Simultaneous mixing processes convert an input frequency to an output frequency via a dark intermediate stage, using a method analogous to atomic stimulated Raman adiabatic passage (STIRAP).

Details: it is commonly assumed that optical wave mixing processes can only take place when the nonlinear medium is transparent at all participating frequencies. We analyze two simultaneous three wave mixing processes, 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 means that even if the medium is opaque at the intermediate frequency, the conversion will remain efficient. 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.