Terahertz radiation
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Radio 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 light spectrum between 10 terahertz and 100 gigahertz, corresponding to the wavelength range 30 micrometres (ending edge of far-infrared light, micrometre wavelength) to 3 mm (starting edge of microwave radiation, millimeter wavelength).
Atmospheric_terahertz_transmittance_at_mauna_kea(simulated).gif.gif
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. As of 2004 the only effective sources of terahertz radiation are the gyrotron, the backward wave oscillator ("BWO"), the far infrared laser ("FIR laser"), and the free electron laser (FEL). The first imaging device based on terahertz radiation was introduced in 1995.
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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 cm 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 provide more accuracy and safety 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.
- Spectroscopy in terahertz radiation could provide novel information in chemistry and biochemistry.
- Some frequencies of terahertz radiation can propagate through metals in a manner similar to fiber optics and could be used in photonics.
- 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 nearinfrared regions of the spectrum. The utility of THz TDS is limited when the sample is very thin, or has a low index and absorption, 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.
References
- "Revealing the Invisible". Ian S. Osborne, Science 16 August 2002; 297: 1097.
- Article in Nature Nov 14, 2002 (local copy of the Jefferson Lab) (http://www.jlab.org/FEL/terahertz/nature.pdf)
- News and Views in Nature Nov 14, 2002 (local copy of the Jefferson Lab) (http://www.jlab.org/FEL/terahertz/nature2.pdf)
See also
External links
- Terahertz radiation: applications and sources (http://www.aip.org/tip/INPHFA/vol-9/iss-4/p27.html) by Eric Mueller
- Blinded By The Light At 20,000 THz (http://www.spacedaily.com/news/laser-03b.html)
- News article on propagation in metals (http://www.eetimes.com/article/printableArticle.jhtml?articleID=54200109&url_prefix=&sub_taxonomyID=4217)
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