Hazard tracker

Coupling a uniquely structured copper material and terahertz radiation (T-rays) could lead to the development of sensors that might revolutionise security screening for dangerous materials.

T-rays, electromagnetic waves in the far infrared part of the electromagnetic spectrum, have a wavelength 500 times longer than visible light.

One of the areas with the most potential to use them is security sensing and scanning, because many molecules in explosives and biological agents such as anthrax strongly absorb this radiation. If T-rays are tightly confined on surfaces in contact with such molecules, the detection sensitivity is greatly increased.

Researchers from Bath University and Imperial College London, with Spanish physicists, have developed a way to control these rays on to a specially designed material, known as a metamaterial.

Simple metallic surfaces have been used to control T-ray waves before, but they offered only weak guides for radiation. The radiation field extended centimetres above the surface so the rays were less effective for sensing.

The new research has shown that a metamaterial surface draws T-rays close to it, creating a strong field less than a millimetre above the surface. This greatly enhances the absorption by molecules on the surface, making highly effective sensing techniques possible.

A metamaterial is a man-made material with designed electromagnetic properties that are impossible for natural materials to possess.

The metamaterial created for this new research consists of a copper surface textured with a two-dimensional array of pits. The researchers chose the dimensions of the pits so that T-rays are drawn closely to them as they travel along the surface.

The T-rays are generated from a small dipole antenna on a semiconductor. The antenna is struck with a fast laser pulse to cause a spark between two electrodes in the semiconductor. Those electrodes cause a pulsed current in the antenna that emits terahertz radiation.

‘T-rays have the potential to revolutionise security screening for dangerous materials such as explosives,’ said Stefan Maier from Imperial College, one of the leaders of the project.

‘Until now it hasn’t been possible to exert the necessary control and guidance over pulses of this kind of radiation for it to have been usable in real-world applications.

‘We have shown with our material that it is possible to tightly guide T-rays along a metal sheet, possibly even around corners, increasing their suitability for a wide range of situations.’

The researchers hope their work will lead to the development of portable sensors for high-security areas.

If there was a security threat in a government building, hazardous material specialists would place the metamaterial surfaces in a room and aim T-rays on to it. If there were hazardous materials present, they would attach to the surface and absorb radiation from the rays. The materials would then be identified with a spectrometer.

‘T-rays are a big area of research. People are trying to find applications for radiation in this part of the spectrum,’ said Steve Andrews from Bath University, the other leader in the project.

He said other projects have attempted to use T-rays in security imaging applications at airports. Unlike today’s metal or X-ray detectors, which can identify only a few obviously dangerous materials, checkpoints that look instead at T-ray absorption patterns could detect and identify a much wider variety of hazardous or illegal substances.

T-rays, which are a safe form of electromagnetic radiation, can penetrate through many common materials such as leather, fabric, and paper. However, they are not the solution to every problem. Terahertz radiation does not penetrate through metals and water.

There are limitations to the effectiveness of T-ray detection, but Maier said the results of their study show that using them for sensors could be of great use in many security applications.

The researchers are now working to refine their metamaterial surfaces before they can be used for sensing applications.

Maier said at present only a small number of the frequencies that make up a pulse of T-ray radiation are closely confined to the metamaterial.

More sophisticated designs are needed in order to make sure the whole pulse is affected by the surface structure, so that absorption features of molecules can be clearly identified.

The sensors are at least two years away from being used outside the laboratory in real-world applications.

‘These are first steps,’ Andrews said. ‘We’ve made an important step in developing the infrastructure you need to push things forward.’