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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.

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We compare the thermal lens (TLS) and the optical transmission (OT) spectroscopy techniques to monitor the kinetic of a photocatalytic reaction. For this, an OT measurement facility was added to a TLS set-up. The TLS was implemented in a microspatial configuration named thermal lens microscopy (TLM). Methylene blue (MB) in Water solutions were used as test samples within a concentration range in which both techniques show good sensibility. Within this range, the limit of detection obtained by TLM was about one order of magnitude lower than that achieved by OT. The methylene blue concentration evolution with photocatalytic reaction time was measured with both techniques, showing a good agreement between their results. A ZnO thin film deposited on a glass substrate by the spray pyrolysis technique was used as catalyst, and the reaction was induced by UV-violet light.

The heat and thermoelastic equations are analytically solved for a material with low optical absorption. This set of equations shows profiles describing the temperature distribution, surface displacement, stresses and optical path for a sample with a thick-disk geometry when is excited by a ring-shaped laser beam. This is done by determining the steady-state because when a laser beam is used, it turns on for several minutes to stabilize before using it in experiments. It is shown that the temperature takes some seconds to reach the steady-state condition, then, this approximation is very useful and simplifies the data processing. These results were also compared with a Gaussian profile, showing that the Gaussian equations are a consequence of the ring-shaped laser beam equations. The finite element method is used as a form of validation of the equations found in this work, obtaining a good agreement between numerical results. Errors are calculated for the temperature, displacement and optical path difference at the center(edge) of the sample; these are around 0.14 %(0 %), 0.11 %(30.50 %) and 0.09 %(4.86 %), respectively, for a BK7 sample. The analytical results obtained could be of great help in the design of the optical components and different experimental configurations. Maybe the principal advantage of a ring-shaped laser beam is to produce a temperature profile with a top-hat form at steady-state, while that a top-hat laser beam does not.