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

Abstract

We report an investigation of third-order optical nonlinearities in Ge nanocrystals (∼ 6 nm radius) embedded in silica matrix using the Z-scan and pump-probe techniques with femtosecond laser pulses at 780-nm wavelength. The nanocrystallite Ge samples were prepared using magnetron co-sputtering and post-thermal annealing at 800°C. The nonlinear absorption coefficient and refractive index of the Ge nanocrystals were determined to be in the range from 1.8 × 10-7 to 6.8 × 10-7cm/W and 1.5 × 10-12 to 8.0 × 10-12 cm2/W, respectively, which are proportional to the Ge atomic fraction in the matrix. Relaxation of the non-linear response was found to have two characteristic time constants, 1.8 ps and 65 ps. The mechanisms responsible for the observed nonlinear response are discussed.