Graphene-based, ultra-thin light detector peeks below the surface

The ultra-thin terahertz band light detector can see just below the surface of clothing, body tissues, plastic, and other non-metallic objects (Photo: Shutterstock)

A new prototype light detector uses graphene's light-absorbing properties to see in a broad band of light wavelengths that includes terahertz waves. These fall between the microwave and infrared bands, thereby making it possible to look just beneath the surface of opaque objects such as skin and plastic.

Whereas most existing terahertz detectors are bulky and slow and must be kept at close to absolute zero (around 4 Kelvin), the prototype developed in a collaboration between the University of Maryland, Australia's Monash University, and the US Naval Research Lab operates at room temperature and at speeds co-author Michael Fuhrer says are "more than a million times faster" than existing room temperature detectors in the terahertz range.

This is possible thanks to a principle called the hot-electron photothermoelectric effect and to a material called graphene, which is composed of pure carbon that's so thin it measures just one atom in thickness.

Graphene has gained a reputation as something of a wonder material for its super strength (it's stronger than steel) and conductive properties, and it's the hero once again here as its electrons absorb the light and retain the resultant energy. "They remain hot while the carbon atomic lattice remains cold," explains Dennis Drew, a physics professor at the University of Maryland and co-author on the study.

The electrons then flee the lattice for one of two metal electrodes. More electrons escape through one than the other, as they possess different conductivities, and this asymmetry produces an electrical signal that detects the presence of terahertz waves.

These are light waves that are invisible to human eyes, but unlike x-rays – which don't stop until they reach something as dense as your bones – terahertz waves cannot penetrate water or metal. They are also non-ionizing, so they won't damage body tissues or DNA.

So where might such a detector be used? In security scanners, for example, it could identify concealed weapons without invading bodily privacy. It could also make medical imaging safer and more effective.

Other applications include chemical sensing, remote bomb detection, night vision goggles/cameras, high-altitude telecommunications, manufacturing quality control (as terahertz waves penetrate cardboard and plastic), preventing premature car rusting, and even 3D printing.

A paper describing the research was published recently in the journal Nature.

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