Terahertz (THz) radiation is located in the spectral region 0.1-10 THz (3 mm - 30 µm, 3 cm-1 - 300 cm-1) between the microwave and the infrared portion of the electromagnetic spectrum. In contrast to visible or infrared light, THz radiation has the potential to penetrate materials such as plastics cloth, or paper products. This feature might be helpful to sense concealed threats inside of clothing, luggage, parcels, and sealed containers. Because THz radiation does not penetrate metals, such a sensor would supplement rather than replace conventional X-ray and metal detectors. It has been shown, that THz imaging systems can be used to image threat items, non-metallic weaponry can be imaged when concealed beneath clothing, and it has been demonstrated that explosive materials have characteristic THz spectra.
The attractive features of THz radiation for security applications are:
These properties make THz systems a promising tool for security applications wherever detection and identification of hidden threats is the task and when human beings are to be scanned.
THz imaging of hidden objects
THz radiation is very well suited for the detection of objects carried by a person underneath its clothes or hidden in a package or an envelope. There exists a significant collection of data regarding transmission, absorption and reflection of different materials. In general, the transmission decreases with increasing frequency. However in many cases it is sufficient for remote detection of hidden objects. A technical advantage is that THz waves, like light waves, can easily be propagated though space, reflected, focused and refracted using THz optics. The short wavelength, much shorter than that of microwaves, allows for a spatial resolution, which is sufficient in many imaging applications.
THz “fingerprint” spectra
THz spectra - either reflective or transmissive - are already known for several explosives and proof-of-principle experiments claimed for bioagent simulants. The THz spectra of explosives also appear to be quite distinguishable from those of human skin and other materials such as metals, plastics, and cloth. By comparing measured spectra with reference spectra, it might be possible to identify different threats. Recent work in THz spectroscopy suggests that THz signatures associated with optical phonon bands are common to many crystalline organic molecules. THz spectroscopy has also been used in basic experiments to study the characteristic spectra of explosives and explosive-related compounds. Substances as diverse as plastic explosives, aspirin and amphetamines have significant absorption and reflection features in the THz range as can be seen from Fig. 2. Because the identification of distinct THz resonances from explosives would pave the way to a reliable chemical-specific sensor, most commonly encountered explosives have recently been investigated. The range of potential applications is likely to grow even further with the increased availability of absorption spectra (i.e. fingerprint spectra) peculiar to specific chemicals, including vitamins, sugars, pharmaceuticals, and agricultural chemicals.
THz radiation is non-ionizing
Due to its low photon energy (about one million times less than X-rays) THz radiation is non-ionising and to our present knowledge not dangerous for human beings. In fact an imaging system for medical applications which illuminates the human skin with THz radiation in order to localize skin cancer has been developed by Teraview Ltd.