Femtosecond growth dynamics of an underdense ionization front measured by spectral blueshifting

Time-resolved spectral blue shifts of 100-fs laser pulses caused by ionization of atmospheric density N₂ and noble gases subjected to high (10¹⁴ W/cm² to 10¹⁶ W/cm²) light intensities are examined. Included are data for two experiments: self-shifting of the ionizing laser pulses for varying peak intensities, pressures (1-5 atm), and gas species; and time-resolved blue shifts of a weak copropagating probe pulse for the same range of ionization conditions. The self-shift data reveal a universal, reproducible pattern in the shape of the blueshifted spectra: as laser intensity, gas pressure, or atomic number increase, the self-blueshifted spectra develop from a near replica of the incident pulse spectrum into a complex structure consisting of two spectral peaks. The time-resolved data reveal different temporal dependence for each of these two features. A quantitative model for a simplified cylindrical focal geometry is presented