Comparison between low optical absorption models for pump-probe transient thermal lens spectroscopy: A theoretical and experimental study

We compare the most widely used low optical absorption models based on the Fresnel diffraction theory for the description of the time-resolved thermal lens signal in a dual-beam pump-probe mode-mismatched configuration. In this work, we will name one of them as the numerical mode-mismatched model. The others are two simplified approximations of it that will be referred to as the analytical mode-mismatched and the reduced mode-mismatched (RMM) model. The limits of application of these approximations are discussed based on computational calculations and experimental measurements performed under conditions that neglect contributions to the thermal lens signal generation of mechanisms different from optical absorption, heat diffusion and the propagation of the excitation and probe laser beams. It is shown that for fractional changes of the probe beam greater than about 15% these approximations must be used carefully, particularly the RMM model.