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Novel approaches for digital data storage are imperative, as storage capacities are drastically being outpaced by the exponential growth in data generation. Optical data storage represents the most promising alternative to traditional magnetic and solid-state data storage. In this paper, a novel and energy efficient approach to optical data storage using rare-earth ion doped inorganic insulators is demonstrated. In particular, the nanocrystalline alkaline earth halide BaFCl:Sm is shown to provide great potential for multilevel optical data storage. Proof-of-concept demonstrations reveal for the first time that these phosphors could be used for rewritable, multilevel optical data storage on the physical dimensions of a single nanocrystal. Multilevel information storage is based on the very efficient and reversible conversion of Sm³⁺ to Sm²⁺ ions upon exposure to UV-C light. The stored information is then read-out using confocal optics by employing the photoluminescence of the Sm²⁺ ions in the nanocrystals, with the signal strength depending on the UV-C fluence used during the write step. The latter serves as the mechanism for multilevel data storage in the individual nanocrystals, as demonstrated in this paper. This data storage platform has the potential to be extended to 2D and 3D memory for storage densities that could potentially approach petabyte/cm³ levels. Refereed/Peer-reviewed

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. Refereed/Peer-reviewed