Upconversion-pumped 2.8–2.9-μm lasing of Er3+ ion in garnets

Upconversion-pumped lasing characteristics near 3 μm on the 4I11/2-4I13/2 transition of the Er3+ ion in Er:YSGG (erbium-yttrium-scandium-gallium garnet) and Er:YAG (erbium-yttrium-aluminum-garnet), laser state spectroscopy, and population kinetics, are the main subjects discussed. The wide difference in lasing patterns of both garnets can be attributed to the difference in population inversion kinetics which was studied by analyzing the rise and decay of fluorescence starting from the laser states. It is shown that the effective decay times, τ1 and τ2 of the lower and upper laser states, are changed during lasing by the nonlinear energy-transfer processes such that the resulting ratio becomes τ1/τ2<1. In Er:YSGG this condition is established during the declining phase of the 4I11/2 population, whereas in Er:YAG it occurs only for a short time during the rising phase. Consequently Er:YAG lases in a self-terminating pulsed mode. The role of various energy-transfer processes in the upconversion-pumped population inversion kinetics is discussed and analyzed. It is experimentally shown that the lower 4I13/2 laser state in both garnets is depopulated by the cooperative energy summing due to the di- and tri-ionic interactions. Because of these interactions, the 4I11/2 and 4S3/2 states are populated with ensuing fluorescence from these states at 980 and 550 nm, respectively. The indication that the latter state is populated directly by the tri-ionic interaction, rather than via the former state by two sequential ion-pair interactions, follows from the experimental fact that the 550-nm fluorescence rises faster and reaches its maximum sooner than the 980-nm fluorescence. The previously proposed kinetic rate equations for Er:YAG and Er:YSGG were modified by adding a cubic term to represent the tri-ionic interaction. The steady-state solutions yielded approximately cubic dependence of green fluorescence intensity on the intensity of pump radiation in agreement with the experiment. These solutions also explained deviations from cubic dependence observed experimentally in several erbium-doped crystals.