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 Sm super(2+) in nanocrystalline BaFCl:Sm super(3+) by exposure in the deep UV (UV-C) below 250 nm. The generated Sm super(2+) can be read out via its distinct fluorescence signature which is efficiently excited around 425 nm by the relatively strong f-d transitions with [epsilon] [approximate]400 (l mol super(-1) cm super(-1)). The generation of Sm super(2+) can also be reversed, erasing the fluorescence signal via two-photon ionization by increasing the power at 425 nm from [< or =] 1 mW/cm super(2) to ~100 mW/cm super(2). It follows that the switching mechanism is based on oxygen impurities that are in close proximity to the Sm super(3+) ions. The photoionization kinetics indicate that the average Sm super(3+)- oxide impurity separation is a few interionic spacings. The level of Sm super(3+)- Sm super(2+) 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:Sm super(3+) as a photoluminescent X-ray storage phosphor.