High-order harmonicgeneration in a strong laser field: an alternativequantum-mechanical model

Abstract

The multiphoton strong-field process of high-order harmonicgeneration (HHG) by an isolated atomic system exposed to anintense laser monochromatic field is considered theoreticallyand studied numerically within the frame of a newly developednon-perturbative fully quantum-mechanicalstrong-field-approximation (SFA) approach. The related proposedalternative HHG model is mainly based on the Keldyshapproximation combined with a specific utilization of the `essential states' method (and associated `pole approximation'), which applied together allow forthe representation of an analytical expression for the total HHG amplitudein the factorized form, enabling transparentinterpretation. The model seems to be extremely useful toadequately describe the general shape of the HHG spectrum andequally appropriate to very effectively calculate its mainfeatures and detailed structure as well as to study theirbehaviour within the most interesting region of the main problemparameters. To demonstrate the model applicabilitycertain HHG spectra generated by various atomic systems (mostlyby noble gas atoms and negative ions) have been calculated andshown to reproduce sufficiently well the conventionalphenomenological rule for the extent of the HHG plateau and the positionof its cut-off frequency. Moreover, they are in a reasonablygood accordance with typical examples calculated by other authorswithin various different (but more analytically sophisticatedor computationally demanding) approaches developed earlier.