Self-action of ultrashort intense laser pulses in dense gases: self-focusing, optical breakdown, supercontinuum generation

Self-action during the propagation of ultrashort intense laser pulses through dense gases is practically unavoidable and leads to a dramatic change of pulse characteristics such as the beam size and frequency spectrum. The authors report results of experiments on propagation of ultrashort intense laser pulses in high pressure (1-40 atm) gas media. Self-focusing (SF) was observed under various conditions of gas pressure and laser power, accompanied by such phenomena as supercontinuum (SC) generation, conical emission (CE) and optical breakdown (OB). Supercontinuum generation (i.e. a nearly white spectrum generated upon propagation of intense short laser pulses through nonlinear media) has been observed in gases only recently, with appearance a new generation of powerful ultrashort lasers. The authors measured for the first time the spectral evolution of the supercontinuum radiation as a function of incident laser power and gas pressure. The single shot SC spectrum was deeply modulated, with this modulation changing with gas pressure and laser power. Beyond the SC generation threshold the authors also observed an intense conical emission with sharp spectral and spatial maxima