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

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.