Cross-relaxation and upconversion processes in Pr3+ singly doped and Pr3+/Yb3+ codoped nanocrystalline Gd3Ga5O12:: The sensitizer/activator relationship

The room temperature luminescence properties of Pr3+-doped gadolimum gallium garnet (GGG:Pr3+, Gd3Ga5O12:Pr3+) nanocrystals (0.1, 1, 5 and 10 mol %) were evaluated. Increasing the Pr3+ concentration in the nanocrystals resulted in a decrease of the P-3(0) emission to lower lying states via the [(3)p(0), H-3(4)] - [H-3(6),D-1(2)] cross-relaxation (CR) process. Similarly, a decrease in the D-1(2) emission was observed and was attributed to the [D-1(2),H-3(4)] -> [(1)G(4),F-3(3),(4)] cross-relaxation mechanism. The increase in cross-relaxation efficiency on increasing the Pr3+ concentration was attributed to the smaller average interionic distances between the dopant ions. Dominant blue/green emission due to the P-3(0) -> H-3(4) and (3)p(0) -> H-3(6) transitions was observed after laser excitation at 457.9 nm for Pr3+/Yb3+ codoped nanocrystalline GGG samples. The blue/green emission decreased as the sensitizer (Yb3+) concentration increased in the GGG samples. The observed near-infrared (NIR) F-2(5/2) -> F-2(7/2) emission from the Yb3+ ion, upon 457.9 nm excitation, suggests the presence of an energy transfer process from the Pr3+ ions to neighboring Yb3+ ions and results in a decrease of room temperature visible emission with increasing ytterbium concentration. Upconversion emission was observed for 0.1 and 1 mol % single doped samples. The nanocrystalline GGG:Pr3+,Yb3+ codoped samples showed a notable increase in upconversion emission intensity relative to the singly doped samples excited upon 980 nm excitation radiation. Upconversion was observed to occur via an ET process for the single and codoped samples.