The millimeter wave (MMW) band of frequencies extends from 30 GHz to 300 GHz, with some fuzziness on both ends of this spectrum. The terahertz (THz) band extends from about 200 GHz to about 30 THz, despite the fact that the lower frequencies in this range are not strictly 10~(12) Hz or higher. These bands are also variously called submillimeter, far-infrared, and near-millimeter. In recent years, there has been some degree of hype associated with the capabilities of systems operating in these bands. Sometimes exorbitant claims have been made relative to the ability of these systems to see through walls, detect buried structures, and detect cancer cells, for example. In this chapter we shall examine some of these clams and assess their validity. We shall find that MMW and THz systems can do some amazing things, some of them not related to the above claims, and that there is substantial promise of even more interesting results. In this chapter we begin by discussing these atmospheric limitations, since they permeate the whole technology of MMW, sub-MMW, and THz technology. We then discuss MMW and THz sources, detectors, optics, and systems in separate sections. Finally, we present some results obtained using sensors operating in these bands. Perhaps the most interesting of these results demonstrate the capability to image objects at resolutions as good as λ/100, where λ is wavelength. These measurements show the connection between this sensor technology and applications to security.
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