Photorefractive diffusion-driven self-focusing and self-trapping in near-transition paraelectric crystals

Reports on the observation of self-focusing and self-trapping in K/sub 1-y/Li/sub y/Ta/sub 1-x/Nb/sub x/O/sub 3/ (KLTN) in the very-near-transition paraelectric phase without any external applied field and without any sort of artificial background illumination. We show that this behavior depends only on the temperature T of the sample and is independent of beam intensity and beam size. In order to explain this peculiar phenomenon we make use of the PR paraelectric diffusion nonlinearity, obtaining solutions that describe self-focusing and that eventually lead to the formation of 'diffusion-driven solitons'.