Nonlinear optical response of Ge nanocrystals in silica matrix with excitation of femtosecond pulses

We report an investigation of third-order optical nonlinearities in Ge nano crystals (similar to 6 nm radius) embedded in silica matrix using the Z-sca n and pump-probe techniques with femtosecond laser pulses at 780-nm wavelen gth. The nanocrystallite Ge samples were prepared using magnetron co-sputte ring and post-thermal annealing at 800 degreesC. The nonlinear absorption c oefficient and refractive index of the Ge nanocrystals were determined to b e in the range from 1.8 x 10(-7) to 6.8 x 10(-7) cm/W and 1.5 x 10(-12) to 8.0 x 10(-12) cm(2)/W, respectively, which are proportional to the Ge atomi c fraction in the matrix. Relaxation of the nonlinear response was found to have two characteristic time constants, 1.8 ps and 65 ps. The mechanisms r esponsible for the observed nonlinear response are discussed.