Graphene Leading the Way to Optical Chips

In a joint research project, researchers from the Massachusetts Institute of Technology (MIT), Columbia University, and IBM’s T. J. Watson Research Center have used graphene as a photodetector for enabling an optical chip.

Graphene has tantalized researchers in photodetector applications with its wide spectral range (from the ultraviolet to the infrared), fast optoelectronic response that is the result of high electron mobility, and its lack of a band gap. However, graphene can absorb only a small fraction of incoming light, so its responsivity has been limited.

While this minimal light sensitivity may limit its use in digital camera applications, the MIT, Columbia, and IBM researchers may have engineered a way to use graphene as a photodetector for converting light into electricity for integrated optoelectronic chips.

The research team, which published its work in the journal Nature Photonics (“Chip-integrated ultrafast graphene photodetector with high responsivity”) developed a method by which they could overcome graphene’s low responsivity to incoming light (measured at between 2 and 3 percent of the light passing through it being converted to electrical current). They turned to creating a bias in the photodetector so that electrons that were disrupted by incoming photons would remain in a higher energy state.

Typically, creating this bias involves maintaining voltage through the photodetector. However, this voltage is a source of noise that compromises the photodetector’s readings. To avoid this noise, the researchers turned to the work of Fengnian Xia and his colleagues at IBM; they produced a bias in a photodetector without the application of a voltage.

This trick is accomplished through an ingenious design in which light is funneled into the photodetector through a channel—or a waveguide—that is capped with a piece of graphene oriented perpendicular to the channel. The graphene has gold electrodes on either side of it, but instead of them being evenly spaced, one of the electrodes is closer to the graphene than the other.

“There’s a mismatch between the energy of electrons in the metal contact and in graphene,” said Dirk Englund, an assistant professor at MIT and the leader of the research team, in a press release. “And this creates an electric field near the electrode.”

So in operation, photons come through the channel and start kicking the electrons up to a higher energy state. These excited electrons are then pulled to the electrodes by the electric field, thereby creating a current—without applying a voltage.

This voltage-free bias boosts the photodetector to the point where it could generate 100 milliamps per watt, a responsivity equal to that of germanium. The researchers believe that with a bit of engineering (i.e., thinner electrodes, and a narrower waveguide), it could be possible to boost these results by a factor of two or perhaps even four.

The impact of chips that use light rather than electricity is clear. They will consume less power and produce less heat. Both of these factors have become ever more critical as chip features get smaller and smaller.

Thomas Mueller, an assistant professor at the Vienna University of Technology’s Photonics Institute, and a co-author of very similar research in the same journal, noted in the press release: “The other thing that I like very much is the integration with a silicon chip, which really shows that, in the end, you’ll be able to integrate graphene into computer chips to realize optical links and things like that.”