Graphene Device Modulates Light

9 May 2011—A device built out of graphene that can encode electrical signals onto light could dramatically increase the speed of optical communications and bring light-based data transfer down to the scale of computer chips, its creators say.

Such a device, called an optical modulator, has the potential to operate at speeds of 500 gigahertz and could be as small as a few square micrometers, say the researchers at the Nanoscale Science and Engineering Center at the University of California, Berkeley. That kind of speed makes for blazingly fast communications. "If you can connect to the Internet with 100 GHz, that means you can download a movie in a second," says Ming Liu, a postdoctoral researcher at Berkeley and lead author of a paper on the device; the paper appears in the 8 May advanced online publication of the journal Nature.

To operate their device, the researchers used voltage to manipulate the fermi level—the energy state of electrons—in a sheet of graphene. Normally, graphene—a single-atom-thick structure of carbon that looks like chicken wire—is opaque to the wavelengths of near-infrared light used in telecommunications, because the electrons in the material absorb most of the photons striking it. Applying a negative voltage draws electrons out of the graphene so that they cannot absorb the photons, which can now pass through. A positive voltage also renders the graphene transparent because it becomes crowded with electrons, leaving no unoccupied energy state for the electrons to fill if they do absorb a photon’s energy. So simply turning the voltage on and off causes the graphene to alternately block or pass incoming light, thus modulating a beam. The trick works, Liu says, because graphene is a sheet of carbon just one atomic layer thick. "It’s a two-dimensional material, so it has relatively few electrons," he explains.

Commercial modulators used in telecommunications networks are made from lithium niobate and operate at speeds up to 40 gigabytes per second. They’re also expensive, at US $4000 to $5000 apiece. Liu believes graphene-based modulators could be made for less than a dollar each.

The Berkeley modulators, created by placing a layer of graphene on top of a silicon waveguide, measure 25 micrometers square, a size that could make them compatible with computer chips and open the door to high-speed optical data transmission between chips and computer boards. Copper wires have long been used to carry that data, but as the number of transistors on a chip continue to climb, they’re producing more data than the wires can transmit. Lithium niobate modulators, as well as silicon modulators in development, tend to be measured in millimeters, a scale that’s too big for chips. Other researchers are working on modulators made with germanium or compound semiconductors, but there are questions as to whether those could be compatible with traditional chip manufacturing processes. Liu says graphene should fit in easily with complementary metal-oxide semiconductor (CMOS) manufacturing. And where other modulators handle only a narrow range of wavelengths, a graphene device should be compatible with light from the visible to well into the infrared, he says.

The device the team demonstrated operated at 1 GHz, but Liu says using better-quality graphene with fewer defects will quickly increase that. "We believe it’s no challenge at all," he says. He expects that a graphene modulator could become commercially attractive at 10 GHz, perhaps within three to five years.

Frank Schwierz, head of the RF & Nanoelectronics Research Group at Technical University of Ilmenau, in Germany, says the graphene optical modulator the researchers describe "looks promising," and he agrees it will likely fit into CMOS manufacturing. But he doesn’t expect commercialization anytime soon. "This is not related to the modulator itself but rather to the fact that the semiconductor industry itself is very conservative," he says. "History tells us that chipmakers introduce new materials when, and only when, it is unavoidable."