Handheld scanners could spot drugs and bombs

2019-03-02 11:03:11

By Tom Simonite Portable scanners that can instantly detect concealed drugs, weapons and explosives are a step closer with the development of a compact method for generating terahertz radiation. Researchers at the University of St Andrews in Scotland, UK, came up with a terahertz generator that could fit into a large shoebox. This could help reduce the size of terahertz scanning devices, which are currently the size of a small car. “This will take the potential of terahertz radiation out of the lab and into the street,” says laser specialist Malcolm Dunn from St Andrews. “Making the components that produce the terahertz beam more compact also means we get more power from less energy, so there’s no need for a huge power source or cryogenic cooling.” Terahertz radiation lies between infra-red and microwaves on the electromagnetic spectrum. It passes straight through plastic, fabric, wood and stone but can be used to spot other compounds, including certain drugs, metals and explosives, as it interacts with the weak bonds that form between larger molecules. Terahetz radiation is also non-ionising and is therefore harmless to people. Most terahertz sources “pump” laser light into a crystal called a parametric generator. This splits the light into two beams – one in the terahertz range and one that is a by-product. Dunn’s team created a smaller terahertz source by putting the parametric generator inside the pump laser itself. This solved two key problems as the generator requires less space and also uses a less powerful laser. Inside a normal laser, light is used to excite electrons within a crystal tube with mirrors at each end. The electrons produce photons, some of which escape as light, and the rest of which bounce back and forth between the two mirrors. Because one of the mirrors is only partially reflective, a laser beam of parallel photons escapes. Dunn’s team places a parametric generator inside the chamber of the laser, among the bouncing photons. “It is a much more efficient use of the power of the pump laser,” he says. It also means that turning the parametric generator makes the photons enter it at a different angle, thereby tuning the frequency of the terahertz beam. Tuning the beam is important as it makes it possible to scan for different compounds. Dunn’s set-up can run from a battery and takes up a space 40cm by 20cm. Packing it into a portable, self-contained unit is the next goal. “A portable terahertz source is one of the requirements for making this technology usable by customs and others in real life,” says Giles Davies, a photonic engineer at Leeds University, UK. “The other is to know what the signals from different substances of interest look like.” Researchers at The Institute of Physical and Chemical Research in Japan have previously shown that terahertz scanners can detect particular drugs including ecstasy and methamphetamine hidden inside envelopes. But the equipment used required a bulky liquid gas cooling system and a sizable power generator. Other researchers have used fibre-optics to pipe a terahertz beam a short distance away from the generating device. But this is impractical for many situations. Davies is using terahertz radiation to identify different illegal substances supplied under licence by the UK government. “We have already shown that it is possible to distinguish the different forms of cocaine, as well as a range of different explosives,” he says. Davies’ current research project involves using samples of drugs and explosives mixed with other chemicals, to mimic “street” compounds. But he also hopes to analyse samples seized by the police. “The information we collect in this way can be used to generate a database of the spectra of these compounds to calibrate field devices when they become available,