Terahertz image of a closed envelope containing the chip of an electronic musical greeting card and a small bag filled with sugar. Other electronic components and substances can be detected in a similar way.
All airplane passengers are familiar with the security checks at the entrance to the restricted area. So far, screening has largely been confined to metallic objects which metal detectors can identify. In addition, authorities and security companies rely on pat-down checks. Now, contact-free examination methods become mature. One of the emerging technologies, terahertz (THz) imaging and spectroscopy, is even capable of discovering non-metallic objects.
Arms and substances like pharmaceuticals, narcotics, and explosives are often hidden on the human body or in letters. In the terahertz spectral range, such dangerous objects and substances can be tracked down particularly well. In the electromagnetic spectrum, the terahertz band is located between the millimetre-wave and the infrared range, which corresponds to frequencies between 300 gigahertz and 10 terahertz and wavelengths of between 1millimetre and 30 micrometres. Thanks to the technological progress made in recent years, the terahertz technology is now ready to make its great leap from purely academic research to practical application.
Shorter than millimetre waves
Terahertz image (in colour) of a dummy weapon hidden under a T-shirt. The terahertz image was superimposed with a visual, black-and-white image of the same scene. The dummy weapon was 18 metres away from the terahertz scanner.
There are several reasons why this should be so. First, terahertz radiation is capable of penetrating clothing as well as many packaging materials. The fact that the wavelength of terahertz rays is shorter than that of millimetre waves permits building systems and scanners that are more compact and have a greater range. This opens up entirely new application potentials, possibilities the stationary body scanners currently on trial using millimetre waves are not capable of. Another advantage that terahertz rays have over millimetre waves: in the terahertz range, many substances, including explosives and pharmaceuticals, have their own characteristic absorption and reflection signatures called fingerprint spectra by which they can be identified. Moreover, unlike X-rays, terahertz radiation has no ionising effect, which, from what we know today, makes it safe to use on humans.
At the Institute of Planetary Research in Berlin-Adlershof, DLR has been systematically investigating the advantages of using Terahertz radiation in security applications for years. Scientists aim to develop serviceable system demonstrators with a focus on Terahertz scanners that are capable of detecting hidden objects from distances in excess of five metres. More than that: under favourable conditions, researchers were able to detect hidden objects from a distance of no less than 20 metres. Another research focus is on developing Terahertz spectrometers to identify explosives. Yet another is to combine Terahertz sensors with other detection methods.