The F3+ color centers in crystals were shown to generate a phase-conjugated 460nm wave. It was demonstrated that this generation was caused by the saturation of optical absorption due to non-radiative electron transitions between the singlet and metastable triplet levels in F3+ centers.

T.Tsuboi, H.E.Gu: Radiation Effects and Defects in Solids, 1995, 134, 349-52