Quantum origin of plateau structures in strong laser-atom processes

The characteristic plateau structures in the high-energy spectra of strong laser-atom phenomena (such as ATI and HHG) are shown to originate from the properties of the exact wave function of an initially bound electron subjected to a strong monochromatic laser field of frequency $\omega$. Using time-dependent effective range theory [1], we obtain the spatial dependence of the Fourier-harmonic components (for frequencies $n\omega$) of the quasistationary, quasienergy state (QQES) corresponding to the initial bound state. This dependence reveals a remarkable plateau-like behavior over spatial distances up to the amplitude of free-electron oscillations in the laser field. This behavior is typical for Fourier-harmonics with energies $n\hbar\omega$ up to 3.17$U_{p}$ (where $U_{p}$ is the ponderomotive energy). Our numerical analysis of ATI and HHG spectra demonstrates that the high-energy plateaus in these spectra (which have been interpreted in terms of the semiclassical rescattering scenario) originate from the above-mentioned plateau features of the Fourier-harmonics of the QQES wave function. [1] M.V. Frolov et al., \textit{Phys. Rev. Lett.} \textbf{91}, 053003 (2003).