Abstract: Colour centres in wide-bandgap semiconductors feature metastable charge states that can be interconverted with the help of optical excitation at select wavelengths. The distinct fluorescence and spin properties in each of these states have been exploited to show storage of classical information in three dimensions, but the memory capacity of these platforms has been thus far limited by optical diffraction. Here we leverage local heterogeneity in the optical transitions of colour centres in diamond (nitrogen vacancies) to demonstrate selective charge state control of individual point defects sharing the same diffraction-limited volume. Further, we apply this approach to dense colour centre ensembles, and show rewritable, multiplexed data storage with an areal density of 21 Gb inch–2 at cryogenic temperatures. These results highlight the advantages for developing alternative optical storage device concepts that can lead to increased storage capacity and reduced energy consumption per operation.
[...] This is possible by multiplexing the storage in the spectral domain.
"It means that we can store many different images at the same place in the diamond by using a laser of a slightly different color to store different information into different atoms in the same microscopic spots," said Delord, a postdoctoral research associate at CCNY. "If this method can be applied to other materials or at room temperature, it could find its way to computing applications requiring high-capacity storage." [...]
"What we did was control the electrical charge of these color centers very precisely using a narrow-band laser and cryogenic conditions," explained Delord. "This new approach allowed us to essentially write and read tiny bits of data at a much finer level than previously possible, down to a single atom."
Optical memory technologies have a resolution defined by what's called the "diffraction limit," that is, the minimum diameter that a beam can be focused to, which approximately scales as half the light beam wavelength (for example, green light would have a diffraction limit of 270 nm).
"So, you cannot use a beam like this to write with a resolution smaller than the diffraction limit because if you displace the beam less than that, you would impact what you already wrote. So normally, optical memories increase storage capacity by making the wavelength shorter (shifting to the blue), which is why we have 'Blu-ray' technology," said Delord.
> For decades, that [Abbe diffraction] limit has operated as a sort of roadblock to engineering materials, drugs, or other objects at scales smaller than the wavelength of light manipulating them. But now, the researchers from Southampton, together with scientists from the universities of Dortmund and Regensburg in Germany, have successfully demonstrated that a beam of light can not only be confined to a spot that is 50 times smaller than its own wavelength but also “in a first of its kind” the spot can be moved by minuscule amounts at the point where the light is confined.
> According to that research, the key to confining light below the previous impermeable Abbe diffraction limit was accomplished by “storing a part of the electromagnetic energy in the kinetic energy of electric charges."
What differentiates the CCNY optical storage approach from others is that it circumvents the diffraction limit by exploiting the slight color (wavelength) changes existing between color centers separated by less than the diffraction limit.
"By tuning the beam to slightly shifted wavelengths, it can be kept at the same physical location but interact with different color centers to selectively change their charges—that is to write data with sub-diffraction resolution," said Monge, a postdoctoral fellow at CCNY who was involved in the study as a Ph.D. student at the Graduate Center, CUNY.
Another unique aspect of this approach is that it's reversible. "One can write, erase, and rewrite an infinite number of times," Monge noted
"Reversible optical data storage below the diffraction limit" (2023) https://www.nature.com/articles/s41565-023-01542-9 :
"Optical data storage breakthrough increases capacity of diamonds by circumventing the diffraction limit" (2023) https://phys.org/news/2023-12-optical-storage-breakthrough-c... :
https://news.ycombinator.com/item?id=35617859 :
"Real-space nanophotonic field manipulation using non-perturbative light–matter coupling" (2023) https://opg.optica.org/optica/fulltext.cfm?uri=optica-10-1-1...
"Optical tweezers"