Self-defocusing of convergent laser beams

The distinctive characteristics of self-defocusing in sharply focused laser beams are investigated theoretically. Both stationary and nonstationary self-defocusing are analyzed according to various physical laws for nonlinear media, and the effect of nonlinear absorption on stationary self-defocusing is determined. It is found that for both stationary and nonstationary self-defocusing, the peak change in permittivity is not dependent on the frequency of the incident radiation but is determined instead by the square of the focusing angle. A cut-off of the transmitted radiation pulse occurs during nonstationary defocusing and the peak light intensity in the focus is determined by the nonlinearity of the medium and by the slope of the leading edge of the incident radiation pulse. The feasibility of observing self-defocusing experimentally is considered.