Terahertz – to see the invisible
Terahertz radiation, located between infrared and microwave radiation (the range is accepted as one from 0.1 THz to 10 THz, namely wavelengths of 30-3000 mm), belongs to the least studied subranges of the electromagnetic spectrum. The reasons behind it include difficulties in constructing adequately sensible detectors and controlled sources of this radiation.
An often used feature of this range of spectrum is low radiation damping by most dry, non-metallic substances, such as: plastic, textiles, paper. Already for some time there are commercially available imaging devices working in the lower terahertz region (100-200 GHz), which allow detection of non-metallic objects hidden, for example, under clothing. However, they raise serious ethical issues related to violation of privacy or with negative influence of the radiation “passing through”. Stronger terahertz radiation may penetrate tissues at the depth of several milimeters. Although it is non-ionizing radiation (in contrast to X-rays, for example), currently it is difficult to evaluate its influence on health.
A phenomenon making measurements of terahertz radiation more difficult is its relatively strong absorption by water vapour present in the air. What is a disadvantage in one of the applications, though, may be an advantage in another. Studies are being conducted on using this wavelength in safe, wireless, short-distance communication. Because of the aforementioned damping, this communication would be impossible to eavesdrop outside the premise in which it takes place.
However, the most interesting (and hitherto rarely used) feature of the terahertz radiation is the fact that many substances deemed dangerous or unwanted (like explosives and illicit drugs) have their most characteristic parts of the spectrum in this particular range. This gives the potential possibility of their remote detection by using spectral measurement of the radiation they emit (naturally) or passing through them (emitted by other sources). Obviously, applications related to prevention of terrorism come into question here, but not only these. Similarly characteristic spectra are exhibited by many foods and microorganisms. Potential applications may thus also include laboratory studies of food composition or medical diagnostics.
Terahertz signatures of some food types