Dynamic lenses in femtosecond filament

We study, both experimentally and numerically, the variations of the refractive index at the filamentation of a femtosecond laser pulse in a fused silica. Using the polarigraphy and shadowgraphy techniques, we investigate the spatio-temporal variations of the refractive index induced by Kerr and plasma nonlinearities. The induced variations of the refractive index play the role of dynamic lenses: focusing for Kerr nonlinearity and defocusing for plasma nonlinearity. We show that intensity saturation occurs at the distance, where the total optical force of Kerr and plasma nonlinear lenses reaches its maximum.