Ultrahigh-Order Wave Mixing in Noncollinear High Harmonic Generation

摘要

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 ω_1 field its second harmonic ω_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 Ω = n_1ω_1 + n_2ω_2. We observe that the photon (Ω) yield follows an n_2 power law on the ω_2 intensity, before saturation. It demonstrates that, although HHG is a highly nonperturbative process, a perturbation theory can still be developed around it.