Low-cost nonlinear optics experiment for undergraduate instructional laboratory and lecture demonstration

This paper presents a simple and affordable experiment on the thermal lens effect, suitable for an undergraduate educational laboratory or as a tabletop demonstration in a lecture on nonlinear optics. Such an experiment exploits the formation of a lens in an absorbing medium illuminated by a laser beam with a Gaussian intensity profile. As an absorber, we use a commercial soy sauce, which exhibits a strong thermal lensing effect. Additionally, we show how to measure the radius of a Gaussian beam using the knife-edge method, and how to estimate the focal length of the induced thermal lens.