New detector sets stage for devices that see through walls

The new detector, created by researchers at the University of Maryland in the USA, is able to pick up an extremely broad band of light wavelengths including notoriously difficult to detect terahertz waves, which are invisible to the human eye.

These waves have a number of emerging applications in fields such as mobile communications, medical imaging, chemical sensing, night vision, and security as they can pass through materials normally considered opaque, such as skin, plastics, clothing, and cardboard. They can also be used to identify chemical signatures that are emitted only in the terahertz range.

Their long wavelengths and low frequencies fall between microwaves and infrared waves, but they are hard to detect and detectors either need to be kept extremely cold – about -270°C – or made bulky, slow, and prohibitively expensive to work at room temperature.

The new detector, developed in collaboration with the US Naval Research Lab and Monash University in Australia, exploits the special properties of atom-thick graphene to increase the speed and maintain the sensitivity of room temperature wave detection in the terahertz range.

Using a new operating principle dubbed the 'hot-electron photothermoelectric effect', the research team created a device that is "as sensitive as any existing room temperature detector in the terahertz range and more than a million times faster," according to Michael Fuhrer, professor of physics at the University of Maryland and Monash University.

In a paper about the new detector published yesterday in journal Nature Nanotechnology the researchers explain that when light is absorbed by the electrons suspended in the honeycomb lattice of the graphene, they do not lose their heat to the lattice but instead retain that energy.

University of Maryland physics professor Dennis Drew said: "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."

These heated electrons escape the graphene through two electrical leads made of different metals, which conduct electrons at different rates, and because of this difference in conductivity more electrons escape through one lead than the other, producing an electrical signal.

This electrical signal detects the presence of terahertz waves beneath the surface of materials that appear opaque to the human eye, but unlike x-rays that go right through materials like skin to the bone missing the layers just beneath the skin's surface entirely, these waves reveal what’s in-between.

The speed and sensitivity of the room temperature detector presented in this research opens the door to future discoveries in this in-between zone, according to the researchers.