Nonlinear optical beam propagation and solitons in photorefractive media

Conclusions  The last few years have witnessed a great renewal of interest in the photorefractive effect thanks to the realization of the possibility of propagating stable 2D spatial solitons at very low optical powers in PR crystals. This result, besides possessing a great scientific relevance in itself, associated with the first experimental demonstration of a non-diffracting two-dimensional pencil of light, has also an interesting applicative potential thanks to the guidance properties of the waveguide associated with the soliton, which survives in the dark also after the soliton has been turned off. In this paper, we have tried to present a self-contained approach to the theory of self-trapped nonlinear propagation, which is far from being definitive, together with the most important experimental demonstrations obtained in the last few years. The novelty of the field, and the fact that it is still undergoing a rapid growth, has made our task not an easy one and we apologize to the readers for the many inevitable omissions both in the subjects we have chosen to emphasize and in the references. We cannot conclude this review without mentioning self-trapping of planar optical beams by the use of the PR effect in a semiconductor (InP:Fe) [94, 95], the main advantages over standard PR materials being associated with its sensitivity in the range of the near-infrared wavelengths and faster response time.