Optimising data storage and beam shaping using ultrafast laser writing
Humans are generating ever-larger information and high-value datasets in their daily activities, creating the urgent need for more efficient forms of data storage offering high capacity, low energy consumption and long lifetime.
To meet the growing data storage demand, commercial cloud providers rely on magnetic media storage technologies such as hard disk drives (HDDs) and tapes.
However, the drawbacks of HDDs are their high energy consumption and short lifespan of less than ten years, while tapes have a long average case response time – hindering the application of both technologies.
More futuristic DNA-based data storage is capable of storing hundreds of terabytes (TB) of data per gram, but with limited durability and a high cost.
Optical data storage has been heralded as an energy-efficient solution with a longer lifetime, but traditional optical data storage technologies of compact disk (CD) and digital versatile disk (DVD) are only capable of holding tens of gigabytes (GB) per disk, with a lifetime of a decade. Recent advances in femtosecond (fs, 10-15s) laser writing in dielectric materials have paved the way for high-density data storage with a long lifespan by rapid and precise energy deposition.
500TB of data on a glass disc
Although five-dimensional (5D) optical data storage in transparent materials has been demonstrated before by laser writing to harness plasmonic properties of metallic nanoparticles  or imprint nanogratings , it is still challenging to achieve high data writing speed and capacity for real-world applications. To overcome this hurdle, we’ve used a 10MHz femtosecond laser to create tiny voxels, each of which contains a single nanolamella-like structure with a size of just 500nm x 50nm . These tiny structures can be used for virtually eternal 5D optical data storage that is more than 10,000 times denser than Blu-Ray optical disc storage technology. The demonstrated readout accuracy of this multi-layer data is close to 100%. The technology can be used to write 1,000,000 data voxels per second, corresponding to about 230 kilobytes (kB) of data (more than 100 pages of text) per second.
Since the nanostructures are anisotropic, the birefringence they produce can be characterised by the slow axis orientation (4th dimension, corresponding to the orientation of the nanolamella-like structure) and strength of retardance (5th dimension, defined by the size of nanostructure). When data is written into glass with an ultrafast laser, the slow axis orientation and strength of retardance can be controlled by the polarisation and intensity of light, respectively.
Figure 1: Laser writing of birefringence structures inside silica glass. (a) Schematic of laser writing setup. (b) Images of the slow axis azimuth of voxels written by 10 laser pulses with (left) and without (right) energy modulation with 250fs pulse duration and 515nm wavelength, at repetition rates of 10MHz
Instead of femtosecond laser direct writing, we harnessed the light to produce an optical phenomenon known as near-field enhancement, in which a nanolamella-like structure is created by a few weak light pulses, from an isotropic nanovoid generated by a single pulse microexplosion. This reduces thermal accumulation and any resultant damage problems that can occur when using high repetition rate lasers.
This new approach improves the data writing speed to a practical level, enabling us to write tens of gigabytes of data in a reasonable time. The highly localised, precision nanostructures enable a higher data capacity because more voxels can be written in a unit volume. In addition, using pulsed light reduces the energy needed for writing.
With the current system, we have the ability to preserve terabytes of data, which could be used, for example, to preserve information from a person’s DNA.
Ultrahigh efficiency geometric phase shaping and polarisation control
Another important application of anisotropic modification via ultrafast laser writing is the fabrication of beam shaping elements.
Conventional nanograting-based elements have a drawback of low optical transmission, especially in the visible and ultraviolet range. At the Optoelectronics Research Centre, we recently demonstrated a new type of birefringent modification with ultrahigh transmittance by ultrafast laser direct writing in silica glass . Randomly distributed oblate nanopores with diameter about 20nm have been observed, which are responsible for the high transmittance and controllable birefringence. For demonstration purposes, ultralow loss spatially variant birefringent optical elements, including a geometrical phase flat prism and lens, vector beam converters and zero-order retarders have been fabricated. The high optical damage threshold of the demonstrated optical elements, comparable to pristine silica glass, overcomes the limitations of geometrical phase and polarisation shaping using conventional materials and fabrication methods, including photo-aligned liquid crystals and meta-surfaces.
The path ahead
In order for the ultrafast laser birefringence patterning technology to become viable for practical applications ranging from optical data storage to beam shaping elements, high throughput is still a critical factor.
Figure 2: Virtually lossless geometric phase flat lens fabricated with nanopore modification in silica glass. Left: Birefringence image of a flat lens and intensity patterns of 488nm beam and the focal lengths are ±208mm. Right: The same lens corrects short -5D and long +5D sightedness.
We are now working to increase the data writing speed to a level of MB/s by using a 40MHz laser and to make the technology usable outside the laboratory. On the other hand, rapid and accurate readout of the stored data is equally important for practical data storage applications, and so a computer-controlled motorised imaging system is being considered to improve readout speed.
Moreover, today’s high transmittance beam shaping elements with high optical damage threshold are expensive due to their relatively long manufacturing time. We recently proposed more efficient ultrafast laser writing with elliptically polarised pulses instead of linear polarisation to reduce the fabrication cost. More complicated beam shaping elements and holograms can be imprinted with such techniques as well. Therefore, we can expect that in the near future, multifunctional beam shaping elements with better performance and lower cost can be used in a wider range of scenarios.
This work was financially supported by Microsoft (Project Silica) and the European Research Council (ENIGMA, 789116).
Yuhao Lei is a PhD candidate at the Optoelectronics Research Centre, University of Southampton.
Professor Peter G. Kazanksy leads the Physical Optics group at the Optoelectronics Research Centre, University of Southampton.
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 Y. Lei, et al. "High speed ultrafast laser anisotropic nanostructuring by energy deposition control via near-field enhancement." Optica 8.11 (2021): 1365-1371.
 M. Sakakura, et al. "Ultralow-loss geometric phase and polarization shaping by ultrafast laser writing in silica glass." Light: Science & Applications 9.1 (2020): 1-10.