Pressure-dependent phase matching for high harmonic generation of Ar and N2 in the tight focusing regime

We experimentally investigate gas pressure-dependent effect on the high harmonic generation (HHG) of Ar and N2 gases in a tight focusing regime with near IR ultrafast laser fields. Different from a loose-focusing condition, the measured data show a shift of the pressure regime for phase matching towards to the lower pressure side with the increasing driven laser intensity. By comparing the results with a theoretical calculation based on the Constant' one-dimensional model, we illustrate that this pressure dependence stems from the Gaussian beam phase gradient and atomic dipole phase which can become the key factors for phase matching in the tight focusing regime, interested in a high-repetition-rate coherent extreme-ultraviolet generation.