Nonlinear optical response of Ge nanocrystals in a silica matrix

Time-resolved degenerate-four-wave-mixing measurements were used to study t he nonlinear optical response (intensity-dependent refractive index! of Ge nanocrystallites embedded in a silica matrix. Nanocrystals were fabricated by ion-implanting silica with 1.0 MeV Ge ions to fluences in the range from 0.6 to 3 X 10(17) Ge cm(-2), followed by annealing at 1100 degrees C for 6 0 min. For the highest fluence, this resulted in nanocrystals with a log-no rmal size distribution, having a geometric mean diameter of 3.0 nm and a di mensionless geometric standard deviation of 0.25. The intensity-dependent r efractive index \n(2)\ was measured at a wavelength of 800 nm and found to increase linearly with increasing Ge fluence. For the highest fluence, \n(2 )\ was determined to be in the range 2.7-6.9 X 10(-13) cm(-2) W-1, dependin g on the duration of the excitation pulse; values were consistently smaller for shorter pulse lengths. Relaxation of the nonlinear response was found to have two characteristic time constants, one <100 fs and the other simila r to 1 ps. (C) 1999 American Institute of Physics. [S0003-6951(99)04802-0].