Femtosecond laser optical gas breakdown microplasma: the ionisation and postionisation dynamics

The formation and evolution dynamics of the laser plasma produced in the microvolumes of gases (air, nitrogen, argon, and helium) upon their multiple ionisation by high-intensity (4×10¹⁶ W cm^-2) tightly focused (to a region 1.7 μm in diameter) 400-nm, 100-fs second-harmonic pulses from a Ti:sapphire laser is studied. The spatiotemporal distribution profiles of the refractive index and electron microplasma density were recorded by ultrahigh-speed interferometry. The postionisation of a femtosecond laser plasma, i.e. the growth of the electron density known after the end of the exciting laser pulse, is detected for the first time. A theoretical model is proposed, which describes the mechanism of plasma postionisation by hot photoelectrons. The results of electron density calculations are in good agreement with experimental data.