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 3.0x10(17) Ge cm(-2) and annealing at 1100 degrees 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 similar to less than or equal to 1 ps, independent of energy. However, For long pulse lengths (600 fs) the signal exhibits a higher order (similar to 4th order) dependence on pulse energy and has a relaxation time (similar to 10 ps) which increases with increasing pulse energy. Extreme pulse energies are shown to cause irreversible changes in the sample. (C) 1999 Elsevier Science B.V. All rights reserved.