Energy transfer in YB sensitized upconversion laser crystals

Most crystalline upconversion visible lasers demonstrated to date operate at cryogenic temperatures. Since many of these laser transitions terminate on a Stark level of the ground state or a long-lived intermediate state, thermally activated population of these laser terminating levels results in substantial absorption losses. This is generally considered to be one of the reasons for the failure of upconversion lasers to operate at room temperature. Yb sensitized upconversion schemes provide some opportunities for material optimization since Yb ions have strong absorption at useful near IR pump wavelengths and can efficiently transfer the absorbed energy to co-dopant ions so that they, in turn, are able to emit in the visible. This makes it possible to reduce the concentration of lasing ions. However, it has been found that energy transfer from the lasing ions, such as Ho, Er, and Tm, to Yb also takes place very efficiently. This back transfer is very sensitive to Yb concentration and temperature and depletes the population of both the blue-green emitting levels and the intermediate levels. Back transfer is largely responsible for the drastic thermal quenching of the upconverted emission. The effects of energy back transfer on the emission spectra and intensity and the implication it has in Yb sensitizer concentration optimization, as well as the mechanisms of these back transfer processes will be discussed in detail