Terahertz radiation

Electromagnetic waves sent at terahertz frequencies, known as terahertz radiation, submillimeter radiation, terahertz waves, terahertz light, T-rays, T-light, T-lux and THz, are in the region of the electromagnetic spectrum between 300 gigahertz (3x1011 Hz) and 3 terahertz (3x1012 Hz), corresponding to the submillimeter wavelength range between 1 millimeter (high-frequency edge of the microwave band) and 100 micrometer (long-wavelength edge of far-infrared light).



Introduction
Like infrared radiation or microwaves, these waves usually travel in line of sight. Terahertz radiation is non-ionizing submillimeter microwave radiation and shares with microwaves the capability to penetrate a wide variety of non-conducting materials. Terahertz radiation can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. It can also penetrate fog and clouds, but cannot penetrate metal or water. The Earth's atmosphere is a strong absorber of terahertz radiation, so the range of terahertz radiation is quite short, limiting its usefulness for communications. In addition, producing and detecting coherent terahertz radiation was technically challenging until the 1990s.

Theoretical and technological uses under development

 * Medical imaging:
 * Terahertz radiation is non-ionizing, and thus is not expected to damage tissues and DNA, unlike X-rays. Some frequencies of terahertz radiation can penetrate several millimeters of tissue with low water content (e.g. fatty tissue) and reflect back. Terahertz radiation can also detect differences in water content and density of a tissue. Such methods could allow effective detection of epithelial cancer with a safer and less invasive or painful system using imaging.
 * Some frequencies of terahertz radiation can be used for 3D imaging of teeth and may be more accurate and safer than conventional X-ray imaging in dentistry.
 * Security:
 * Terahertz radiation can penetrate fabrics and plastics, so it can be used in surveillance, such as security screening, to uncover concealed weapons on a person, remotely. This is of particular interest because many materials of interest, such as plastic explosives, have unique spectral "fingerprints" in the terahertz range. This offers the possibility to combine spectral identification with imaging. Passive detection of Terahertz signatures avoid the bodily privacy concerns of other detection by being targeted to a very specific range of materials and objects.
 * Scientific use and imaging:
 * Spectroscopy in terahertz radiation could provide novel information in chemistry and biochemistry.
 * Recently developed methods of THz time-domain spectroscopy (THz TDS) and THz tomography have been shown to be able to perform measurements on, and obtain images of, samples which are opaque in the visible and near-infrared regions of the spectrum. The utility of THz-TDS is limited when the sample is very thin, or has a low absorbance, since it is very difficult to distinguish changes in the THz pulse caused by the sample from those caused by long term fluctuations in the driving laser source or experiment. However, THz-TDS produces radiation that is both coherent and broadband, so such images can contain far more information than a conventional image formed with a single-frequency source.
 * A primary use of submillimeter waves in physics is the study of condensed matter in high magnetic fields, since at high fields (over about 15 teslas), the Larmor frequencies are in the submillimeter band. This work is performed at many high-magnetic field laboratories around the world.
 * Submillimetre astronomy.
 * Communication:
 * Potential uses exist in high-altitude telecommunications, above altitudes where water vapor causes signal absorption: aircraft to satellite, or satellite to satellite.
 * Manufacturing:
 * Many possible uses of terahertz sensing and imaging are proposed in manufacturing, quality control, and process monitoring. These generally exploit the traits of plastics and cardboard being transparent to terahertz radiation, making it possible to inspect packaged goods.

Terahertz versus submillimeter waves
The terahertz band, covering the wavelength range between 0.1 and 1 mm, is identical to the submillimeter wavelength band. However, typically, the term "terahertz" is used more often in marketing in relation to generation and detection with pulsed lasers, as in terahertz time domain spectroscopy, while the term "submillimeter" is used for generation and detection with microwave technology, such as harmonic multiplication.

References and notes

 * "Revealing the Invisible". Ian S. Osborne, Science 16 August 2002; 297: 1097.
 * Article in Nature 14 November 2002 (local copy from the Jefferson Lab)]
 * News and Views in Nature 14 November 2002 (local copy from the Jefferson Lab)
 * Instrumentation for millimeter-wave magnetoelectrodynamic investigations... Review of Scientific Instruments, 2000

Books on millimeter and submillimeter waves and RF optics

 * Quasioptical systems: Gaussian beam quasioptical propagation and applications, Paul F. Goldsmith, IEEE Press
 * Millimeter wave spectroscopy of solids, edited by G. Grüner, Springer
 * Detection of light: from the ultraviolet to the submillimeter, George Rieke, Cambridge
 * Modern millimeter-wave technologies, Tasuku Teshirogi and Tsukasa Yoneyama, eds, IOS press
 * Optoelectronic techniques for microwave and millimeter-wave engineering William Robertson, Artech