However, the popular perception of the dangers of microwave radiation may cause public concern over this imaging technique.
Until recently, most terahertz sources were either low-brightness emitters, such as thermal sources, or cumbersome, single-frequency molecular vapor lasers. Detection usually relied on bolometric methods, which required cryogenic operation, or Golay cells, which have little dynamic range. Recently, however, a revolution has occurred in terahertz technology as a number of newly discovered or rediscovered generation and detection schemes have revitalized the field. These techniques rely either on frequency conversion using nonlinear optics or on quantum cascade lasers, which depend on devices formed from superlattices of semiconductors. They are often simpler, more reliable, and potentially much less expensive than the more traditional approaches. Chapter 3 presents a general description of these devices.
The promise is that terahertz imaging will provide an orthogonal detection capability that will allow the identification of explosive materials concealed on a person’s body or in nonmetallic carry-on luggage. In simple terms, the terahertz waves can penetrate materials such as clothing and leather with enough residual energy to excite molecular vibrations, rotations, and phonon-band resonances in solid materials. This excitation can be detected using spectroscopy techniques and the substance identified by an analysis of the unique signatures from different molecules. While spectroscopy will provide additional capability, it is expected that the transmissivity of explosives to x-rays is different from their transmissivity to millimeter-wavelength/terahertz technology.
The technology challenge for active imaging rests on the development of components that operate in the millimeter-wavelength/terahertz region. Poor performance and high cost have historically limited research and development efforts at millimeter-wave/terahertz wavelengths. New techniques for wave generation and detection that are unique to the terahertz spectrum are being developed in research laboratories worldwide. A literature search will yield hundreds of recent citations covering all aspects of terahertz research.
In order to form a terahertz image using time domain spectroscopy, the terahertz beam is brought to an intermediate focus using a pair of lenses or parabolic reflectors, which are inserted into the region where the terahertz beam is collimated. An object is placed at the focus of the terahertz beam, and then the amplitude and delay of the wave that has traversed through the object is measured. By translating the object and measuring the transmitted terahertz waveform for each position of the object, one can build an image pixel by pixel.
An imaging system in which the terahertz beam is reflected from the sample rather than being transmitted through it can be used for tomographic imaging. If the sample consists of several separate dielectric layers, the interface between each pair of layers reflects a portion of the terahertz beam. The reflected waveform therefore consists of a series of isolated pulses. Each pulse in this pulse train contains information about each of the layers through which it propagated, as well as about the interface from which it originated. With this information, a tomographic image may be constructed.