When a high-power laser beam is focused into an absorbing medium, the medium is locally expanded. The phenomenon of the changing angle of convergence and divergence according to changes in the refraction of light caused by the variation in the density in the expanded region is called the thermal lens effect. The measurement methods for the thermal lens effect can be divided into a single-beam method and a dual-beam method that uses pump and probe beams. In this study, the thermal lens effect in liquids such as ethanol and methanol was measured by using a single-beam method. A Nd:YVO$_{4}$ laser beam was used as the light source, and the intensity variation of the beam transmitted through the cell that contained liquid samples was measured by using several lenses with different focal lengths as the cell was moved $\pm$3.0 cm from the focal point. The result showed that the thermal lens effect was clearly produced in ethanol and methanol that the effects for both liquids were quite similar.

Keywords: Thermal lens, Thermal expansion, Z-scan