Relating Localized Electronic States to Host Band Structure In Rare-Earth-Activated Optical Materials

crystal’s electronic band states relative to the 4f N or 4f N-1 5d 1 states responsible for the ion’s optical transitions is important for understanding the properties and performance of each material since energy and electron transfer between these states influences the material’s efficiency and stability. 1 Little is known about the relationships between these states, but there is growing motivation to explore these properties for developing ultraviolet laser materials, phosphors for applications including plasma displays and mercury-free lamps, scintillator materials for medical imaging, and optical data processing and storage technologies based on photorefractivity or photon-gated photoionization holeburning. Continued advances in optical technologies require knowledge of the systematic trends and behavior of rare-earth energies relative to crystal band states so that the properties of current materials may be fully understood and new materials may be logically developed. We have recently initiated a systematic study of the relative energies of the rare-earth ions’ electronic states and the host band states in optical materials using resonant electron photoemission spectroscopy (RPES). 2,3 RPES directly determines the energies of all occupied electronic states relative to a common energy reference and can unambiguously separate and assign spectral features to a particular electronic state. 4 Figure 1 presents results for yttrium aluminum garnet (YAG), the most important host crystal for solidstate lasers. Circles represent measured binding energies of the rare-earth 4f N ground state relative to the valence band maximum (the host’s highest energy occupied state). These results have led to an empirical model that successfully describes the rareearth binding energies in optical materials with two parameters: one describes a constant shift experienced by all rare-earth ions and the second describes a smaller dependence on the rare earth’s ionic radius. These empirical parameters may be determined from measurements on just two different rare-earth ions, or, in certain cases, simply from measurements on the host crystal itself. With parameters determined from our measured