利用单光子和双光子共振电离增强He原子谐波强度

理论研究了He原子在吸收紫外光子后对高次谐波的强度的影响。结果表明：当紫外光光子能量满足He原子基态与激发态的单光子和双光子共振跃迁时，谐波强度有超过2个数量级的增强。当紫外光光子能量满足He原子基态与激发态的多光子共振跃迁时，谐波强度增强明显减弱。分析电离几率表明，单、双光子共振电离在紫外共振电离中起主要作用，因此导致谐波强度的明显增强。同时，在啁啾调频场的作用下谐波截止能量也明显延伸，可以获得一个强度较高的谐波光谱连续区。选取该谐波连续区可获得脉宽为40 as的孤立脉冲。

Enhancement of harmonic intensity by using single-photon and two-photon resonance ionizations of He atom

By absorbing the ultraviolet photons, the intensity of high order harmonics of He atom has been theoretically studied. The results show that when the ultraviolet photon energy satisfies the single-photon and two-photon resonance transitions between the ground state and the excited state of He atom, the harmonic intensity can be enhanced by over 2 orders of magnitude. While, when ultraviolet photon energy satisfies the multi-photon resonance transitions between the ground state and the excited state of He atom, the harmonic enhancement is remarkably decreased. The analyses of ionization probability shows that the single and double photon resonance ionizations play the dominant role in resonance ionization process, which is responsible for the remarkable harmonic enhancement. Moreover, with the introduction of frequency-chirping, the harmonic cutoff can also be extended, showing an intense spectral continuum. By choosing this spectral continuum, the isolated pulses with the durations of 40 as can be obtained.