Experimental demonstration of novel effects on the far-field diffraction patterns of a Gaussian beam in a Kerr medium

We report on the experimental observation of the effects on the far-field patterns obtained when a divergent or convergent beam interacts with a thin self-defocusing material. The measurements were performed using a CW laser beam propagated through a cell containing a colloidal system that presents a very large nonlinear refractive index. The high nonlinearity of the colloid allowed for the observation of patterns never investigated before, revealing a quite different behaviour on the far field from a convergent or a divergent beam due to the opposite interaction with the negative Kerr nonlinearity of the medium. An unpredicted relation between the number of generated rings and the nonlinear phase-shift is obtained in the case of a convergent beam. Our results are in good agreement with a model for thin nonlinear media.