Spectral-angular patterns and energy threshold for linear-to-nonlinear femtosecond laser pulse focusing in air

Abstract

Focused propagation of a high-power ultrashort laser pulse is strongly influenced by optical medium nonlinearity that can significantly affect pulse parameters in the focusing region as compared to the linear focusing regime. We present the results of our experimental study and numerical simulations of frequency-angular patterns and laser plasma distribution of a gigawatt-power femtosecond Ti-sapphire laser pulse propagating in air under various focusing conditions. For the first time to the best of our knowledge, we found experimentally and confirm theoretically the range of pulse numerical aperture (NA), from approximately NA = 2 ⋅ 10−3 to 5 ⋅ 10−3, when both frequency and angular spectra broadening during supercritical laser pulse focusing in air is minimal. We attribute this NA-range to the transition region between quasilinear and nonlinear pulse focusing regimes. Additionally, the energy threshold for different pulse focusing regimes in the wide range of focusing conditions is obtained for the first time through the numerical simulations. This threshold is demonstrated to decrease with pulse NA increase.