Influence of spatial coherence of a laser field on the efficiency of two-photon absorption

An investigation was made of the dependence of the two-photon absorption of ruby laser radiation in alcohol solutions of rhodamine 6 G on the intensity of the exciting field in the case of single-frequency emission and emission of a large number of transverse modes. The efficiency of the two-photon absorption in the case of multimode excitation was 1.8 times (determined with a relative error of 25%) greater than the efficiency of the absorption as a result of single-frequency excitation. The measured absorption coefficient was close to that calculated on the assumption that the single-frequency field was spatially coherent and the spatial fluctuations of the multimode field were described by a Gaussian distribution.