Ionization-free femtosecond UV pulse filamentation resulting from non-perturbative Kerr effect saturation due to transient photoexcitation of molecules

Our experimental and numerical study of filamentation of the 248 nm 100 fs pulses in air, nitrogen, and oxygen at atmospheric pressure shows that the densities of photoelectrons in the filaments are not sufficient to limit Kerr self-focusing.

The simulations explain this result, showing that the nonlinear refractive index growth with the UV intensity is saturated due to transient photoexcitation of a molecule, not the photoionization.

Correct simulation of the observed filament intensity ratio in different gases validates the suggested mechanism of the femtosecond UV pulses filamentation.