Stephan Heer, Olaf Lehmann, Markus Haase and Hans-Ulrich Güdel
Blue, Green, and Red Upconversion Emission from Lanthanide-Doped LuPO₄ and YbPO₄ Nanocrystals in a Transparent Colloidal Solution
Angew. Chem. 115, 3288-3291 (2003)                 
Angew. Chem. Int. Ed. 42, 3179-3182 (2003)                 

Abstract: (Article without Abstract)
Graphical Abstract: Light converter: The emission of visible light (Vis) upon excitation in the nearinfrared (NIR) has been observed for the first time in a transparent solution. Excitation at 975 nm (marked red in the picture) of optically clear colloidal solutions of LuPO₄:Yb³⁺, Tm³⁺, and YbPO₄:Er³⁺ nanocrystals in chloroform leads to visible upconversion luminescence in the blue, green, or red spectral regions.