Terahertz Radiation Detector May Open New Possibilities in Medicine

Terahertz radiation has the potential to revolutionize certain aspects of medical imaging because the energy is non-ionizing and so shouldn't have the side effects of X-rays. (It doesn't penetrate tissue nearly as deep as X-rays, though.) The big problem with putting this technology into practice is the lack of practical detection technology. Now Japanese scientists from the in Wako have developed a detector based on a carbon nanotube transistor that can sense small numbers of terahertz frequency photons.

One component, a structure known as a two-dimensional electron gas (2DEG), absorbs THz radiation. A 2DEG is a thin layer of electrons that are confined between two different semiconducting materials—in this case, GaAs and AlGaAs—but are free to move within the layer. A 2DEG is highly conducting and therefore highly absorbing of THz radiation.
On top of the 2DEG, the researchers built a carbon nanotube single electron transistor—a device that behaves like a switch with electrical characteristics controlled by the presence or absence of individual electrons in its channel. They found that when the 2DEG beneath this transistor absorbed a burst of THz radiation, it shifted the voltage at which the transistor switched. At the lowest levels of radiation, they also observed intermittent switching behavior known as telegraph noise, suggesting that only a few photons were needed to induce switching.

As well as achieving unprecedented detection sensitivity, the device operates at temperatures much higher than most previously demonstrated detectors. This means it needs much simpler refrigeration techniques, making it cheaper and easier to use in practical applications.