A shadowgraph model for the thermal lens effect: a theoretical and experimental study

In the thermal lens technique, based on the effect having the same name, energy from a focused laser beam heats a sample generating a local gradient in the refractive index that causes a change in the phase of a second laser beam (called a probe) passing through the same region of excitation. On the other hand, the shadowgraph technique allows us to visualize the deflections of a light beam caused by refractive index disturbances of a medium it passes. In this paper, we use the methodology widely used in shadowgraph techniques to propose a new model describing the temporal evolution of the probe laser intensity due to the thermal lens effect. Thermal diffusivity measurements in liquid samples demonstrated the usefulness of the model.