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 =1×10–16 m2/W. The nonlinear absorption shows an intensity-independent coefficient of =4×10–10 m/W related to fast processes. In addition, we measure a second 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.