Thermo-optical effects in Z-scan measurements using high-repetition-rate lasers

In this paper the effects on Z-scan measurements of thermo-optical nonlinearities due to cumulative heating of the sample are investigated. A model calculation of the Z-scan signal induced by thermo-optical nonlinearities is presented which takes into account absorption processes involving an arbitrary number of photons as the sources of nonlinearity. It is quantitatively shown how weak linear or nonlinear absorption coefficients can give rise to large signals depending on the repetition rate of the laser and on the experimental conditions. Conversely, very weak nonlinear absorption coefficients can be measured exploiting the effect of cumulative heating when using a long train of laser pulses. The possibility of separating such thermal contributions from fast optical nonlinearities in Z-scan signals is discussed.