Effect of nonlinear absorption on self-focusing of a laser beam in a plasma

Considering both the real and imaginary parts of the dielectric constant to be intensity dependent, we have investigated the self-focusing of a Gaussian laser beam in a plasma. The mechanism or nonlinearity considered herein is the ponderomotive force or heating mainly determined by collisions of electrons with heavier particles. An equation for the beamwidth parameter and an expression of the axial intensity have been obtained in the WKB(J) and paraxial approximations. It is seen that the effect of nonlinear absorption on self-focusing is significant. Consideration of nonlinearity in absorption predicts focusing of a laser beam (under certain conditions) even when the linear-absorption approximation would predict defocusing of the beam. The results reduce to the corresponding analytical results reported earlier, when the intensity dependence of the dielectric constant is neglected. The technique adopted in the present investigation for solving the wave equation in the presence of nonlinear absorption is equally applicable to media other than a plasma.