Theoretical description of low divergence Gaussian fields in self-defocusing photorefractive media

Experimental evidence is presented that shows that low intensity optical fields preserve their Gaussian transverse amplitude distribution as they propagate through self-defocusing Ce:BaTiO₃ photorefractive media. The Gaussian nature of the field is used in a theoretical treatment to derive conditions under which bright solitons are formed in photorefractive media that have a light induced refractive index that is approximately quadratic. This analysis shows that while it is not possible to produce a single bright soliton in self-defocusing media that it is possible to minimize the field’s divergence such that the change in beam radius is small (<1%) over large propagation distances (∼1 m). An imaginary light induced refractive index component is necessary to generate the low divergence fields in both self-focusing and self-defocusing media when illuminated with Gaussian fields that have a non-planar wavefront.