Resonant high harmonic generation: Beyond the three-step model |
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| Vasily Strelkov | |
| Prokhorov General Physics Institute of Russian Academy of Sciences | |
| High harmonic generation (HHG) via interaction of intense laser radiation with matter provides a unique source of coherent collimated xuv femto- and attosecond pulses. Simple, but very fruitful three-step model [1,2] of the HHG describes it as result of tunneling ionization, classical free electronic motion in the laser field, and recombination accompanied by the XUV emission upon the return to the parent ion. In this paper we consider two cases where the HHG study assumes essential development of this model.
We present a four-step HHG model describing generation of the harmonic resonant with the transition between the ground and an autoionizing state of the generating atom or ion [3]. The first two steps are the same, but instead of the last step (radiative recombination from continuum to the ground state) the free electron is trapped by the parent ion, so that the system (parent ion + electron) lands in the autoionizing state, and then it relaxes to the ground state emitting XUV. The results of the numerical and analytical calculations based on this model are in good quantitative agreement with the experiments (see reviews [4,5] and references therein) showing HHG enhancement up to two orders of magnitude when the harmonic is resonant with the transition frequency. Our simulations predict the phase-locking of the resonantly-enhanced harmonics. This allows production of an attosecond pulse train using such harmonics, recently demonstrated experimentally [6]. The second example deals with polarization properties of the HH generated from atoms and molecules. The harmonics generated by atoms in elliptically-polarized laser field are elliptically polarized, and the polarization ellipse of the harmonic is rotated by a certain angle with respect to that of the fundamental. This rotation can be well described with the three-step model considering a CLASSICAL electronic motion after the photoionization. The harmonic ellipticity itself, however, can be hardly understood taking into account only classical motion properties. We show [7] that this ellipticity originates from quantum-mechanical uncertainty of the electron motion. The analytical theory is verified with the exact numerical TDSE solution. The results reasonably agree with experimentally measured polarization properties of harmonics both in case of atomic and molecular HHG. The outlook of this work is study of the polarization properties of resonant HH. [1] P. B. Corkum, Phys. Rev. Lett., 71, 1994 (1993). |