Researchers build first permanent optical memory on a chip

Picture courtesy of Wikimedia CommonsA team of boffins has created the first permanent optical memory on a chip, a critical step towards photonic chips.

Harish Bhaskaran, a nanoengineering expert at the University of Oxford and electrical engineer Wolfram Pernice at the Karlsruhe Institute of Technology in Germany, have worked out a way to solve to the disappearing memory problem using material at the heart of rewritable CDs and DVDs.

Rewritable material is made from a thin layer of an alloy of germanium, antimony, and tellurium. When zapped with an intense pulse of laser light, GST film changes its atomic structure from an ordered crystalline lattice to an “amorphous” jumble.

These two structures reflect light in different ways, and CDs and DVDs use this difference to store data. To read out the data—stored as patterns of tiny spots with a crystalline or amorphous order—a CD or DVD drive shines low-intensity laser light on a disk and tracks the way the light bounces off.

The researchers noticed that the material affected not only how light reflects off the film, but also how much of it is absorbed. When a transparent material lay underneath the GST film, spots with a crystalline order absorbed more light than did spots with an amorphous structure.

Having worked that out the researchers wanted to see whether they could use this property to permanently store data on a chip and later read it out.

Using standard chipmaking technology they built a chip with a silicon nitride device, known as a waveguide, which contains and channels pulses of light. They then placed a nanoscale patch of GST atop this waveguide. To write data in this layer, the scientists piped an intense pulse of light into the waveguide. The high intensity of the light’s electromagnetic field melted the GST, turning its crystalline atomic structure amorphous. A second, slightly less intense pulse could then cause the material to revert back to its original crystalline structure.

They dramatically increased the amount of data they could store and read. For starters, they sent multiple wavelengths of light through the waveguide at the same time, allowing them to write and read multiple bits of data simultaneously, something you can’t do with electrical data storage devices.

According to Nature Photonics, which we get for the spot the proton competition, by varying the intensity of their data-writing pulses, they were also able to control how much of each GST patch turned crystalline or amorphous at any one time.

Photonic memories are a long way from being better than their electronic counterparts. Ultimately, Bhaskaran believes that if a more advanced photonic memory can be integrated with photonic logic and interconnections, the resulting chips could be 50 to 100 times the speed of today’s computer processors.