What was previously known at the sub-millimeter microwave band (and considered part of the radio spectrum) has been moved to the imaging camp and renamed the terahertz band. This term roughly identified the band starting at the high end VLWIR and extending to down to a few hundred GHz – about 0.3 to 6 THz, corresponding to wavelengths of from 50 µm to 1 mm. Imaging at frequencies from around 100 GHz up has a long history. 94 GHz, for instance, has been used for passive imaging through fog because atmospheric attenuation is low at that frequency. The optics used are symmetrical spherical structures of graded refractive index polyethylene foam called Luneburg lenses. However, the molecular bonds in materials have no signatures at this frequency so the images tend to show featureless shapes of objects. While this may be sufficient to monitor movement of aircraft around airport concourses, it is insufficient to support identification of smaller objects.
At higher frequencies, especially above 1 THz, the composition of objects begins to become visible in images. Unfortunately, the self-radiation of the objects has insufficient power to provide these images so active sources are required. Unlike 94 GHz, where high-powered sources using standard microwave components and techniques are readily available, terahertz frequencies have no easily-implemented generators. Quantum cascade lasers and other devices are used but better sources are the continuing subject of research. In addition, atmospheric absorption is very high in the terahertz band, so the applicability of terahertz imaging is limited to short-range situations.
Terahertz radiation can be detected with a variety of small-geometry antenna arrays or by wavelength-independent detectors. Pyroelectrics and thermopiles, for instance can detect radiation at any wavelength. Unfortunately, the sensitivity of these is too low for practical imaging systems. Microbolometer arrays can be used as can more esoteric detectors such as Pockels cells, which convert electric field strength to polarization, essentially converting the terahertz image into a visible one. Research on better, faster detectors continues.
The varying signatures of organic materials have made terahertz imaging valuable in security screening. Clothing is largely transparent while metal objects are opaque and many plastics, including those used in explosives, have unique terahertz signatures. In addition, the relatively long wavelengths of terahertz radiation make detection of phase feasible. This may enable measure thickness or other properties not easily detected just by transmission amplitude.
Although many potential industrial inspection applications have been suggested, little implementation is yet in evidence. Aside from the cost and size of the detectors necessary for terahertz imaging, the relatively low resolution, fractions of a millimeter, stands in the way of qualifying this technique for detailed inspection tasks. Perhaps, there will be important applications in detecting materials below other materials, such as in laminates or in detecting organic contaminants in surface treatments. If an application seems intractable in the visible or infrared, considering the contrast the might be generated with terahertz illumination is worth considering.
As with any new field, important development can occur at any time. Frequent information searches are recommended as part of any terahertz development project.