Nonlinear evolution of cosh-Gaussian laser beams and generation of flat top spatial solitons in cubic quintic nonlinear media

In this Letter, we have investigated the propagation characteristics of a cosh-Gaussian laser beam both in Kerr and cubic quintic nonlinear media. An appropriate nonlinear Schrödinger equation has been solved numerically to reveal the beam dynamics. It is found that these beams are unstable and convert into sech/Gaussian type beam. In Kerr media, they undergo collapse above certain virtual threshold power. In defocussing quintic nonlinear media too, with low and moderate power these beams transform into sech/Gaussian beam. At high power they form flat top beam, where the length of the flat top and sharpness of flatness increase with the increase in power. At very large power the beam first splits into two and again coalesces and this phenomenon recurs.