Cascaded high harmonic generation in mixture of argon and helium: Achieving a broad photon spectrum

In this study, we explore high harmonic generation (HHG) in pure argon (Ar) gas and a mixture of argon and helium (He). We investigate phase-matching conditions and interference effects in both single and mixed-gas systems. By varying the gas pressure in the Ar–He mixture, we optimize the harmonic spectrum, achieving a broad range from H17 to H43, corresponding to photon energies of 25 eV to 75 eV. We attribute the spectrum broadening to a cascaded HHG process: argon-generated extreme ultraviolet photons excite helium's outer electrons, which are then driven by the fundamental laser field to contribute to HHG. This finding aligns with previous studies, showing that mixing gases with low and high ionization potentials can enhance HHG. The results offer a broad HHG spectrum ideal for ultra-fast spectroscopy and high-resolution imaging applications.