Efficient frequency conversion through absorptive bands of the nonlinear   crystal
  Two simultaneous three wave mixing processes are analyzed, where an input frequency is converted to an output frequency via an intermediate stage. By employing simultaneous phase-matching and an adiabatic modulation of the nonlinear coupling strengths, the intermediate frequency is kept dark throughout the interaction, while obtaining high conversion efficiency. This feat is accomplished in a manner analogous to population transfer in atomic stimulated Raman adiabatic passage (STIRAP). Applications include conversion between remote frequencies, e.g. mid-IR to visible, and study of electronic crystal properties in the UV absorption band. 
Gil Porat and Ady Arie
physics.optics