Abstract
The self-compression and spatiotemporal evolution of a Gaussian laser pulse propagating in a double-ionized helium gas are investigated. The numerical model is formulated by solving the nonlinear Schrödinger equation using the paraxial like approach. The beam width parameter and pulse width parameter are estimated to investigate the laser pulse advancement in a tunnel ionizing gas. Transverse focusing and longitudinal compression are examined by characterizing the beam spot size in space and time, incorporating the gas ionization processes, relativistic mass variation, and ponderomotive effects. The results show that the inclusion of laser-induced double ionization of helium gas modifies the plasma density, which significantly affects the laser pulse evolution. For intense laser pulse, relativistic-ponderomotive nonlinearity enhances the pulse compression and consequently the self-focusing of the laser pulse. The compression mechanism and the localization of the pulse intensity both are boosted by the modified electron density via a dielectric function. At a helium gas pressure of 1.4 bar, we observed that 100 fs long laser pulse with intensity I0=8.5×1016 W/cm2 is compressed to 20 fs and the initial laser spot size 10 μm focused to 2 μm. These results promise to be a method for the generation of table-top light sources for ultrafast high-field physics and advanced optics.
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