Development of optically-active materials for high-capacity optical data storage
Creators: name: GAO, Le
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 510204
subject: Photonics, optoelectronics and optical communications

description: In the current information revolution, in which big data, cloud computing, and artificial intelligence are being developed rapidly, an unprecedented amount of information is generated in the digital world at an ever-increasing annual rate. The current data centres worldwide are facing tremendous and constant challenges in storing the rapidly increasing amount of information and retaining data integrity over long periods of time for future retrieval. Hence, researchers are attempting to address the formidable tasks of developing data storage media and technical solutions that support higher data capacities, extended data lifetimes, cost-effectiveness for large-scale industrialisation, and low power consumption.

The latest results achieved in optical data storage (ODS) studies with multi-dimensional data storage, long-term ODS, and superresolution optical microscopy techniques such as stimulated emission depletion (STED) and superresolution photoinduction-inhibited nanolithography (SPIN) have displayed promising potential for next-generation ODS studies.

The STED and SPIN techniques permit nanoscale level imaging and writing of subdiffraction size features that can assist in increasing the data storage density significantly. However, a suitable medium is still lacking in terms of power consumption and costeffectiveness in material preparation.
The research work described in this thesis included two sections in which two technical pathways were employed to develop optically active materials for next-generation ODS.

The first research work in this PhD project (chapter 3) was focused on developing a solidstate optically active polymeric material with high thermal stability that supports fourdimensional (4D) ultra-high-density storage with a centuries-long data lifetime. A solid-state polymeric material was developed, and 4D ODS was made possible through a galvo-dithered direct laser writing system with an assignment of 16 grayscale levels for each recorded bit from multiple bit-layers. Further, 4D storage with 80 layers of bit recording and 16-level greyscale coding suggests that 10 TB per standard digital video disc can theoretically be supported by the material, and the data lifetime was estimated to be approximately 250 years at room temperature by performing an accelerated aging test. Rare-earth-doped up-conversion nanoparticles (UCNPs) possess an extended fluorescent emission lifetime hundreds of times longer than that of average fluorophores and can be photo-activated at extremely low power, providing a pathway for ultra-low power ODS.
 
The objective of the second part of this PhD thesis (chapter 4-5) was to explore and theoretically model a new type of monomer-based nanocomposite doped with UCNPs with distinct fluorescence emission lines for ultra-low power three-dimensional (3D) superresolution nanoscale ODS using the SPIN technique assisted by UCNPs. A series of core-shell structured NaYF4:Yb3+, Tm3+/NaYF4 UCNPs with optimised fluorescence emission bands in the ultraviolet (UV) and visible (VIS) range were custom devised for a two-beam co-irradiation superresolution writing system. The initiator and inhibitor systems were designed and configured at optimised concentration ratios based on theoretical simulation. The nanocomposite was subsequently developed with UCNPs, an initiator-inhibitor system, and a selected monomer host matrix. The fluorescence emission from UCNPs induced by an external excitation source (980 nm continuous-wave (CW) laser) activates the initiator, initiating the polymerisation process, and an inhibition source (375 nm CW laser) activates the inhibitor, initiating the inhibition process. Theoretical simulation demonstrates that the sub-diffraction (~50nm) writing occurs within the nanocomposite in the two-beam superresolution system with a Gaussian-shaped 980 nm CW laser as the excitation beam and a doughnut-shaped 375 nm CW laser as the inhibition beam within the nanocomposite with the optimized initiator to inhibitor ratio and doping concentration of monomer host matrices.

The 980 nm CW laser writing beam for conducting superresolution writing is estimated to be tens to hundreds of watts per square centimetre, and the inhibition beam 375 nm CW laser is on the order of tens to hundreds of milliwatts per square centimetre based on the theoretical simulation and previous reports (see in chapter 5). As a result, the intensity values of the writing and inhibition beam presented in this work are comparable to others previously reported using the SPIN technique and are several orders lower than those reportedly achieved with the nanoscale ODS in UCNPs-graphene oxide (GO) under the co-irradiation of a two-beam superresolution writing scheme with writing and inhibition beam intensities of 0.13 MW/cm2 and 11.25 MW/cm2, respectively.

In summary, the first part of this PhD thesis reports on a solid-state optically active polymeric material based on commercially available materials and the use of a well-developed optical writing technique to achieve ultra-high-density ODS with a centuries-long data lifetime. The results provide a practical data storage medium solution for next-generation data centres with large-scale industrialisation potential. The second part of the PhD thesis describes the achievement of designing and theoretical modelling ultra-low-power 3D nanoscale ODS in a novel nanocomposite based on UCNPs and a monomer host matrix via traditional low-cost CW laser sources. This work presents a conceptual breakthrough in exploring superresolution writing with the incorporation of UCNPs into the current SPIN technique. The power consumption demonstrated in this work is comparable to the state-of-the-art results from SPIN and is estimated to be several orders lower than those of other UCNP-involved subdiffraction writing techniques such as UCNPs-GO based on the theoretical analysis. Furthermore, this scientific innovation can be readily translated into other research domains and exhibits great potential for ultra-high-resolution imaging and 3D nanoscale fast prototyping with distinct fluorescent structures.