Ultrahigh-Order Wave Mixing in Noncollinear High Harmonic Generation

We show that noncollinear high harmonic generation (HHG) can be fully understood in terms of nonlinear optical wave mixing. We demonstrate this by superposing on the fundamental omega(1) field its second harmonic omega(2) of variable intensity in a noncollinear geometry. It allows us to identify, by momentum conservation, each field's contribution (n(1,) n(2)) to the extreme ultraviolet emission at frequency Omega = n(1)omega(1) + n(2)omega(2). We observe that the photon (Omega) yield follows an n(2) power law on the omega(2) intensity, before saturation. It demonstrates that, although HHG is a highly nonperturbative process, a perturbation theory can still be developed around it.