First light-based memory chip to store data permanently

London: Scientists led by an Indian-origin researcher have developed the world’s first entirely light-based memory chip that stores data permanently, an advance that could dramatically improve the speed of modern computing.

Current computers are held back by the relatively slow transmission of electronic data between the processor and the memory. “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 from Oxford University who led the research. “But we think using light can significantly speed this up,” he said. Simply bridging the processor-memory gap with photons isn’t efficient, though, because of the need to convert them back into electronic signals at each end. Instead, memory and processing capabilities would need to be light-based too.

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. For many applications – such as computer disk drives – it’s essential to be able to store data indefinitely, with or without power. Now, an international team of scientists including researchers from Oxford University’s Department of Materials has produced the world’s first all-photonic nonvolatile memory chip.

The new device uses the phase-change material Ge2Sb2Te5 (GST) – the same as that used in rewritable CDs and DVDs – to store data. This material can be made to assume an amorphous state, like glass, or a crystalline state, like a metal, by using either electrical or optical pulses. In a paper published in Nature Photonics, the researchers describe the device they have created, which uses a small section of GST on top of a silicon nitride ridge, known as a waveguide, to carry light.

The team has shown that intense pulses of light sent through the waveguide can carefully change the state of the GST. An intense pulse causes it to momentarily melt and quickly cool, causing it to assume an amorphous structure; a slightly less-intense pulse can put it into an crystalline state. Later, 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.

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