The big data era has witnessed a significant increase in data volume, necessitating additional storage devices to handle the continuous growth in information. High-density optical storage technology offers advantages such as non-contact operation, resistance to electromagnetic interference, and high storage density, suggesting an excellent solution for storing, processing, and analyzing big data. However, traditional optical storage technologies encounter limitations regarding storage density improvement owing to the diffraction limit, which restricts the size of recording points.
Advancements on the diffraction limit to improve optical storage density represent an important research topic. However, the reported near-field optical storage techniques require evanescent wave detection, which requires precise motion control during optical writing. Additionally, it is difficult to form multi-layer structures, which limits the increase in storage capacity. Recently, the development of multi-parameter optical field modulation technology has enabled the creation of novel small light field structures under the tightly focusing of high numerical aperture objectives. This advancement can be used to generate small sized recording points, which provides new possibilities for achieving high-density optical storage. This study focuses on the latest advancements in optical field modulation technology, particularly in tight focusing. It includes theoretical designs, simulations, experiments, efficient generation devices, and spatial tighter focal field applications.
The study highlights the significance of small-sized sub-diffraction focal spots for improving optical storage density. It discusses light field modulation theory, including mathematical descriptions of optical diffraction and the focal spot size limit. Diffraction depictions such as scalar and vectorial diffraction theories, along with the concept of optical super-oscillation, are explored to address the diffraction limit and achieve super-resolution focal spots. Scalar diffraction theory is a simplified form of vectorial diffraction theory. The latter has further applicability and provides higher precision in optical calculations. In contrast to scalar beams, vector beams consist of more complex light field distributions and are better suited to address the diffraction limit to generate smaller focal spots. This study also examines the sidelobe size variation concerning the main lobe size. Generally, as the main lobe size decreases, the strength of the sidelobe increases. When the size of the main lobe meets the super-oscillation criterion (0.38 λ/NA), the sidelobe ratio is 16.2%. At this point, the intensity of the sidelobe maintains a good balance with the main lobe size, and its effect on the actual focused spot is negligible, which is acceptable for practical application.
Furthermore, new optical field structures and modulation technologies are introduced, featuring the excellent focusing performance of cylindrical vector, non-diffraction (Bessel beams), and nonparaxial abruptly autofocusing beams. Moreover, the distinct optical field distributions produce unique characteristics for tight focusing. Cylindrical vector beams featuring radially polarized components, particularly radially polarized light, generate smaller focal spots owing to their stronger longitudinal component during focusing. Non-diffracting beams (Bessel) are capable of elongating focal spots and enhancing the focus depth. Nonparaxial abruptly autofocusing beams concentrate more energy within a shorter distance to produce a tightly focused spot. These advancements enable greater freedom for customizing the structural light field in the focal plane and are compatible for additional application scenarios.
A discussion is presented on focusing devices based on new optical field modulation technology, such as binary lenses, photon sieves, and metalenses. This research highlights that super resolution focusing can be achieved and additional functions can be realized by combining new light fields, such as modulating Bessel beams to achieve a super resolution optical needle. Furthermore, each method has distinct advantages. For instance, binary lenses have a simple structure with straightforward processing, photon sieves effectively suppress high-order diffraction and the sidelobe effect with a wide range of spectral responses. Metalenses, with subwavelength characteristic sizes, demonstrate significant potential in light field modulations owing to their high accuracy and multiple dimensions.
In summary, this study emphasizes the importance of super-resolution focal spots in achieving high-density optical storage that surpasses the shortcomings imposed by the diffraction limit. The results provide effective solutions for reducing recording point size and improving storage density. Different focusing devices are evaluated for their unique characteristics, and their application in high-density optical storage is discussed. These advancements address the diffraction limit restriction and offer a variety of options for the development and application of new high-density optical storage technologies.
