Excited state dynamics of thulium ions in yttrium aluminum garnets

Abstract The processes that take place in the excited states of a trivalent Thulium (Tm) ion in an Yttrium Aluminum Garnet (YAG) crystal, being relevant to the use of this system for laser applications, have been the object of several studies. We have reexamined this system, focusing our attention on the dynamics of Tm following its excitation in the 3H4 level. Under these conditions the system relaxes through a cross-relaxation process, 3H4→3F4, 3H6→3F4, whose rate depends upon both the concentration of the Tm ion and the temperature of the crystal. The excitation spectrum obtained by monitoring the 1.8 μm emission of Tm (due to the 3F4→3H6 transition) indicates an increase in the contribution to this emission from the 3H4 level relative to the 3H5 level as the Tm concentration increases; this shows the increased role played by the 3H4 level in pumping the infrared emission. Correspondingly the duration of the luminescence originating in the 3H4 level is shortened as the concentration of Tm increases. The concentration quenching of this lifetime can be fit to a model which assumes that the cross-relaxation is due to a dipole-dipole interaction; from this fit the intrinsic Tm lifetime in absence of cross relaxation can be derived. We have used this lifetime to calculate the rate of the cross-relaxation process. We have evaluated this rate as function of temperature and found it to be fastest at 77 K. We have also calculated the microscopic interaction parameters for the cross-relaxation process by using two independent experimental features: (i) the time evolution of the emission from the 3H4 level, and (ii) the spectral overlap between the 3H4→3F4 emission and the 3H6→3F4 absorption. We have also considered the migration of excitation among the Tm ions in the 3F4 level and calculated the relevant microparameter by the use of the relevant spectral overlap. The data are consistent with the model in which the Tm ions, once excited into the 3H4 level decay by cross relaxation to the 3F4 level, and then transfer rapidly their energy to other Tm ions.