 Depletion of the generating media due to ionization also means that harmonic generation is mainly confined to the leading edge of the driving pulse.High harmonics are emitted co-linearly with the driving laser and can have a very tight angular confinement, sometimes with less divergence than that of the fundamental field and near Gaussian beam profiles.The maximum photon energy producible with high harmonic generation is given by the cut-off of the harmonic plateau.