There is a need for ever more effective security screening to detect an increasing variety of threats. Many techniques employing different parts of the electromagnetic spectrum from radio up to X- and gamma-ray are in use. Terahertz radiation, which lies between microwave and infrared, is the last part to be exploited for want, until the last few years, of suitable sources and detectors. Terahertz imaging and spectroscopy has been shown to have the potential to use very low levels of this non-ionising radiation to detect and identify objects hidden under clothing. This paper describes recent work on the development of prototype systems using terahertz to provide new capabilities in people screening, both at security checkpoints and stand-off detection for remote detection of explosives and both metallic and non-metallic weapons.
We describe the latest progress on two new terahertz (THz) prototype systems for security screening applications. Firstly, we show a terahertz stand-off detection system, working at a distance of 0.5 m, and with a dynamic range of ~60 dB. This allows the measurement of spectral features from 0.1 to 2.5 THz. Using this prototype, we present the reflection spectrum of an explosive material collected at 0.5m stand-off and compare it with the predicted spectrum. Secondly, a hand held wand using multiple terahertz detectors has been assembled. This resembles a metal detector wand in operation, and is designed for detecting explosives as well as both metallic and non-metallic weapons hidden under clothing or possibly in bags. The first data from this system is presented, showing that objects can be distinguished using their spectral features.
We demonstrate an all-optoelectronic continuous-wave terahertz (cw-THz) imaging system using technology based on low-cost and compact diode lasers. THz radiation is generated by photomixing two near-infrared lasers (830 nm) in a photoconductive emitter and is tunable from 0.1-2 THz. Images are captured in reflection geometry, and a phase-sensitive photoconductive detection scheme is used, which operates at room temperature. We have optimised the growth and annealing of low-temperature gallium arsenide (LT-GaAs), and achieved state-of-the-art material with 100 fs carrier trapping lifetimes. The photomixers load resonant antennas, which efficiently couple out monochromatic THz radiation. Images are captured in the time-domain using a real-time rapid scan delay line capable of data acquisition at 15 Hz, with both amplitude and phase information available. There are a number of advantages in using continuous-wave imaging systems, compared to the more established pulsed technologies. In particular, the combination of diode lasers and photoconductive detection demonstrates, for the first time, an imaging system that is compact, robust, genuinely turn-key and of low cost. Such a system would be well suited for routine THz imaging in both medical and non-medical applications.
Recent events have led to dramatic changes to the methods employed in security screening. For example, following the failed shoe bombing, it is now common for shoes to be removed and X-rayed at airport checkpoints. There is therefore an increasing focus on new Recent events have led to dramatic changes to the methods employed in security screening. For example, following the failed shoe bombing, it is now common for shoes to be removed and X-rayed at airport checkpoints. There is therefore an increasing focus on new technologies that can be applied to security screening, either to simplify or speed up the checking process, or to provide additional functionality. Terahertz (THz) technology is a promising, emerging candidate. In previous publications we have shown how our THz pulsed imaging systems can be used to image threat items, and have demonstrated that explosive materials have characteristic THz spectra. We have also demonstrated that nonmetallic weaponry can be imaged when concealed beneath clothing. In this work we examine more closely the properties of barrier and potential confusion materials. We demonstrate that barrier materials have smooth spectra with relatively low attenuation. We further demonstrate that the terahertz spectra of several common chemicals and medicines are distinct from those of threat materials.
Recent events have accelerated the quest for ever more effective security screening to detect an increasing variety of
threats. Many techniques employing different parts of the electromagnetic spectrum from radio up to X- and gammaray
are in use. Terahertz radiation, which lies between microwave and infrared, is the last part to be exploited for want,
until recently, of suitable sources and detectors. This paper describes practical techniques for Terahertz imaging and
spectroscopy which are now being applied to a variety of applications. We describe a number of proof-of-principle
experiments which show that Terahertz imaging has the ability to use very low levels of this non-ionising radiation to
detect hidden objects in clothing and common packing materials and envelopes. Moreover, certain hidden substances
such as plastic explosives and other chemical and biological agents may be detected from their characteristic Terahertz
spectra. The results of these experiments, coupled with availability of practical Terahertz systems which operate outside
the laboratory environment, demonstrate the potential for Terahertz technology in security screening and counterterrorism.