A collaboration between terahertz-radiation specialist TeraView (Cambridge, England) and security-detection provider Smiths Detection (Pine Brook, NJ) has resulted in terahertz components that may soon make security-screening systems based on terahertz radiation a reality. The team at TeraView has developed high-performance continuous-wave (CW) terahertz sources and detectors that open up the possibility of practical terahertz cameras for security screening and suicide-bomber detection-for example, one that is in the form of a hand-held wand (see figure).
Terahertz radiation passes through clothing, paper, and plastics to detect metal, ceramic, or plastic weapons, as well as bulk and sheet explosives that are concealed beneath clothing, inside shoes, or in other objects (see Laser Focus World, April 2003, p. S10). It also allows substances such as explosives to be identified from their characteristic spectral fingerprints (see Laser Focus World, July 2002, p. 23).
Previously, most terahertz light sources were based on relatively sophisticated ultrafast pulsed lasers. TeraView’s sources and detectors are based on low-cost laser diodes and are suitable for integration into imaging arrays for camera applications. The company uses commercial research-grade laser diodes operating in the near-IR. The outputs from two diodes, with a difference frequency in the terahertz region, are combined in a photoconductive semiconductor to generate terahertz radiation. TeraView has used pulsed lasers, but recent trials show that CW lasers could soon be a practical alternative. Much better CW performance resulted from improvements to the design of the difference-frequency- generation element.
“Continuous-wave techniques based on photomixing have been around for some time, but sources have always been rather weak,” said Bill Tribe, research group leader at TeraView. “We have been working to develop better sources and detectors for three years and have found a number of ways to improve performance. Our latest results give three-orders-of-magnitude improvement over what has generally been regarded as state-of-the-art.”
“The hand-held scanner has the terahertz sources and detectors in the wand head itself and, initially, will use pulsed techniques,” explained Mike Kemp, vice president of business development and head of TeraView’s security program. “The CW devices, which are starting to come out of the lab, can be engineered into real-time imaging arrays for applications such as portals at security checkpoints and so-called stand-off detection to detect suicide bombers. With support to develop these devices further and integrate them into practical systems, we would be looking at providing some very valuable new capabilities.”
Terahertz technology has the potential to overcome the limitations of new security-screening techniques that use ionizing radiation, for which safety concerns have been raised about using on people. Terahertz radiation is nonionizing and can be used safely for screening of humans. The wand will provide improved security over hand-held metal detectors by reducing the number of false alarms that lead to invasive manual searches. It will also enable in situ screening of luggage for explosives