Pressure optimization and phase matching of high harmonics generation in CO₂ and C₂H₂ molecular gases

Highlights: • The phase matching of high harmonic generation (HHG) with variation of pressure at the interaction region. • Enhancement and extension of HH yield around optimum pressure point. • Phase matching relies on atomic dispersion, electronic dispersion, Gouy phase, and atomic dipole phase. - Abstract: We experimentally investigate the effect of the varying gas pressure on high harmonic generation (HHG) in carbon dioxide (CO₂) and acetylene (C₂H₂) gases with femtosecond laser fields. The pressure variations cause changes of the concentration of gas molecules and also affect the phase matching conditions and absorption. The experimental results have demonstrated appearance of well pronounced peaks of the yields of high harmonics at certain pressure values maximizing the coherence length of HHG and were confirmed with the theoretical calculations based on one-dimensional model, providing good agreement.