Influence of transient self-defocusing on the propagation of high-power femtosecond laser pulses in gases under ionisation conditions

A transient three-dimensional model is used in a numerical investigation of the spatiotemporal and spectral characteristics of a femtosecond pulse of 10¹⁴–10¹⁶ W cm^-2 intensity propagating in rare gases. It is shown that nonlinear defocusing caused by the ionisation of a gas may result in a 20-fold reduction of the peak intensity in the beam waist. The transverse distribution of the intensity beyond the waist is a ring with a minimum on the beam axis at a pressure of 1 bar and with a local maximum on the axis at 5 bar. The frequency spectra of such a pulse are in quantitative agreement with experimental data.