Coherent buildup of high-order harmonic radiation: The classical perspective

We present a classical model for high-order harmonic generation during the propagation of an intense laser pulse through an atomic gas. Numerical simulations of the model show excellent quantitative agreement with the corresponding quantum model for the blueshift and intensity reduction of the propagating laser pulse over experimentally realistic propagation distances. We observe a significant extension of the high-order harmonic cutoff due to propagation effects. A phase-space analysis of our classical model uncovers the mechanism behind this extension.