Unusual metal could enable mid-infrared detectors

1 min read

A new phenomenon, found by MIT physicists in an unconventional metal, could provide a new way of making highly sensitive detectors – including night vision, thermal sensing, and environmental monitoring – for mid-infrared wavelengths.

According to the researchers, Weyl fermions are particles which potentially could be used to build more efficient circuits and other devices.

Even though Weyl fermions have never been observed directly, researchers have recently observed a phenomenon that mimics essential aspects of their theorised properties, in a class of metals known as Weyl semimetals.

Theory predicts that the fermions should move at the speed of light, and, at the same time, spin about the direction of motion. They come in two varieties depending on whether their rotation around the direction of motion is clockwise or counter clockwise. This property is known as the chirality of Weyl fermions.

The researchers found that a metal called tantalum arsenide, or TaAs, exhibits an interesting optoelectronic property called the circular photogalvanic effect.

Conventionally, electrical conduction requires applying an external voltage across the two ends of a metal. By contrast, the researchers found that, by shining circularly polarised light in the mid-infrared wavelength range, the TaAs can produce an electrical current without applying external voltages.

In addition, the direction of the current is dictated by the chirality of Weyl fermions and can be switched by changing the light polarisation from left-handed to right-handed.

The amount of current generated is said to be 10 to 100 times stronger than the response of other materials used for detecting this kind of light.

According to the team, nobody had previously been able to measure the Weyl fermions’ chirality, meaning its left- or right-handed spin.