Enhanced optical nonlinearity in amorphous silicon and its application to waveguide devices.

We present the first measurements of optical nonlinearity due to free carrier effects in amorphous silicon films using z-scan technique, demonstrating enhanced nonlinearity due to existence of midgap localized states. We also introduce, fabricate and experimentally characterized a new composite waveguide structure consisting of amorphous and crystalline silicon. The fabricated composite rib waveguide confirms to have enhanced free-carrier nonlinearity at the estimated value of 4cm/GW, i.e., seven times larger than that of a crystalline silicon waveguide. Due to existence of the midgap localized states in amorphous silicon, the measured free-carrier lifetime in the composite rib waveguide was about approximately 300ps which is shorter than the values reported in the literature for similar geometries made of silicon.