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We numerically solved the time-dependent Schrödinger equation (TDSE) for a hydrogen atom interacting with intense near-infrared laser fields to investigate the mechanism of below-threshold high-harmonic generation (HHG). The primary focus was on understanding the spectral features, particularly resonant structures, arising in the fifth harmonic region under specific driving conditions. Our simulations utilized a laser wavelength of 608 nm. At this wavelength, hydrogen atoms can resonantly absorb five photons, promoting electrons from the ground state |1s⟩ to the excited state |2p⟩. Concurrently, atoms can absorb additional photons leading to ionization. Crucially, due to the AC Stark shift induced by the intense laser field (laser dressing), certain laser-dressed continuum states |c⟩ become energetically degenerate with the laser-dressed |2p⟩ state. High-harmonic radiation at the fifth harmonic frequency can then be emitted via two distinct quantum paths: (1) Bound-bound recombination: Direct recombination from the laser-dressed |2p⟩ state back to the ground state |1s⟩. (2) Continuum-bound recombination: Recombination from the laser-dressed continuum states |c⟩ (reached via ionization) back to |1s⟩. Both pathways emit photons of identical energy corresponding to the fifth harmonic. Our key finding is the pronounced quantum interference between these two recombination channels. This interference manifests spectrally as a characteristic asymmetric Fano lineshape in the intensity profile of the fifth harmonic. Furthermore, we demonstrate that the shape of this Fano resonance exhibits a strong and controllable dependence on the intensity of the driving laser field. This study provides clear evidence that Fano quantum interference, typically associated with multi-electron correlations or autoionizing states in complex systems, can emerge in the fundamental single-electron hydrogen atom system under intense laser fields. The interference arises directly from the coherent superposition of the bound-bound and continuum-bound recombination pathways enabled by laser-induced degeneracy. Importantly, the spectral profile of the Fano resonance can be actively manipulated by tuning the laser intensity, highlighting a novel avenue for coherent control of harmonic emission in simple atomic systems.
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Keywords:
- Hydrogen atom /
- Intense laser field /
- Below-threshold high-harmonic generation /
- Fano resonance
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