Luminescence and photoconductivity of cerium compounds

The study of the luminescence properties of rare earth doped systems historically focuses on systems which exhibit strong luminescence. More recently, extensive studies on materials with high quantum efficiency are in part motivated by the search for new phosphor and scintillator materials. However, a thorough study of certain systems which show very low quantum yield will certainly lead to a better understanding of phosphor materials and rare earth systems in general. As an example of recent studies which address both the fundamental question of relaxation processes in rare earth doped systems and phosphor applications we present studies on cerium-doped lutetium oxide crystals which are characterized by a complete quenching of the 5d-4f luminescence and compare its optical properties to that of very efficient cerium doped phosphor material, lutetium oxyorthosilicate. To find the mechanisms which lead to the different quantum efficiency in these systems, extensive absorption, photoexcitation and photoconductivity studies were performed on single crystals. We demonstrate that the radically different emission properties of the investigated systems originate in small but crucial differences in the location of the emitting 5d level of the cerium ion with respect to the conduction band of the host - a general result which can be applied to a broad range of materials.