20/11/2009 : Sub-terahertz imaging for security
by Adrian Dobroiu Terahertz Sensing and Imaging Laboratory, RIKEN, Japan
The terahertz radiation has two important qualities that make it especially attractive for security and safety applications. One such quality is its ability to penetrate materials which are opaque in other frequency ranges, in particular the visible range. Paper, cardboard, plastics, textiles, wood, and many dry materials are transparent and non-scattering for the terahertz radiation. The other quality is the fact that many chemicals, especially those that are crystalline, have each a specific absorption fingerprint spectrum, which allows the identification of the chemical in question. Additionally, the short wavelength of the terahertz radiation, roughly between 0.1 and 1 mm, is useful where imaging needs to be performed, because it allows a sufficient resolution in many practical situations. Soon after sources and detectors for terahertz radiation became affordable and more widely available, applications in security became one of the most motivating research directions. Many groups invested important amounts of effort in applications such as : stand-off imaging for use in airports and other crowded places, to detect concealed weapons and explosives ; identification of drugs, explosives, and biochemical warfare substances hidden in mail envelopes ; identification of liquids that may be used as precursors for explosives ; and others.