Harmonic generation beyond the Strong-Field Approximation: Phase and temporal description

Despite its general success, the validity of the Strong-Field Approximation to reproduce some particular aspects of the harmonic generation and electron photoionization has been recently put into question. In particular, the decrease of the high-order harmonic generation yield with the wavelength is found to be much sharper than the prediction from this approximation. New theoretical developments demonstrate that the scaling law for this quantity can be well reproduced with approaches either alternative to the standard S-matrix, or extending the Strong-Field Approximation to include the relevant dynamics of the bound state. This later method is demonstrated to give a quantitative accurate description of the high-order harmonic spectra in a wide variety of situations, including different pulse shapes, pulse lengths, intensities and wave-lengths. In this paper we will analyze the accuracy of this extended Strong-Field Approximation to reproduce the phase characteristics of the spectra and, consequently, to the full characterization of the high-order harmonics (amplitude and phase) in comparison with the standard strong-field approximation and with the exact integration of the time-dependent Schrödinger equation.

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