Terahertz radiation

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Electromagnetic waves sent at terahertz frequencies, known as terahertz radiation, terahertz waves, 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 wavelength range starting at submillimeter (<1 millimeter) and 100 micrometres (ending edge of far-infrared light).

Image:Atmospheric terahertz transmittance at mauna kea(simulated).gif.gif

1 Introduction
2 Sources
3 Theoretical and technological uses under development
4 References
5 See also
6 External links

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Introduction

Like infrared radiation or microwaves, these waves usually travel in line of sight. Terahertz radiation is non-ionizing and shares with microwaves the capability to penetrate a wide variety of non-conducting materials. They can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. They 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. In addition, producing and detecting coherent terahertz radiation was technically challenging until the 1990s.

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Sources

While terahertz radiation is emitted as part of the black body radiation from anything with temperatures greater than about 10 kelvin, this thermal emission is very weak. As of 2004 the only effective stronger sources of terahertz radiation are the gyrotron, the backward wave oscillator ("BWO"), the far infrared laser ("FIR laser"), quantum cascade laser, the free electron laser (FEL), synchrotron light sources, and single-cycle sources used in Terahertz time domain spectroscopy. The first images generated using terahertz radiation date from the 1960's; however, in 1995, images generated using terahertz time-domain spectroscopy generated a great deal of interest, and sparked a rapid growth in the field of terahertz science and technology. This excitement, along with the associated coining of the term "T-rays," even showed up in a contemporary novel by Tom Clancy.

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Theoretical and technological uses under development

Medical imaging:
- Terahertz radiation is non-ionizing, and thus is not expected to damage DNA, unlike X-rays. Some frequencies of terahertz radiation can penetrate several centimeters of tissue and reflect back. Terahertz radiation can also detect differences in water content and density of a tissue. Terahertz imaging could allow effective detection of epithelial cancer and replace the mammogram with a safer and less invasive or painful imaging system.
- Some frequencies of terahertz radiation can be used to make 3D imaging of teeth and could be more accurate and safer than conventional X-ray imaging in dentistry.
Because of terahertz radiation's ability to penetrate fabrics and plastics 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, exhibit unique spectral fingerprints in the terahertz range. This offers the possibility of combining spectral identification with imaging.
Spectroscopy in terahertz radiation could provide novel information in chemistry and biochemistry.
The recently developed techniques of THz time-domain spectroscopy (THz TDS) and THz tomography have been shown to be capable of performing measurements on, and obtaining 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. On the other hand, the fact that THz-TDS produces radiation that is both coherent and broadband means that such images can contain far more information than a conventional image formed with a single-frequency source.
satellite telecommunications, and high-altitude communications (aircraft to satellite or satellite to satellite)
Many possible applications of terahertz imaging have been proposed in manufacturing, quality control, and process monitoring. These generally exploit the fact that plastics and cardboard are transparent to terahertz radiation, so that it is possible to inspect packaged objects.