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 ε ≈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