Anomalous high-order harmonic generation

Abstract : We present experimental harmonic spectra showing anomalously high orders generated at low laser intensities (around 2 × 10^13 W cm^−2) in argon, krypton and xenon. These high orders cannot be explained with the usual three step model that predicts the existence of a sharp cut-off after the region of constant efficiency called the plateau: the observed orders are far above the cut-off limit inferred from the laser intensity in the medium. The systematic study of these spectra are presented, and some possible theoretical models are explored.