| Abstract | 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. |
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