Tunable mid-IR pulses in the range 1300-4200 cm -1 (7.7-2.4 µm) are generated through a phase-matched four-wave mixing process in ordinary mid-IR window materials such as CaF2 and BaF2. In this process the difference frequency v3 =2v2 - v1 is generated from pump fields v1 and v2. The process can be phase matched to different frequencies by adjustment of the angle between the pump fields.

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Journal Opt. Lett.
Nienhuys, H.-K, Planken, P.C.M, van Santen, R.A, & Bakker, H.J. (2001). Generation of mid-infrared pulses by c(3) difference frequency generation in CaF2 and BaF2. Opt. Lett., 26, 1350–1352.