Energy Transfer and Upconversion in YTTERBIUM:YAG and YTTERBIUM:ERBIUM:YAG

Energy transfer and upconversion have been studied in ytterbium, and ytterbium-erbium doped YAG (yttrium-aluminum garnet rm Y_2 Al_5 O_ {12}). The energy-transfer probabilities were calculated for Yb-Yb, Yb-Er, and Er-Yb interactions, as well as for Er-Er and Yb-Er upconversion. The transfer of excitation from ytterbium to erbium was found experimentally to be efficient for Er concentrations above about 1%, with a Yb concentration of 6.5%. The Burshtein hopping model for donor migration-assisted transfer was applied to the Yb-Er system for different Er concentrations. Fitting the decay predicted by the Burshtein model to the Yb fluorescence gave a value for the Yb-Er dipole-dipole interaction strength which was in very close agreement with the calculated value, but discrepancies were noted for the donor-donor interaction. Upconversion was observed during pulsed measurements, and studied in detail with intense CW pumping. The Yb -Er upconversion was observed to dominate the Er-Er upconversion in the Yb-Er samples. Values for the Yb-Er and Er-Er rm(^4 I_{11/2}) upconversion coefficients were determined from quantitative measurements of the ^4 S_ {3/2} fluorescence which these processes generate, and measurements of the change in ^4 I_{11/2} population with increasing pump density. The quenching of nominally pure Yb:YAG by trace rare-earth impurities was also studied. The time dependence of the Yb fluorescence decay in some high-concentration samples suggested a fast quenching process active at high excited-ion densities, and a slower one at lower excited -ion densities. Study of the cooperative luminescence emitted by excited Yb pairs suggested a quenching mechanism acting preferentially on pairs. Sensitive Ar^+-ion laser -pumped fluorescence measurements showed the presence of high ppm levels of several impurities, including erbium, thulium and chromium. Pumping the ytterbium at 940 nm using pulsed and CW lasers revealed erbium and thulium upconversion. The latter is a three-step process, and its time-dependence illustrated the importance of Yb-Yb migration to the impurity quenching.