The device is based on the classical chalcogenide material Ge2Sb2Te5 (GST) that is also used in compact discs and DVDs and which can adopt either an ordered crystalline or disordered amorphous state.
In a paper published in Nature Photonics, the researchers describe how they included a small patch of GST on top of a silicon-nitride waveguide and used an intense pulse of light causing the thin film to melt. On cooling it assumes an amorphous structure. A less-intense pulse and slower cooling puts the film into a crystalline state and light with much lower intensity can be used to read the state of the film, the researchers report.
A scanning electron microscope image of optical memory GST layer, highlighted in yellow, on top of the silicon nitride waveguide, highlighted in red. Source: Oxford University.
"There’s no point using faster processors if the limiting factor is the shuttling of information to-and-from the memory – the so-called von-Neumann bottleneck," said Professor Harish Bhaskaran, in a statement. Professor Bhaskaran is the Oxford engineer who led the research along with Professor Wolfram Pernice from the University of Muenster.
Using optical transport can be used to transfer data but carries an overhead if the optical signals need to be converted back into electronic signals at the point of storage. By making the memory system optical this overhead has been reduced. Researchers have tried to create this kind of photonic memory before, but the results have always been volatile, requiring power in order to store data, according to Oxford University.
"This is the first ever truly non-volatile integrated optical memory device to be created," said Carlos Rios, one of two lead authors of the paper along with Matthias Stegmaier. "And we’ve achieved it using established materials that are known for their long-term data retention — GST remains in the state that it’s placed in for decades."
Next: Multi-level cell and wavelength multiplexing
The researchers have also shown that it is possible to use wavelength multiplexing – sending multiple light pulses of different wavelengths at the same time – it is possible to write and read to the memory at the same time. "In theory, that means we could read and write to thousands of bits at once, providing virtually unlimited bandwidth," said Professor Pernice, in the same statement.
The research also report that different light intensities can be used to accurately and repeatedly create different proportions of amorphous and crystalline within the GST film. When lower intensity light pulses are sent through the waveguide the researchers were able to detect these different mixes by changes made to the transmitted light, allowing the possibility of reading up to eight different memory levels allowing the storage of 3 bits of data per memory site.
Professor Bhaskaran said the team has demonstrated novel functionality using proven existing materials and the optical bits can be written with frequencies of up to one gigahertz. The team is now working on an electro-optical interconnect, which will allow such optical non-volatile memory chips to connect with other components using light, rather than electrical signals.
The paper ‘Integrated all-photonic nonvolatile multi-level memory’ is published in Nature Photonics.
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