Energy transfer upconversion in Ho3+ and Ho3+, Yb3+ doped CdF2 crystals

A comparative spectroscopic study for Ho3+ and Ho3+, Yb3+ doped CdF2 crystals has been realised at liquid nitrogen temperature. The Ho3+ single-doped sample shows principally blue emission corresponding to the F-5(3) --> I-5(8) transition, whereas the Ho3+, Yb3+ codoped sample gives rise to a very efficient green emission which is ascribed to the F-5(4), S-5(2) --> I-5(8) transition. The total integrated upconversion emission intensity (including blue and green emissions) for the codoped crystal is three times more intense than that of the Ho3+ doped one, while the ratio for green emission rises to 27. The green emission in the codoped sample results mainly from the Ho3+-Yb3+ mixed centre, whereas the blue emission is essentially due to the Ho3+-Ho3+ pair. Two energy transfer upconversion mechanisms have been proposed and are discussed in the paper.