Saving Data with Light: Light-Based Storage Chips

The world's first fully light-based storage chip that can store data permanently was developed by materials scientists at the University of Oxford in collaboration with scientists in Exeter, as well as Karlsruhe and Münster in Germany. The chip built with materials used in rewritable CDs and DVDs could dramatically improve the data transfer speed in modern computers.

Today's computers are slowed down by the relatively slow data transfer between the processor and memory. "There is no point in installing faster processors when a kind of brake is in transferring information to and from the memory - the so-called Neumann bottle neck", says Professor Harish Bhaskaran, who headed the research. "But we think light can speed up the transfer considerably."

However, the simple bridging of the processor-memory bottleneck, by means of photons, is not efficient since the data must first be converted into optical signals and, after the transfer, into electronic signals. To increase the transfer speed, the memory and the processor would have to work with light, too.

Researchers have tried repeatedly to develop such a photonic memory, but the stored data was always volatile, which means that energy is needed to maintain the data, but a computer hard drive must be able to store data indefinitely with or without power supply.

An international team of researchers, including scientists from the Department of Materials at Oxford University, has produced the world's first photonic non-volatile memory chip. The new chip uses the Ge2Sb2Te5 (GST) phase change material, which is also used for rewritable CDs and DVDs. This material may adopt an amorphous state (such as glass) or a crystalline state (such as metal), regardless if electrical or optical pulses are used. The researchers use a small area of the GST known as a “waveguide” to transport the light pulses.

The team showed that intense light pulses sent through the waveguide can change the state of the GST. An intense pulse causes the material to melt abruptly and cool down rapidly, thereby adopting an amorphous structure; a slightly less intense pulse can transform it into a crystalline state. The two states result later, when light with much lower intensity is transmitted through the waveguide, which is transmitted more or less depending on the state, in the information of a “1” or “0”. "This is the first true non-volatile optical memory", explains Clarendon Scholar and DPhil student Carlos Ríos. "And we have even been able to achieve our goal with common materials known for long-term data retention."

By simultaneously transmitting light with different wavelengths, the team also showed that they could use a single pulse to write and read data simultaneously.  "In theory, this means that we can read and write thousands of bits at the same time, which promises nearly unlimited bandwidth”, explains Professor Wolfram Pernice of the University of Münster, Germany.

The researchers have also found that different intensities of strong pulses produce different mixtures of the amorphous and crystalline structure within the GST. When weak pulses were sent through the waveguide to read out the contents of the memory, they could detect subtle differences in transmitted light. They were able to produce eight different compositions from completely crystalline to completely amorphous and store them on these data and read them out again. This multi-state capability could provide storage units with more than the usual binary information of 0 and 1 so that a single bit of the memory can store multiple states or even perform calculations itself, thus relieving the processor.

"This is a whole new kind of functionality with proven materials," explains Professor Bhaskaran. "These optical bits can be described at frequencies of up to one gigahertz, delivering gigantic bandwidths, which is the kind of ultra-fast data storage that modern computing needs."

In the meantime, the team is working on a series of projects that will use the new technology. They are particularly interested in the development of a new electro-optical link that allows the memory chips to connect directly to other components using light rather than electrical signals.

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