Propagation in Underdense Plasmas and Pre-pulse Action

Recently, a systematic experimental study has clearly proved the effect on a gaseous medium of both ASE and picosecond pedestal prior to the arrival of the ultrashort intense laser pulse [26]. The study has been based on sequences of electron density maps obtained from optical interferograms with femtosecond resolution and has been supported by numerical simulation of the ionization of the medium. 

The experimental data obtained at the same time (Fig. 8.3) show similar electron density distribution, except for the preformed plasma due to the local early gas breakdown induced by the nanosecond ASE pre-pulse (visible in the left part of the ionized region). The investigation was completed with imaging and spectroscopy of the transmitted laser pulse after the propagation in the gas. 

The study confirmed that the action of the precursors of the main pulse has to be carefully considered in the ultrashort intense laser pulse interactions. Nevertheless, a regime of rather stable propagation [34] of a laser pulse of tens of femtoseconds was found in a broad window of the laser intensity/medium density diagram, in which (1) the ionization occurs in one or a few optical cycles, avoiding effects of self-phase modulation and defocusing for most of the 

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