  The emergence of strong-field nanoplasmonics brings extreme laser
field-matter interaction into the realm of nanoscale science, unveiling
exciting new physics. Highly nonlinear interaction is enabled by tightly
confined electric fields in nanoplasmonic structures, permitting use of optical
fields from low-power laser oscillators. Here, we report the first
demonstration of visible 517nm third harmonic generation in ultracompact
nanoplasmonic waveguides on a silicon-on-insulator platform at an unprecedented
conversion efficiency of ~10^{-5}. Exponential growth of broadband white light
generation confirms a new strong-field phenomenon of ponderomotive force-driven
electron avalanche multiplication. Using time-resolved experiments, we show
that the strong nanoplasmonic field confinement allows nonlinear interaction to
occur on an ultrafast timescale of 1.98 +/- 0.40 ps, despite the long
free-carrier lifetime in silicon. These findings uncover a new strong-field
interaction that can be used in sensitive nanoplasmonic modulators and hybrid
plasmonic-electronic transducers.
