Dual-beam extension of a Z-scan technique has been developed to study the thermal changes in the refractive index resulting from linear and nonlinear absorption in liquids. The experiments were carried out with nanosecond-pulsed Nd:YAG radiation and its second harmonic as the excitation sources and a He-Ne laser as the probe beam. The cw probe radiation enabled us to observe temporal dynamics of the refractive index variations, to select the thermal signal for Z-scan measurement and to measure absolute changes in the refractive index as small as 10<sup>-6</sup> due to heating in a number of organic liquids. The temporal shape of the thermal signal is analyzed experimentally and theoretically both in weak and strong phase distortion of the probe wave. Gaussian approximation for the pump and probe beams and the Taylor series decomposition of nonlinear phase distortion term at the exit of the sample are used to describe the thermal signal and to fit experimental data to theoretical models.
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