Dynamics of the spatial spectrum of a self-focusing optical wave in a nonlinear medium

A new nonlinear equation for the dynamics of the spatial spectrum of a self-focusing monochromatic wave in a medium with cubic nonlinearity is derived in the nonparaxial approximation. The formation of optical beams with cross section on the order of a wavelength is considered. Backward self-reflection is found to be the fundamental cause for the limitation of optical self-focusing