Model studies on possible physical origin of plateaus in high- order harmonic generation

Three common features, namely, an initial decrease at low orders, a plateau up to some high order and abrupt cutoff, are well known to characterize typical spectra of high-order harmonic generation (HHG). And yet, the physical origin of the plateau, extending up to surprisingly high orders at times, is not clearly understood. We have been studying HHG by numerically solving the time-dependent Schroedinger equations (TDSE) in one-dimensional atomic and ionic model potentials. We ave also solved the corresponding classical equation of motion numerically to understand the physical origin of the typical features of HHG spectra. When we compare the two results we find a clue which suggests that a plateau is formed in the region of the characteristic frequencies of the system.