View source for Terahertz radiation

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[[Electromagnetic wave]]s sent at [[Hertz|terahertz]] [[frequency|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]] (3x10<sup>11</sup> [[Hertz|Hz]]) and 3 [[terahertz]] (3x10<sup>12</sup> Hz), corresponding to the submillimeter [[wavelength]] range between 1 millimeter (high-frequency edge of the [[microwave]] band) and 100 [[micrometre|micrometer]] (long-wavelength edge of [[far-infrared]] [[light]]). 

[[Image:Atmospheric terahertz transmittance at Mauna Kea (simulated).png|thumb|right|400px|Plot of the zenith atmospheric transmission on the summit of Mauna Kea throughout the range of 1 to 3 THz of the electromagnetic spectrum at a precipitable water vapor level of 0.001 mm. (simulated)]]

==Introduction==

Like [[infrared radiation]] or [[microwaves]], these waves usually travel in [[line of sight (telecommunications)|line of sight]].  Terahertz radiation is non-[[ionizing radiation|ionizing]] submillimeter microwave radiation and shares with microwaves the capability to penetrate a wide variety of non-[[conductor (material)|conducting]] materials. Terahertz radiation can pass through [[clothing]], [[paper]], [[cardboard]], [[wood]], [[masonry]], [[plastic]] and [[ceramic]]s. It can also penetrate [[fog]] and [[cloud]]s, 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 [[coherence (physics)|coherent]] terahertz radiation was technically challenging until the 1990s.

==Sources==
Terahertz radiation is emitted as part of the [[black body]] radiation from anything with temperatures greater than about 10 [[kelvin]]. While this thermal emission is very weak, [[Submillimetre astronomy|observations at these frequencies]] are important for characterizing the cold 10-20K dust in the [[interstellar medium]] in the Milky Way galaxy, and in distant [[starburst galaxy|starburst galaxies]].  Telescopes operating in this band include the [[James Clerk Maxwell Telescope]], the [[Caltech Submillimeter Observatory]] and the [[Submillimeter Array]] at the [[Mauna Kea Observatory]] in Hawai'i, the [[BLAST (telescope)|BLAST]] balloon borne telescope, and the [[Heinrich Hertz Submillimeter Telescope]] at the [[Mount Graham International Observatory]] in Arizona.  Planned telescopes operating in the submillimeter include the [[Atacama Large Millimeter Array]] and the [[Herschel Space Observatory]].  The opacity of the Earth's atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, or to space.

As of 2004 the only viable sources of terahertz radiation were the [[gyrotron]], the [[backward wave oscillator]] ("BWO"), the [[far infrared laser]] ("FIR laser"), [[quantum cascade laser]], the [[free electron laser]] (''FEL''), [[synchrotron light]] sources, [[photomixing]] sources, and single-cycle sources used in [[Terahertz time domain spectroscopy]]. The first images generated using terahertz radiation date from the 1960s; 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]].

There have also been solid-state sources of millimeter and submillimeter waves for many years.  AB Millimeter in Paris, for instance, produces a system that covers the entire range from 8 GHz to 1000 GHz with solid state sources and detectors.  Nowadays, most time-domain work is done via ultrafast lasers.

In mid-2007, scientists at the U.S. Department of Energy's Argonne National Laboratory, along with collaborators in Turkey and Japan, announced the creation of a compact device that can lead to a portable, battery-operated sources of T-rays, or terahertz radiation. The group was led by Ulrich Welp of Argonne's Materials Science Division.<ref>Science News: [http://www.sciencedaily.com/releases/2007/11/071126121732.htm New T-ray Source Could Improve Airport Security, Cancer Detection], ScienceDaily (Nov. 27, 2007).</ref>

The new T-ray sources created at Argonne use high-temperature superconducting crystals grown at the University of Tsukuba in Japan. These crystals comprise stacks of so-called [[Josephson junction]]s that exhibit a unique electrical property: when an external voltage is applied, an alternating current will flow back and forth across the junctions at a frequency proportional to the strength of the voltage; this phenomenon is known as the [[Josephson effect]].

