Researchers at Purdue University seek to enhance optical storage with higher densities and faster reads. A success in the technology development could replace the Blu-ray disc for storage.
Through plasmonic color generation, in conjunction with metallic nanoparticles and ultrathin metasurfaces, the storage medium operates at a nanoscale level. According to the researchers, they've already achieved a readout speed of 18.3 Gbps -- 143 times faster than the state-of-the-art Blu-ray disc. They have also been able to store 5% more data than a Blu-ray disc.
Why colored digital characters
Plasmonics -- or nanoplasmonics -- is a young field that deals with the interaction between lightwaves and metallic nanoparticles. Plasmonics works with nanoscale structures to exploit the electron oscillations that occur as photons hit a metal surface. The researchers believe that they can use this phenomenon for data storage that delivers higher densities and faster readouts than conventional optical discs.
To store data at this scale, the scientists use plasmonic nanostructures that vary in their relative positioning. The nanostructures serve as tiny antennae that enhance light interactions. When light hits the nanostructures, they reflect different colors based on their positioning. Changing those positions alters the reflected color spectrum. In other words, the researchers vary the orientation states of the nanoantennae in order to produce different reflected colors, which they can then translate into binary information.
In conjunction with the nanostructure positioning, the researchers use a surface-relief metasurface that consists of aluminum nanoantenna elements. The metasurface offers broader color variability and higher spatial resolution than other materials, which are often based on composite metallic resonators. According to the researchers, the aluminum metasurface is free from antireflection complexities and supports a smaller thickness. It's also more compatible with commercial nanoimprint processes.
The researchers have proposed a frame-based storage structure in which each frame is made up of 16 nanopixels. Each nanopixel has four-unit cells, or nanoantennae. Every nanoantenna orientation has a unique color. In this way, an imaging system can retrieve the corresponding color sequence from each frame. Those colors can then be translated to binary data.
In their prototype, the researchers stored 3 bits of data per nanopixel, which enables higher densities and read speeds than a Blu-ray disc. Based on their early simulations, the researchers believe that they could store 4 bits of data in each nanopixel, which results in 40% greater density than Blu-ray and achieves a read speed that is 191 times faster.
Uses of enhanced optical storage
By repositioning the nanoantennae, plasmonic storage can generate a broad range of distinguishable colors, so there's more data storage in a finite space. An optical analyzer can then capture the unique color sequences as the disc rotates and then translate those readings into binary data. Several analyzers together can read multiple storage units simultaneously, resulting in much faster read operations.
Storage admins could potentially use the new technology as a replacement for today's prerecorded audio and video optical discs, particularly Blu-ray. These discs are read-only, nonrecordable and mass-produced on a large scale, with the data stamped into the underlying media. Plasmonic discs can be manufactured in much the same way, in part because of the aluminum metasurface, according to the researchers. The higher capacities and read speeds could prove especially beneficial for the increasing densities of video content.
In its current form, the plasmonic disc is a write-once, read-many (WORM) medium that promises higher read performance than other optical storage. It still offers the high durability and archival capabilities of traditional optical discs. This makes plasmonic storage well suited to read-intensive workloads that rely on cold or archived storage, such as analytical applications that incorporate machine learning, predictive analytics and other advanced technologies.
Plasmonic discs, in general, could be a good fit for archiving an assortment of structured and unstructured data, whether or not organizations mine the data for advanced analytics. A medium with fast read access can save an organization a lot of time when it tries to find specific information. For example, IT might need to access archived data to address a compliance issue or in the event of a cyber attack.
Differences from traditional optical storage
The Purdue researchers clearly have their sights on the optical storage market, particularly as a replacement to the Blu-ray disc. The medium they used for their demonstration was the same size as a standard optical disc -- 120 millimeters in diameter. It rotated at 5,000 rpm, a comparable speed to a Blu-ray disc.
Today's optical disks are flat and circular. Data is stored as microscopic pits and lands, with the pits etched into a reflective layer of recording material. The data is written in radial patterns starting near the disc's center. Lasers then read from the discs as they spin. The differences in reflectivity determine the 0 and 1 bits that represent the data. A CD can store up to 700 MB of data, a DVD can store up to 8.5 GB and a Blu-ray disc can store up to 128 GB.
The material for recording data depends on use of the disc. Prerecorded disks, such as those used to distribute full-length motion pictures, can use cheaper material, like aluminum foil, to store data. However, a disc for WORM data storage often includes an additional organic dye layer for writing data. A rewritable disk uses a phase-change material that can be erased and rewritten multiple times.
The plasmonic disc being developed by the Purdue scientists is still in the early stages of research and has a single material for WORM data storage. The plasmonic disc includes a passive aluminum metasurface that reflects polarization-tunable plasmonic colors. The metasurface is made up of periodic arrangements of rectangular nanoantennae attached to optically thick aluminum film. Researchers estimated that this configuration could support storage densities up to 40% higher than conventional Blu-ray discs.
To record data in the plasmonic prototype, the researchers used electronic-beam lithography to burn the information into the metasurface, but they believe that more commercially viable means can record the data. The use of plasmonics rather than a pit-and-land architecture makes it possible to enhance optical storage, achieve larger densities and speed up read performance.
Challenges of colored digital storage
There appears to have been little progress toward commercializing the technology since the Purdue scientists published their research. Undoubtedly, they face an uphill battle in meeting the challenges of implementing a new type of data storage medium in today's market.
Plasmonic storage is limited to prerecorded and WORM storage, with no support for rewritable storage. The technology is also up against the continued advancements in Blu-ray storage. Although Blu-ray might not achieve the densities and read performance promised by plasmonic storage, it's a well-established medium that organizations can use for prerecorded, WORM and rewritable storage.
Even if plasmonic storage research makes significant strides, manufacturers cannot simply start producing discs. The manufacturing process might be like Blu-ray discs, but manufacturers would need to adjust their operations to accommodate the new format at production scale. Refitting equipment and modifying operations are not small tasks, and a technology must be able to demonstrate a great deal of promise to grab the industry's attention.
Plasmonic discs face some of the same capacity limitations as other optical disks. Although they promise to deliver larger densities than Blu-ray, the medium itself must still contend with the upper limits of a single rotating disk. Other storage media, though, can consolidate into much larger systems and present as a single repository.
The plasmonic disc is also up against the significant advances in solid-state storage, which continues to improve in capacity, performance and cost. Vendors invest a significant amount of time and energy in pursuing stronger SSDs, which pushes other forms of storage to the back burner. On-site storage systems have been steadily giving way to cloud storage and streaming services.
The plan to enhance optical storage through plasmonic discs must also compete against other efforts that address future requirements. For example, Microsoft's Project Silica is researching the feasibility of using ultrafast laser optics to store data in quartz crystal, which promises higher densities and resiliency than anything that currently archives data. IBM has worked on a concept called racetrack memory, which applies electrical current to nanowires to create opposite magnetic regions that can store large quantities of data. Scientists are also actively pursuing DNA memory, an approach to storage that encodes data into the genetic material.