High harmonic generation in a Xe–He gas mixture driven by kHz tightly focused laser pulses

High harmonic generation and its spectral properties are studied by using kHz tightly focused laser pulses in a Xe–He gas mixture. The cutoff region of harmonics in mixed gases is extended from H21 to H25. The results are attributed to the harmonic generation from the excited He atoms assisted by harmonic radiation from the Xe atoms. We illustrate that the pressure dependence of harmonic H19 shows a flat intensity profile in mixed gases. The results are explained by the harmonic H19 being moved from the cutoff region to a plateau region. (paper)