Tunable source converts DC electric field to terahertz radiation

August 05, 2015 // By Jean-Pierre Joosting
Terahertz radiation, the no-man's land of the electromagnetic spectrum, has long stymied researchers.

Optical technologies can finagle light in the shorter-wavelength visible and infrared range, while electromagnetic techniques can manipulate longer-wavelength radiation like microwaves and radio waves. Terahertz radiation, on the other hand, lies in the gap between microwaves and infrared, whether neither traditional way to manipulate waves works effectively. As a result, creating coherent artificial sources of terahertz radiation in order to harness it for human use requires some ingenuity.

Difficulties of generating it aside, terahertz radiation has a wide variety of potential applications, particularly in medical and security fields. Because it's a non-ionizing form of radiation, it is generally considered safe to use on the human body. For instance, it can distinguish between tissues of different water content or density, making it a potentially valuable tool for identifying tumors. It could also be used to detect explosives or hidden weapons, or to wirelessly transmit data.

In a step towards more widespread use of terahertz radiation, researchers have designed a new device that can convert a DC electric field into a tunable source of terahertz radiation. Their results are published in the Journal of Applied Physics, from AIP Publishing.

This device exploits the instabilities in the oscillation of conducting electrons at the device's surface, a phenomenon known as surface plasmon resonance. To address the terahertz gap, the team created a hybrid semiconductor: a layer of thick conducting material paired with two thin, two-dimensional crystalline layers made from graphene, silicene (a graphene-like material made from silicon instead of carbon), or a two-dimensional electron gas. When a direct current is passed through the hybrid semiconductor, it creates a plasmon instability at a particular wavenumber. This instability induces the emission of terahertz radiation, which can be harnessed with the help of a surface grating that splits the radiation.