Despite being just one atom thick, graphene is a strong absorber of electromagnetic radiation over a wide range of wavelengths – particularly in the far infrared and terahertz parts of the spectrum. This is extraordinary because conventional materials normally need to be thousands of atoms thick to be as effective. This high absorption is a result of graphene's unusual electronic properties, which result in its electrons moving extremely fast and behaving like relativistic "Dirac" particles with virtually no rest mass.

Phaedon Avouris and colleagues at IBM's TJ Watson Research Center in New York have created a film comprising alternating layers of graphene and an insulator. The team has shown that a film containing just five layers of each material can shield electromagnetic radiation in the terahertz and microwave ranges by up to 97.5%, while remaining transparent to visible light.

Discs and ribbons

What is more, by making films that contain arrays of tiny graphene discs or ribbons, the researchers have found that they can create tuneable terahertz filters and polarizers. The discs and ribbons are just a few microns across and absorb light by confining it to regions that are hundreds of times smaller than the wavelength of the light by exploiting plasmons – quantized collective oscillations of electrons inside the structures – that interact strongly with light.

The team began by stacking wafer-scale graphene sheets alternately with thin insulating layers to form a five-layer structure and then patterning the ensemble into a 3D photonic-crystal-like construct. Once the device was ready, the scientists measured its terahertz wave transmission spectrum using a Fourier transform infrared spectrometer.

The active elements in such a device are the graphene layers, explains team member Hugen Yan, lead scientist of this project, and it is the plasmons in the material that absorb and enhance the reflection of terahertz and microwave radiation. As a result, the radiation transmitted through the device diminishes. The plasmons inside the nanodisks and ribbons oscillate with the terahertz light and, at certain frequencies, the two oscillate in resonance. "It is at this resonance that the shielding efficiency reaches its maximum," Yan told nanotechweb.org.

"Graphene could be used as a transparent terahertz and microwave shielding material and so help prevent external electromagnetic interference in high-accuracy electronic equipment," he said. "Such a shield could even help protect people from radiation at these wavelengths, which are hazardous to human health, according to the World Health Organization."

Patterned graphene could also be used to make terahertz frequency filters and polarizers for photonics and optoelectronics applications, he adds.

The team is now looking at how the patterned graphene nanostructures respond to terahertz frequencies under high applied magnetic fields. "We shall also study graphene plasmonics over a broader frequency range, including mid-infrared regions," said Yan.

The current work is detailed in Nature Nanotechnology.