Temporal and Angular Resolution of the Ionization-Induced Refraction of a Short Laser Pulse in Helium Gas

The refraction of a short and intense laser pulse focused in helium gas has been studied both experimentally and numerically. Using the time-frequency correspondence of a 120 fs laser pulse linearly chirped to 1.8 ps, the ionization-induced refraction is resolved both temporally and angularly. Two-dimensional numerical simulations are performed to calculate the laser propagation, from the focusing lens to the detector. With such a simulation, we get a quantitative agreement with the experiment, both for the refraction of the chirped pulse and for the ionization-induced blueshifted spectrum of the compressed pulse.