ENERGY-TRANSFER IN (CA-ZR)-SUBSTITUTED GD3GA5O12 SINGLE-CRYSTALS DOPED WITH CR-3+, TM-3+, AND HO-3+ IONS

(Ca-Zr)-substituted Gd3Ga5O12 single crystals doped with Cr3+, Tm3+ and Ho3+ ions were grown and optical studies have been made. Cr3+ --> Tm3+ energy transfer and Tm3+ --> Tm3+ cross-relaxation processes have been analyzed in (Cr, Tm) doubly-doped crystals and their efficiencies calculated as a function of the Tm3+ concentration. In (Cr, Tm, Ho) triply-doped crystals, it is shown that whatever the excitation, the 5I7 level of Ho3+ is mostly populated by energy transfer from the 3F4 level of Tm3+. The 5I7(Ho3+) --> 3F4(Tm3+) back-transfer is shown to occur at room temperature and to increase with the Tm3+ concentration. This has a deleterious effect on the Tm-->Ho energy transfer efficiency.