Self-focusing and frequency broadening of an intense short-pulse laser in plasmas

An intense ultrafast laser pulse propagating through a plasma undergoes sel f focusing and self-phase-modulation as a result of relativistic mass nonli nearity. The inclusion of a quartic (r(4)) term in the expansion of the eik onal in the radial coordinate r allows the modification of the shape of the radial intensity profile. The front of the pulse, under the combined effec ts of time-dependent self-focusing and frequency downshifting, acquires a s everely distorted temporal shape. The radial profile for I lambda (2)(mu) < 2.8 x 10(18) W/cm(2), where I is the axial laser intensity and <lambda>(mu ) is the laser wavelength in micrometers, is transformed from a Gaussian to a super-Gaussian because of the faster convergence of the marginal rays th an the paraxial rays. In the opposite case of I lambda (2)(mu) > 2.8 x 10(1 8) W/cm(2) when nonlinear plasma permittivity approaches saturation, the ra dial profile in the axial region becomes broader than the Gaussian. (C) 200 1 Optical Society of America.