Graphene Offers a Better Way to Capture T-rays

Researchers are already aware of the potential benefits of electronic devices that send and receive digital pulses at frequencies in the terahertz region of the electromagnetic spectrum. Devices for airport security, medical imaging, drug and food inspection, and high-speed communication, will be much more sensitive than today’s versions—that is, if researchers can develop better sources and detectors for that type of radiation. Now a team of scientists at the University of Maryland reports that it has used graphene to build a terahertz device that is at least as sensitive, and many times as fast, as existing detectors.

Graphene, a sheet of carbon atoms only one atomic layer thick, works well as a terahertz detector because of its ability to absorb radiation, from the ultraviolet to the terahertz regions, equally well. Meanwhile, terahertz radiation, also known as T-rays, can penetrate a wide variety of materials without the ionizing effects of x-rays, and can spectrographically identify materials, making it ideal for applications such as identifying drugs or explosives without harming people.

“[The graphene-based device] as good as any room-temperature detector in this spectral range, and potentially much better,” says Dennis Drew, a research scientist at the University of Maryland’s Center for Nanophysics and Advanced Materials. Drew and his colleagues presented their findings in the latest issue of Nature Nanotechnology.

The detector relies on the photothermoelectric effect. Photons striking the graphene cause electrons in the material to jump to a higher energy level. The affected graphene molecules want to dissipate the resulting thermal energy, but because the electrons lose the heat to the surrounding molecules rather slowly, placing metal contacts on the graphene allows the material to shed excess energy by pushing electrons to the metal. If the contacts are made of two different metals with different conductivity—in this case, gold and chromium—the result is a current. Measuring the current reveals how much terahertz power is being absorbed by the graphene.

Drew says the new detector is as sensitive as the Golay cell, another device used to detect terahertz rays. But while the Golay cell has a response time on the order of a second, the graphene detector makes the measurement in 0.1 nanosecond. Another alternative, a pyroelectric detector, has response times measured in milliseconds, and tends to be somewhat less sensitive.

The graphene detector’s ability to pick up terahertz rays might be further improved by various means, Drew says. Using multiple layers of the material may allow it to capture more radiation. Adding voltage gates to create P-N junctions could also raise such a detector’s performance. Contacts made from metals other than the ones used in the experiments detailed in the paper—aluminum, for example—might also increase the efficiency, though it’s harder to get aluminum to adhere to graphene. Drew says optimizing the performance is a relatively easy engineering challenge.