Evaluation of the upconversion mechanisms in Ho3+-doped crystals:: Experiment and theoretical modeling -: art. no. 235119

The paper compares the mechanisms that enable the upconverted green emission (S-5(2)-->I-5(8)) of the Ho3+ ion under infrared excitation (700-920 nm) in several crystalline hosts (YAlO3, YLiF4, Y3Sc2Ga3O12, and BaY2F8). Parameters involved in the upconversion such as excited-state absorption and cross-relaxation rates were determined from spectroscopic measurements. A system of differential equation (rate equations) was used to describe the upconversion mechanism and was numerically solved. The results were compared with experimental data. A reduction of this system to a three-level simplified system is presented, which includes only the ground level, the emitting level, and the intermediate level. The differences between the photon-avalanche mechanism and the looping mechanism are discussed and analyzed according to this simplified system.