the interaction of femtosecond optical pulses with plasmonic nanocavities. We show that such cavities have two distinct types of modes, namely, plasmonic cavity modes and multipole plasmon modes generated via the hybridization of modes of single nanowires. Our analysis reveals that the properties of the latter modes depend only weakly on the cavity geometry, whereas the lifetime and quality factor of plasmonic cavity modes vary considerably with the system parameters.