FRESNEL DIFFRACTION THEORY FOR STEADY-STATE THERMAL LENS MEASUREMENTS IN THIN-FILMS

Fresnel diffraction theory is combined with a three-dimensional temperature model to predict accurately the beam intensity profile produced by a steady-state thermal lens in 250-μm liquid films. In the comparison of theory and data, the optical parameters of the thermal lens instrument and the thermo-optical properties of the sample were introduced into the model; it was not necessary to adjust any parameter in the theory to produce good agreement between the predicted and observed beam profile. Because no parameters are adjusted to match theory to data, the thermal lens measurement provides an absolute measurement of sample absorbance. The model is used to predict the behavior of the thermal lens in thin films with respect to sample absorbance, sample thickness, laser power, and optical alignment. The new model is compared with several models from the literature; it is necessary to utilize the three-dimensional heat-flow analysis and Fresnel diffraction theory to obtain an accurate description of the behavior of the thermal lens in measurements of thin films.