Upconversion effect on fluorescence quantum efficiency and heat generation in Nd3+-doped materials

The thermal lens technique was carried out to experimentally determine the influence of the energy transfer upconversion (ETU) processes on fluorescence quantum efficiency (eta) in Nd3+-doped materials. The samples with high Nd3+ concentration present a considerable reduction in eta?with the increasing excitation power due to the efficient ETU processes. Besides, the energy migration was identified as the mechanism responsible for the upconversion losses. In addition, it was verified that the critical inversion density is not concentration independent, as previously stated, but it decreases with the Nd concentration. Our results point out the approach based on TL technique as a valuable alternative because of its sensitivity allowing the measurements to be performed in a pump power regime that avoids damages in the investigated material.