Researchers at the University of Maryland, College Park, say that their work could lead to a generation of light detectors that can see below the surface of bodies, as well as other objects. Using graphene, their prototype detector is able to view an extremely broad band of wavelengths, including terahertz waves, which are invisible to the human eye.

Top-down view of broadband, ultra-fast graphene detector capable of detecting terahertz frequencies at room tempera- ture. Note: μm = micrometer, Cr = chromium, Au = gold.
(Credit: Thomas Murphy)

While the light we see illuminating everyday objects is actually only a very narrow band of wavelengths and frequencies, terahertz light waves’ long wavelengths and low frequencies fall between microwaves and infrared waves. The light in terahertz wavelengths can pass through materials that we normally think of as opaque, such as skin, plastics, clothing, and cardboard. It can also be used to identify chemical signatures that are emitted only in the terahertz range, the researchers explain.

Few technological applications for terahertz detection are currently realized, however, in part because it is difficult to detect light waves in this range. In order to maintain sensitivity, most detectors need to be kept extremely cold, around -452°F. Existing detectors that work at room temperature are bulky, slow, and expensive.

The new room temperature detector, developed by the UMD team and colleagues at the U.S. Naval Research Lab and Monash University, Australia, gets around these problems by using graphene, a single layer of interconnected carbon atoms. By utilizing the special properties of graphene, the research team has been able to increase the speed and maintain the sensitivity of room temperature wave detection in the terahertz range.

Light is absorbed by the electrons in graphene, which heat up but don’t lose their energy easily. So they remain hot while the carbon atomic lattice remains cold. The heated electrons escape graphene through electrical leads. The prototype uses two electrical leads made of different metals, which conduct electrons at different rates. Because of the conductivity difference, more electrons will escape through one than the other, producing an electrical signal that detects the presence of terahertz waves beneath the surface of materials, including what’s between your skin and bones.