Critical behavior of dynamic thermal properties of liquid mixtures determined from the spatial Fourier spectrum analysis of an induced thermal lens

The authors report here what they believe to be the first observation based on a nonlinear optics technique of a singular behavior for both D/sub th/ and Λ/sub th/ in this system. The original optical technique that the authors have developed for the purpose of measuring D/sub th/ and Λ/sub th/ in liquids is an improved version of the well-known induced thermal lens (ITL) arrangement. The basis of the method is the time analysis of the transverse spatial Fourier spectrum of a thermal lens induced in the sample by a time-modulated YAG laser. The spectrum of a probe field (He-Ne) perturbed by the ITL is formed in the focal plane of an auxiliary lens. Wave vectors of the same modulus are selected through an annular diaphragm. As a result, the filtered probe intensity is proportional to Λ/sub th//sup -2/ and relaxes as a single exponential with a time constant tau = 1/2D/sub th/q/sup 2/. Practically, this method should be considered as low-angle forced Rayleigh scattering (FRS) corresponding to low values of the wave vector. The usual FRS arrangements are convenience only for larger values of q with the drawback of small interaction volumes. (This often requires the use of auxiliary absorbers which are not appropriate in experiments where sample purity is essential.)