Single crystal growth of RE doped thiogallates and their optical properties Original Research Article

Rare earth (RE) elements of the lanthanide series are well known as light emitting sources when doped into transparent host materials. The mechanism of light emission due to these centers is rather well understood from both experimental and theoretical, but the detailed information concerning emitting efficiencies, spectral widths and quenching characteristics due to the RE ion concentration has not been fully obtained yet. To improve the emission efficiency and to control the functionality, now become the main issues to be addressed for their practical application to the light emitting devices. As the host materials for the RE centers above, alkaline earth thiogallates have long been expected as promising ones. However, most of the researches on these materials are carried out using polycrystalline thin films or polycrystals because of the difficulty in the single crystal growth arising from the high chemical reactivity along with high melting points of these compounds, so that the physical characteristics fundamental for the requirements mentioned above has not been fully clarified. Here, first we would like to introduce the details of our single-crystal growth method applied to Ca, Sr thiogallates, and then present the results of optical assessments for CaGa2S4 single crystals doped with several RE elements (Ce3+, Pr3+, Eu2+, Tb3+, Er3+, Tm3+). The optical transition levels of these ions are revisited in view of the calculated RE 4fn energy level now easily available through the Internet (http://www.phys.canterbury.ac.nz/crystalfield/).