Excited-state dynamics and nonlinear optical response of Ge nanocrystals embedded in silica matrix.

We use a dedicated Z-scan setup, arranged to account for cumulative effects, to study the nonlinear optical response of Ge nanocrystals embedded in silica matrix. Samples are prepared with plasma-enchanced chemical-vapor deposition and post-thermal annealing. We measure a third-order nonlinear refraction coefficient of γ=1×10^-16 m²/W. The nonlinear absorption shows an intensity-independent coefficient of β=4×10^-10 m/W related to fast processes. In addition, we measure a second β component around 10^-9 m/W with a relaxation time of 300 μs that rises linearly with the laser intensity. We associate its origin to the absorption of excited carriers from a surface-defect state with a long depopulation time. [ABSTRACT FROM AUTHOR]