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Data boffins see the light with extravagantly bit-blessed optical PCM

Ideal for when you want a cup of tea and a bit of light reading

As if PCM (phase change memory) wasn’t esoteric enough, we now have optical PCM, thanks to sterling boffinry efforts at Oxford and other universities.

Material scientists at Oxford University, Karlsruhe, Munster and Exeter have produced the world’s first all-photonic, non-volatile memory chip. It uses Ge2Sb2Te5 (GST), a chalcogenide substance, with two states – crystalline or amorphous – which offer differing resistance levels.

State switching has always been electrical until now, but now boffins have done it with light.

The work is described in a Nature Photonics paper, Integrated all-photonic non-volatile multi-level memory, in which the research team stated:

By using optical near-field effects, we realise bit storage of up to eight levels in a single device that readily switches between intermediate states.

Our multi-level, multi-bit devices provide a pathway towards eliminating the von Neumann bottleneck and portend a new paradigm in all-photonic memory and non-conventional computing.

What they did was to have a small band of their PCM stuck to the top of a light waveguide, a silicon nitride ridge. Light pulses sent through the wave guide can change the PCM state, with an intense pulse causing it to momentarily melt and then quickly cool, making it assume an amorphous structure.

A slightly less intense pulse can put it into a crystalline state.

Then the digital value bit: “When light with much lower intensity is sent through the waveguide, the difference in the state of the GST affects how much light is transmitted. The team can measure that difference to identify its state – and in turn read off the presence of information in the device as a 1 or 0.”

Optical_PCM_waveguide

Pink lines are waveguides with the yellow band being the PCM

It’s certainly non-volatile, researcher, Clarendon Scholar and DPhil student Carlos Ríos explained: “GST remains in the state that it’s placed in for decades.”

Sending differing colours of light through the waveguide enabled them to read and write at the same time, which sounds weird. Professor Wolfram Pernice of the University of Munster said: “In theory, that means we could read and write to thousands of bits at once, providing virtually unlimited bandwidth.”

Optical_PCM_Schematic

A schematic of the optical PCM device, showing its structure and the propagation of light through it

The researchers also found that different intensities of strong light pulses can accurately and repeatedly create different mixtures of amorphous and crystalline structure within the GST.

Low-intensity read pulses detected subtle differences in the transmitted light, allowing the researchers to reliably write and read eight different levels of state composition, from entirely crystalline to completely amorphous.

So single bits of memory could store several states or even perform calculations themselves instead of at the processor.

That’s probably a step too far in computing esoterica right now, as one binary bit would be enough. What the heck would we do with an octuple bit?

This band of boffins is now developing a new kind of electro-optical interconnect, to allow memory chips to directly interface with other components using light, rather than electrical signals. However, it will probably be a decade or more before general-purpose computers become photonic, if they ever do. ®

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