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 ω₁ field its second harmonic ω₂ of variable intensity in a noncollinear geometry. It allows us to identify, by momentum conservation, each field's contribution (n₁,n₂) to the extreme ultraviolet emission at frequency Ω=n₁ω₁+n₂ω₂. We observe that the photon (Ω) yield follows an n₂ power law on the ω₂ intensity, before saturation. It demonstrates that, although HHG is a highly nonperturbative process, a perturbation theory can still be developed around it.