High-order harmonic generation in an active grating

Abstract : We study theoretically and experimentally high-order harmonic generation (HHG) using two noncollinear driving fields focused in gases. We show that these two fields form a nonstationary blazed active grating in the generation medium. The intensity and phase structure of this grating rule the far-field properties of the emission, such as the relative amplitude of the diffraction orders. Full macroscopic calculations and experiments support this wave-based picture of the process, complementing and extending its standard “photon” picture. This insight into the HHG process allows us to envision structuration schemes to convert femtosecond lasers to attosecond pulses with increased efficiency.