Dispersive subband-resonant nonlinearity in amorphous Si/SiO2 quantum well structures

Dispersive nonlinearity in amorphous Si/SiO₂ quantum well structures (QW´s) has been investigated. The refractive index changes obtained from the intensity-dependent reflection spectra are nonlinearly dependent on the excitation intensity and can be described by the model of the saturating nonlinearity at low pump intensities. The nonlinear refractive index reveals resonant behavior associated with the subband structure of the QW´s. The saturated nonlinear index and the saturation intensity have been obtained as Δn_s=-0.11 and I_s=1.9 MW/cm² at the transitions between the lowest subbands, and Δn_s~0.3 and I_s~0.5 MW/cm² at the transitions between the second subbands of the valence and conduction bands. The nonlinearity for the second subband transitions has been found high enough to provide potentially bistable operation, but the bistability is not expected at the transitions between the ground subbands. Carrier lifetime less than 1 ps restricting the switching time of the nonlinearity has been estimated from the saturation intensity