Nonlinear optical properties of semiconducting nanocrystals in fused silica

Degenerate four-wave mixing (DFWM) is used to examine the nonlinear optical response of Ge nanocrystals in a silica matrix. The nanocrystals are formed by implanting 1.0 MeV Ge ions into silica to a dose of 3.0×10¹⁷ Ge cm^-2 and annealing at 1100°C. The particle size is well described by a log normal distribution with a mean particle diameter of 3.0 nm and a dimensionless geometric standard deviation of 0.25. The nonlinear optical response, measured at 800 nm with pulse lengths in the range 100-1000 fs, is found to exhibit two patterns of behaviour. For short (180 fs) pulses the DFWM signal is shown to exhibit a third-order dependence on pulse energy (Kerr nonlinearity) and to have a relaxation time of ∼≤ 1 ps, independent of energy. However, for long pulse lengths (600 fs) the signal exhibits a higher order (∼4th order) dependence on pulse energy and has a relaxation time (∼10 ps) which increases with increasing pulse energy. Extreme pulse energies are shown to cause irreversible changes in the sample.