Spectrum modification of high-order harmonic generation in a gas mixture of Ar and Kr

In this paper, we experimentally investigate a high-order harmonic generation (HHG) process in a gas mixture of Ar and Kr. Compared with HHG in a pure gas medium, strong modification of the HHG spectrum is observed when a second gas is injected. The final overall HHG spectrum is determined by the initial pressure of individual gas before mixing. Further analysis and simulation show that the observed HHG spectrum modification is possibly contributed by two processes. First, the broadband phase-matching condition among the same type of gas atoms is shifted by increasing or decreasing the ionization ratio of Kr and Ar in HHG from the gas mixture, respectively. Second, HHG generated from Ar and Kr interfere with each other and result in wavelength-dependent bandwidth narrowing and intensity fluctuation in the overall HHG spectrum. Our experiment shows that the relative phase modulation among the same and different types of gas atoms play various but important roles in the HHG process, which needs to be carefully considered in the high-flux extreme ultraviolet source generation or HHG-based molecular structure retrieval from a noble gas mixture.