Reversible Mn valence state switching in submicron α-Al2O3:Mn by soft X-rays and blue light – a potential pathway towards multilevel optical data storage

The data storage capacity of conventional devices such as digital versatile discs and compact discs has significantly evolved from megabyte to gigabyte storage levels while achieving a significant reduction in device size. However, the data densities of these storage devices are limited by the diffraction-limited spot size. This spot size is the smallest attainable beam radius at a specific beam focus and is determined by the laser wavelength and numerical aperture.

Although larger numerical apertures and shorter laser wavelengths can be employed to enhance data storage capacity, it is not an economically viable option because of the high cost of UV lasers. With continuous research and development in this area, alternative approaches have been proposed. Among them, multilevel encoding has been identified as a viable approach for improving the storage capacity due to the possibility of encoding more than one bit per point. Effective realization of multilevel encoding requires a high signal-to-noise ratio and a large dynamic range of the physical property undergoing the photoinduced change.

Generally, multilevel data storage is based on varying the energy being deposited on a material, and the maximum number of bits that can be stored at a single point depends on the number of levels encoded. Despite the extensive research on multilevel encoding, sufficient data storage capacity is yet to be achieved. Further work is required in developing efficient multilevel data storage technologies. Previous results have shown that adopting reversible valence state switching is a promising approach for multilevel optical data storage using X-rays for the write step.

Herein, PhD candidate Norfadira Wahib and Professor Hans Riesen from The University of New South Wales together with Dr Nicolas Riesen from the University of South Australia developed a reversible valence state switching system consisting of submicron corundum, α-Al₂O₃, doped with Mn³⁺. The α-Al₂O₃ comprised a rigid lattice structure to provide a strong crystal field for dopant ions and close-packed planes of oxygen anions stacked in the hexagonal closed-packed structure. Samples with different nominal Mn concentrations (0.05,0.1, 0.2, 0.4, 0.6 and 1.2 atom%) were synthesized through a facile combustion method. The reversible valence state switching reaction in α-Al₂O₃:Mn³⁺ submicron crystals exposed to soft X-ray was investigated. The work as been published in the journal, Physical Chemistry Chemical Physics.

The authors demonstrated the generation of Mn⁴⁺ in α-Al₂O₃:Mn³⁺ with a large dynamic range of the luminescence signal, suggesting the potential application of α-Al₂O₃:Mn³⁺ for multilevel optical data storage. Among the studied samples, the sample with 0.4 atom% exhibited the highest luminescence intensity. A writing step with high data storage density was realized by leveraging the benefits of the extremely fine pitch enabled by the small diffraction limit of the X-rays. Around 1.7% of the Mn³⁺ could be converted to Mn⁴⁺ ions by soft-Xray excitation.

The stored information could be read out through the R-lines by Mn⁴⁺ luminescence at about 678 nm using either red or blue excitation, with 630 nm being recommended as it minimizes photobleaching. Interestingly, a reverse reaction could fully erase the Mn⁴⁺ valence state back to Mn³⁺ and Mn⁵⁺ via blue light excitation at around 462 nm – a process also known as the erase step. It was worth noting that the stored information could be written and erased repeatedly with no significant deterioration over five cycles with less than 5% uncertainty.

In summary, the research team reported a fully reversible switching of the Mn valence state between tri-, tetra- and pentavalent states, providing in-depth insights into the write-read-erase system in submicron α-Al₂O₃:Mn³⁺ powder. The findings also revealed that the α-Al₂O₃:Mn³⁺ could be potentially used as a dosimetry and computed radiography material in the soft X-ray region. The write-read-erase steps showed good reproducibility and reversibility without significant degradation. In a statement to Advances in Engineering, Professor Hans Riesen, the lead and corresponding author expressed confidence that their findings will contribute to new approaches for very high-density optical data storage capacity.