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.

Opt. Lett.
Ultrafast Spectroscopy

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