Highly efficient valence state switching of samarium in BaFCl:Sm nanocrystals in the deep UV for multilevel optical data storage
We report on the highly efficient generation of stable Sm2+ in nanocrystalline BaFCl:Sm3+ by exposure in the deep UV (UV C) below 250 nm. The generated Sm2+ can be read out via its distinct fluorescence signature which is efficiently excited around 425 nm by the relatively strong f d transitions with e 400 (l mol 1 cm 1) The generation of Sm2+ can also be reversed, erasing the fluorescence signal via two photon ionization by increasing the power at 425 nm from 1 mW/cm2 to ~100 mW/cm2. It follows that the switching mechanism is based on oxygen impurities that are in close proximity to the Sm3+ ions. The photoionization kinetics indicate that the average Sm3+ oxide impurity separation is a few interionic spacings. The level of Sm3+ Sm2+ conversion in BaFCl is shown to be tunable over a large dynamic range, and therefore could serve as a platform for rewritable ultra high density multi level optical data storage. The present study also sheds light on BaFCl:Sm3+ as a photoluminescent X ray storage phosphor.