Spontaneous self-trapping of optical beams in metastable paraelectric crystals

Summary form only given. Spontaneous symmetry breaking in systems undergoing a phase-transition represents one of the most intriguing subjects of statistical physics. Here we study light propagation in a metastable crystalline system that undergoes a first-order phase transition, and observe nonlinear propagation effects that seed the ordering of the system itself. Specifically, we launch a very narrow optical beam into a sample of photorefractive potassium-lithium-tantalate-niobate (KLTN) (KLiTaNbO/sub 3/) and examine its diffraction. When the crystal is at a temperature slightly above the paraelectric-ferroelectric transition (in the paraelectric phase), the beam is strongly distorted. After a temporal transient, the initially diffracting and heavily distorted beam self-focuses and eventually self-traps: exhibiting stationary (non-diffracting) propagation, resembling a spatial soliton.