Excited state effect of He+ ion on intense harmonic spectrum and attosecond pulse generation

Abstract

We investigate the excited state effect of [Formula: see text] ion on intense harmonic spectrum and attosecond pulse (AP) generation. First step, by using the superposition state model, we find that as the excited state ratio increases, the intensity of harmonic spectrum can be enhanced, and a near-stable intense harmonic yield can be obtained when the ground state (GS) and the excited state remains within a certain ratio range. However, as the excited state ratio further increases, the intensity of harmonic spectrum is decreased. Further, with the introduction of a down-chirp modulation, the harmonic cutoff can be extended, showing a broader spectral continuum with a higher efficiency. Second step, by using the single GS model and with the help of the pump-probe-UV-resonance-ionization mechanism, the high-intensity and broader spectral continuum obtained in the first step can be realized in the experiment. Moreover, this method is not very sensitive to the delay time between the fundamental field and the UV pulse, which is beneficial to experimental implementation. As a result, 3 APs as short as 40 as can be obtained.