High-order harmonic generation with a strong laser field and an attosecond-pulse train: the Dirac Delta comb and monochromatic limits

摘要

In recent publications, it has been shown that high-order harmonic generationcan be manipulated by employing a time-delayed attosecond pulse trainsuperposed to a strong, near-infrared laser field. It is an open question,however, which is the most adequate way to approximate the attosecond pulsetrain in a semi-analytic framework. Employing the Strong-Field Approximationand saddle-point methods, we make a detailed assessment of the spectra obtainedby modeling the attosecond pulse train by either a monochromatic wave or aDirac-Delta comb. These are the two extreme limits of a real train, which iscomposed by a finite set of harmonics. Specifically, in the monochromaticlimit, we find the downhill and uphill sets of orbits reported in theliterature, and analyze their influence on the high-harmonic spectra. We showthat, in principle, the downhill trajectories lead to stronger harmonics, andpronounced enhancements in the low-plateau region. These features are analyzedin terms of quantum interference effects between pairs of quantum orbits, andcompared to those obtained in the Dirac-Delta limit.