Generation of spatial subharmonics by two-wave mixing in a nonlinear photorefractive medium

Self-generation of spatial subharmonics when two coherent, almost collinear, pump beams propagate in a nonlinear photorefractive crystal of Bi12 SiO20, is reported. The intensity of the self-generated light beam at the half-angle between the two pumps is as high as 40% of the pump beam intensity. This phenomenon is probably a spatial analog of the temporal subharmonics obtained in a cavity containing a nonlinear medium. Application of the subharmonic of half-spatial frequency to collinear Bragg diffraction of a near infrared beam is demonstrated.