Frequency conversion of laser pulses in gold plasma: blueshift and splitting of high-order harmonics

We report the optimization of the high-order harmonic generation (HHG) in the laser-induced gold plasma. It includes determination of the characteristics of heating pulses allowing formation of the gold plasma possessing suitable characteristics (plasma density, electron density, electron temperature, velocity of plasma), as well as determination of the optimal delay between heating picosecond pulses and driving femtosecond pulses. Time-resolved Au plasma spectra and images allowed determining the velocity of plasma spreading (2 × 10⁴ m s^-1), electron density (3 × 10¹⁶ cm^-3), and electron temperature (0.8 eV) of gold plasma at the optimal fluence of heating 150 ps pulses allowing HHG up to the 53rd order at the conversion efficiency of 1 × 10^− 5. We also report the modification of the blueshift of harmonics at different regimes of HHG and plasma formation. The harmonic splitting onto strong blue component and weak red component appeared at 3 × 10¹⁴ W cm^-2 intensity of 806 nm pump.