Fluorescence and upconversion in CaF2:Ho3+

Laser excitation studies of CaF2 crystals doped with 0.01 to 10% Ho3+ using the second harmonic wavelength 532 nm of a pulsed Nd:YAG laser are carried out. Three fluorescence bands belonging to the 5F4, 5S2 → 5I8, 5F4, 5S2 → 5I7 and 5F5 → 5I8 transitions are detected. Fluorescence lifetimes of the first two transitions are measured and used for site-identification of new lines. Four upconverted fluorescence bands belonging to the 5F3 → 5I8, 5G4 → 5I8, 5G5 → 5I8, 5H5, 5H6 → 5I8 transitions are also detected. A comparative study is made of two pairs of energy transfer mechanisms, which are multiphonon relaxation versus ion-pair relaxation and two-photon absorption versus ion-pair relaxation. The analysis of the concentration dependence of fluorescence intensities shows that ion-pair relaxation is the chief mechanism responsible for the fluorescence and upconversion behaviour of CaF2:Ho3+. (orig.)