Spectral splitting in high-order harmonic generation

Using superintense 46-fs Ti:sapphire laser pulses, high-order harmonic generation in argon gaseous medium is experimentally studied. When the driving laser intensity is pushed well above the ionization threshold (saturation intensity) of the medium applied, the double-peak spectral splitting of high harmonics has been clearly observed. This splitting is mainly due to the propagation effects in the rapidly ionizing medium, which is verified by means of one-dimensional calculations. The numerical simulations agree well with the experimental results.