We demonstrate the effectiveness of a simple method for using Z-scan technique with high repetition rate lasers managing cumulative thermal effects. Following Falconieri [J. Opt. A, 1 (1999) 662], time evolution of Z-scan signal is recorded. We use data time correlation to extrapolate with accuracy the instantaneous nonlinear optical response of the sample. The method employed allows us to clearly evaluate the order of the absorption process underlying the thermo-optical nonlinearities. Using a 76 MHz repetition rate laser with 120 fs pulsewidth we measure third order nonlinearities and thermal properties of CS2 and toluene in accordance with values obtained with low repetition rate light sources.

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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 mW related to fast processes. In addition, we measure a second Β component around 10-9 mW 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. © 2006 American Institute of Physics

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