High-order harmonic generation in a strong laser field: an alternative quantum-mechanical model

The multiphoton strong-field process of high-order harmonic generation (HHG) by an isolated atomic system exposed to an intense laser monochromatic field is considered theoretically and studied numerically within the frame of a newly developed non-perturbative fully quantum-mechanical strong-field-approximation (SFA) approach. The related proposed alternative HHG model is mainly based on the Keldysh approximation combined with a specific utilization of the 'essential states' method (and associated 'pole approximation'), which applied together allow for the representation of an analytical expression for the total HHG amplitude in the factorized form, enabling transparent interpretation. The model seems to be extremely useful to adequately describe the general shape of the HHG spectrum and equally appropriate to very effectively calculate its main features and detailed structure as well as to study their behaviour within the most interesting region of the main problem parameters. To demonstrate the model applicability certain HHG spectra generated by various atomic systems (mostly by noble gas atoms and negative ions) have been calculated and shown to reproduce sufficiently well the conventional phenomenological rule for the extent of the HHG plateau and the position of its cut-off frequency. Moreover, they are in a reasonably good accordance with typical examples calculated by other authors within various different (but more analytically sophisticated or computationally demanding) approaches developed earlier.