A bolometer is an instrument that absorbs light and turns it into heat. This heat can be detected by measuring the change in resistance of the material making up the device.

Being able to detect infrared light is important in a wide variety of areas, such as security screening, in medicine, biological imaging, astronomy and materials science. Graphene could come into its own here because it can absorb light over a very wide range of wavelengths, from the ultraviolet to the infrared parts of the electromagnetic spectrum, and especially in the infrared. What is more, the fact that electrons in graphene only couple weakly to the lattice vibrations (phonons) in the material means that when these electrons get hit by light, they heat up, but the atoms stay cool. “This allows the electrons to become much hotter than they otherwise would if they simply gave up their heat to the phonons,” explains team leader Michael Fuhrer.

Resistance depends strongly on temperature

In its normal state, however, graphene’s resistance is almost independent of temperature – not a good thing if a bolometer is to be made from the material. The Maryland researchers overcame this problem by using bilayer graphene in their device with electrical gates above and below each layer to apply an electric field perpendicular to the layers. Doing this opens up a small bandgap in the graphene, thus turning it from a metal to a semiconductor. “The semiconducting bilayer graphene has a resistance that strongly depends on temperature and so makes for an excellent bolometer,” Fuhrer told nanotechweb.org.

The current version of the bolometer only works at low temperatures, so it would initially be used in applications where high sensitivity is required – such as in submillimetre-wave (terahertz) radio astronomy, for example. The researchers are busy working on a new graphene bolometer, however, that should work at room temperature.

For starters, though, the team would like to improve its low-temperature device. “The problem we have is that the bilayer graphene only absorbs a few percent of incoming light and the semiconducting material is highly resistive, which makes high-frequency readout difficult,” explains Fuhrer. “We are trying to enhance the absorption of light by taking advantage of plasmonic resonance in graphene and are also looking at ways to lower the resistance of the device, such as by using superconducting electrodes.”

The researchers presented their work in Nature Nanotechnology.