Small-scale self-focusing of nonparaxial laser beams

We use the linear stability theory to study the small-scale self-focusing o f intense laser beams in the presence of nonparaxiality. An analytic expres sion for the gain spectrum is obtained. A comparison of the gain spectrum o btained with the paraxial and non-paraxial methods shows that nonparaxialit y reduces the cutoff frequency, the fastest growing frequency and the maxim um growth of the perturbation. Nonparaxiality exerts more influence on the cutoff frequency and the fastest growing frequency than on the maximum grow th.