Thermally induced nonlinear optical effects in an isotropic liquid crystal at 10.6 mu m

We investigate thermally induced nonlinear optical effects on a CO2 laser beam that passes through an isotropic liquid crystal film. We evaluate the thermal nonlinear coefficient of the refractive index at a wavelength of 10.6 mum by means of a thermographic technique combined with an optical method based upon the measurement of the self-defocusing effect caused by the heating of the liquid crystal induced by the laser beam. The novelty of our work is the application of thermography in order to measure the temperature field on the liquid crystal film caused by the partial absorption of the laser radiation by the liquid crystal. These measurements provide a precise evaluation of the thermal diffusion length of the liquid crystal under investigation. Moreover, thermography results in a straightforward experimental technique that can be used to investigate the thermal properties of a wide class of other fluids. (C) 2002 Optical Society of America.