For several years terahertz scanners have been talked of as the great new hope in airport surveillance, but to date a number of technical hurdles have limited their application. In theory these scanners could be used from tens of metres away to detect illegal items, such as guns or contraband concealed within clothing, without exposing a passenger to harmful radiation or revealing their naked body on a TV monitor. Now, a breakthrough by a UK–US collaboration may enable terahertz scanners to finally live up to their potential.
Terahertz radiation lies between the microwave and mid-infrared regions of the electromagnetic spectrum and is emitted by almost all objects. Crucially, terahertz waves can pass through clothing and packaging, but they are strongly absorbed by metals and other inorganic substances.
The most common sources in terahertz applications are semiconductor lasers, but they tend to produce beams that are widely divergent – similar to how light is emitted from a lamp. So one of the challenges with building these scanners is to find a way of directing the terahertz radiation onto a specific target. Now, a team of researchers at Harvard University, led by Federico Capasso, along with colleagues at the University of Leeds in the UK, have developed a cover that can fit on the front of a laser to act as a waveguide.
A special cover
The device is made from metamaterials, which are specially engineered structures that can respond to light and other electromagnetic radiation very differently than conventional materials. While metamaterials have potential use in novel applications such as cloaking, negative refraction and high-resolution imaging, their use in semiconductor devices has been very limited to date.
In Capasso’s device, photons emitted from a quantum cascade laser – a type of semiconductor laser that can emit terahertz radiation – are made to pass through the metamaterial. At the metamaterial surface the photons interact with electrons to create temporary states known as surface plasmons, which subsequently break down with the re-emission of terahertz radiation.
With a careful design, consisting of a series of narrow grooves that can confine plasmons, the researchers were able to create a geometry that could re-emit terahertz radiation, outwards from the laser, in a tight beam. “In our case, the metamaterial serves a dual function: strongly confining the terahertz light emerging from the device to the laser facet and collimating the beam,” explains Nanfang Yu, a member of the Harvard team.
Although this research is still at the laboratory stage, Yu predicts that this type of terahertz scanning could become commercially available within the next five years. His team already has a broad patent on the technique and parts of the apparatus.
One of the challenges is that even the most practical quantum cascade lasers today operate at below 200 K, usually requiring liquid nitrogen as a coolant, which may not be feasible for widespread application.
This research is described in Nature Materials.
