Ultrafast, green third-harmonic generation and strong-field phenomena in silicon-on-insulator nanoplasmonic waveguides

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.