Optimization of high-order harmonic generation in the space and time domains

Abstract

For strong harmonic generation an intense femtosecond laser pulse is applied to a high-density gas media. The ionization of gas medium during high-order harmonic generation cannot be avoided, therefore, it should be controlled in both the space and time domains in order to generate optimized high-order harmonics. In the time domain, ionization of the medium induces self-phase modulation(SPM), which affects the harmonic spectrum. By a precise control of the chirp of the incident laser pulse, the spectral broadening induced by SPM was properly removed at various laser intensities from $1 \times 10^{15}$ to $5 \times 10^{15} \, W/cm^2$. To verify SPM due to ionization, the temporal characteristics of laser pulses, after they pass through the ionizing medium, were measured using the method of the temporal analysis by dispersing a pair of light electric-fields. The measurements clearly showed the modification of laser pulses due to ionization of medium. In the space doma