High-order harmonic generation in crossed laser beams

We investigate laser high-order harmonic generation in the presence of interfering light. A relatively weak interfering pulse intersects the primary harmonic-generating laser pulse at the focus. The influence on the harmonic-generation process is studied at near-counterpropagating and at perpendicular angles. The interfering beam creates a standing intensity and phase modulation, which disrupts microscopic phase matching and shuts down local high-harmonic production. Simple quasi-phase matching is demonstrated in which the interfering light disrupts harmonic generation in a portion of the laser focus. Under poor phase-matching conditions, perpendicularly propagating light is shown to enhance the 23rd harmonic generated in argon.