Updated: 500 terabytes of data on a CD, almost

Data is written as miniature shapes through the bulk of silica glass using the three spatial dimensions. The shapes have two separate optical characteristics – adding two more dimensions – making this so-called five-dimensional (‘5d’) data storage. The Southampton team can write at 1,000,000 voxels per second, equivalent to ~230kbyte/s.

They used it to record 6Gbyte into a silica glass substrate, in four squares each 8.8 x 8.8mm, as well as draw the university logo (see photo). Read-out accuracy was close to 100%.

“The physical mechanism we use is generic,” said researcher Yuhao Lei. “Thus, we anticipate that this writing method could also be used for fast nanostructuring in transparent materials for applications in 3D integrated optics and microfluidics.”

The shapes, described as ‘nanolamella-like’, are around 500 x 50nm . “Nanolamella-like nanostructure means a nano-void in the centre and two short nano-cracks perpendicular to the laser polarisation direction,” Lei told Electronics Weekly.

These cracks are opposite one another around the void, and can be written in different directions to encode data.

The structures, explained Lei, produce birefringence in glass, which can be characterised by two parameters:

• slow axis orientation (the 4th dimension, coinciding with the orientation of the structures)
• strength of retardance (the 5th dimension, coinciding with the size of the structures)

During recording, the slow axis orientation and strength of retardance are controlled respectively by the polarisation and intensity of light.

Together these were used to encode 4 bits of data into each structure, and they were written in pairs to get 8bits, allowing text to be encoded as ASCII.

To write quickly, high repetition lasers were used. But they could not be used directly as the total power would locally over-heat the substrate.

“We manage to reduce the thermal damage by efficient energy deposition via near-field enhancement, which is a phenomenon of light field increase near nanostructures defined by polarisation direction,” said Lei. “More specifically: an isotropic nano-void is initially generated with pulse energy above the micro-explosion threshold in silica glass. With subsequent few laser pulses with reduced energy, it is elongated to an anisotropic nanolamella-like structure via near-field enhancement, eliminating the detrimental thermal effects from megahertz-rate femtosecond pulses.”

This approach improves the data writing speed to a practical level, according to Lei: “We can write tens of gigabytes of data in a reasonable time.”

The work is covered in ‘High speed ultrafast laser anisotropic nanostructuring by energy deposition control via near-field enhancement‘, published in Optica by Optica (formerly OSA). The full paper can be read without payment.

Although the method could get 500Tbyte of data into a CD-sized patch, it will take a while: “With upgrades that allow parallel writing, the researchers say it should be feasible to write this amount of data in about 60 days,” according to Optica.

Image credit: Yuhao Lei and Peter G. Kazansky, University of Southampton