AVALANCHE UP-CONVERSION IN LAF3TM3+

Strong blue fluorescence corresponding to the 1G4 --> 3H6 transition in LaF3:Tm3+ is generated by excitation with a red laser at 635.2 nm. The upconversion mechanism is shown to be photon avalanche. The temporal behavior of the upconverted fluorescence and the dependence of the blue fluorescence intensity on excitation laser power are described by a four-level system of coupled rate equations. A unique set of of parameters is found which describes the behavior at several different excitation levels, both below and above the avalanche threshold. Two cross-relaxation paths are shown to be important to the upconversion process.