Storing a petabit on a single optical disc

Artists impression

This is a far higher than current magnetic recording techniques, and equates to writing 1.6Pbit on a double-sided disc.

“Optical data storage encounters a fundamental limitation in the spacing of adjacent recorded features, owing to the optical diffraction limit,” according to USST. “This physical constraint not only impedes the development of direct laser writing machines but also affects optical microscopy and storage technology.”

The researchers used a dual-beam writing technique to get over this limit – more of this later – and demonstrated writing 100 layers of physical bits that are nine times smaller that the wavelength of light producing them.

Among the things that had to be invented for the demonstration was a suitable recording medium, for which the team turned to polymer mixes related to those used as etch-resists in semiconductor lithography.

To this they added a fluorescent dye picked for its ability to retain its emissive properties even after it is locked into a solid, and its ability to be optically-modified while in that solid.

The result was a highly transparent material with optically-adjustable fluorescent properties that can be made on a substrate in a few minutes by spin-coating.

Writing voxels of data into this with sub-wavelength resolution was performed using coaxial laser beams, one from a 515nm green femtosecond laser and the other from a continuous wave 639nm red laser.

The tricks here are that the fluorescent dye was also picked for its spectral sensitivity – green light leaves it more fluorescent while red light leaves it less fluorescent – and that the green beam has a Gaussion energy distribution across its width while the red beam is made to have a doughnut distribution.

These factors combine with the result that the green light sensitises and area of the dye, and the red doughnut removes the added sensitivity from the outside of that area, leaving a small sensitive spot in the middle.

Minimum spot (bit) width along a track was 54nm (~ λ/12) and minimum track-to-track pitch 70nm (~λ/9).

The reading process is similar, with a 480nm blue pulsed laser and a 592nm orange continuous laser.

Up to 100μm depth of the transparent layer could be accesses by the objective lens of the optical system used, into which the 100 layers of data were written with ~1.6λ spacing and no cross-talk between layers.

Bit error rate is ~0.33% in the prototype – subtly changing this 8bit image

Details of this research can be found in the paper ‘A 3D nanoscale optical disk memory with petabit capacity‘, published in Nature – this is a clearly-written paper, which makes extensive use of abbreviations, so maybe write yourself a glossary as you go.

Image: Non-factual artists impression provided by USST
Due to a misunderstanding, this article originally contained images that were possibly used without the owners permission – apologies to Nature if this was the case.