By Robert Service – Science Magazine
Today’s electronic computer chips work at blazing speeds. But an alternate version that stores, manipulates, and moves data with photons of light instead of electrons would make today’s chips look like proverbial horses and buggies. Now, one team of researchers reports that it has created the first permanent optical memory on a chip, a critical step in that direction.
“I am very positive about the work,” says Valerio Pruneri, a laser physicist at the Institute of Photonic Sciences in Barcelona, Spain, who was not involved in the research. “It’s a great demonstration of a new concept.”
Interest in so-called photonic chips goes back decades, and it’s easy to see why. When electrons move through the basic parts of a computer chip—logic circuits that manipulate data, memory circuits that store it, and metal wires that ferry it along—they bump into one another, slowing down and generating heat that must be siphoned away. That’s not the case with photons, which travel together with no resistance, and do so at, well, light speed. Researchers have already made photon-friendly chips, with optical lines that replace metal wires and optical memory circuits. But the parts have some serious drawbacks. The memory circuits, for example, can store data only if they have a steady supply of power. When the power is turned off, the data disappear, too.
Now, researchers led by Harish Bhaskaran, a nanoengineering expert at the University of Oxford in the United Kingdom, and electrical engineer Wolfram Pernice at the Karlsruhe Institute of Technology in Germany, have hit on a solution to the disappearing memory problem using a material at the heart of rewritable CDs and DVDs. That material—abbreviated GST—consists of 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.
In their work with GST, 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.
Next, the researchers wanted to see whether they could use this property to permanently store data on a chip and later read it out. To do so, they used standard chipmaking technology to outfit 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.
When the researchers wanted to read the data, they beamed in less intense pulses of light and measured how much light was transmitted through the waveguide. If little light was absorbed, they knew their data spot on the GST had an amorphous order; if more was absorbed, that meant it was crystalline.
Bhaskaran, Pernice, and their colleagues also took steps to dramatically increase 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. And, as they report this week in Nature Photonics, 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. With this method, they could make one patch 90% amorphous but just 10% crystalline, and another 80% amorphous and 20% crystalline. That made it possible to store data in eight different such combinations, not just the usual binary 1s and 0s that would be used for 100% amorphous or crystalline spots. This dramatically boosts the amount of data each spot can store, Bhaskaran says.
Photonic memories still have a long way to go if they ever hope to catch up to their electronic counterparts. At a minimum, their storage density will have to climb orders of magnitude to be competitive. Ultimately, Bhaskaran says, if a more advanced photonic memory can be integrated with photonic logic and interconnections, the resulting chips have the potential to run at 50 to 100 times the speed of today’s computer processors.