AVALANCHE UP-CONVERSION IN LAF3TM3+

Strong blue fluorescence corresponding to the 1G4 --> 3H6 transition i n LaF3:Tm3+ is generated by excitation with a red laser at 635.2 nm. T he upconversion mechanism is shown to be photon avalanche. The tempora l behavior of the upconverted fluorescence and the dependence of the b lue fluorescence intensity on excitation laser power are described by a four-level system of coupled rate equations. A unique set of of para meters is found which describes the behavior at several different exci tation levels, both below and above the avalanche threshold. Two cross -relaxation paths are shown to be important to the upconversion proces s.