These alternating currents then produce electromagnetic fields whose frequency is tuned by the applied voltage. Even a small voltage – around two millivolts per junction – can induce frequencies in the terahertz range, according to Welp.

In 2008 engineers at Harvard University announced they had built a room temperature semiconductor source of coherent Terahertz radiation. Until then sources had required cryogenic cooling, greatly limiting their use in everyday applications. <ref>[http://www.physorg.com/news130385859.html Engineers demonstrate first room-temperature semiconductor source of coherent Terahertz radiation] Phsorg.com.  May 19, 2008. Accessed May 2008</ref>

==Theoretical and technological uses under development==

*Medical imaging:
**Terahertz radiation is non-ionizing, and thus is not expected to damage [[Tissue (biology)|tissue]]s 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 [[Tooth|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 [[Airport security|security]] screening, to uncover [[concealment device|concealed]] [[weapon]]s 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. <ref>{{cite news |first= |last= |authorlink= |coauthors= |title=Camera 'looks' through clothing  |url=http://news.bbc.co.uk/1/hi/technology/7287135.stm |work= |publisher=BBC News 24 |date=10 March 2008 |accessdate=2008-03-10 }}</ref> 
*Scientific use and imaging:
**[[Spectroscopy]] in terahertz radiation could provide novel information in [[chemistry]] and [[biochemistry]].
**Recently developed methods of [[Terahertz time domain spectroscopy|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 [[infrared|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 [[Tesla (unit)|teslas]]), the [[Larmor frequency|Larmor]] frequencies are in the submillimeter band. This work is performed at many [[National High Magnetic Field Laboratory|high-magnetic field laboratories]] around the world.
**[[Submillimetre astronomy]].
*Communication:
**Potential uses exist in high-altitude [[telecommunication]]s, 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 [[Manufacturing process management|process monitoring]]. These generally exploit the traits of [[plastic]]s and [[Paperboard|cardboard]] being transparent to terahertz radiation, making it possible to inspect [[Packaging and labelling|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.{{Fact|date=July 2007}}

==References and notes==
* "Revealing the Invisible". Ian S. Osborne, ''Science'' [[16 August]] [[2002]]; 297: 1097.
*[http://www.jlab.org/FEL/terahertz/nature.pdf Article in ''Nature''] [[14 November]] [[2002]] (local copy from the Jefferson Lab)]
*[http://www.jlab.org/FEL/terahertz/nature2.pdf News and Views in ''Nature''] [[14 November]] [[2002]] (local copy from the Jefferson Lab)
*[http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=RSINAK000071000001000186000001&idtype=cvips&gifs=Yes Instrumentation for millimeter-wave magnetoelectrodynamic investigations...  Review of Scientific Instruments, 2000]
{{reflist|2}}
==Books on millimeter and submillimeter waves and RF optics==
* [http://www.amazon.com/dp/0780334396/ Quasioptical systems: Gaussian beam quasioptical propagation and applications, Paul F. Goldsmith, IEEE Press]
* [http://www.amazon.com/dp/3540628606/ Millimeter wave spectroscopy of solids, edited by G. Grüner, Springer]
         
* [http://www.amazon.com/dp/0521017106/ Detection of light: from the ultraviolet to the submillimeter, George Rieke, Cambridge]
* [http://www.amazon.com/dp/1586030981/ Modern millimeter-wave technologies, Tasuku Teshirogi and Tsukasa Yoneyama, eds, IOS press]
* [http://www.amazon.com/dp/0890067112/ Optoelectronic techniques for microwave and millimeter-wave engineering William Robertson, Artech]

==See also==
*[[Terahertz time domain spectroscopy]]
*[[Heterojunction bipolar transistor]]
*[[HEMT]]
*[[Picarin lens]]

== External links ==
*[http://www.aip.org/tip/INPHFA/vol-9/iss-4/p27.html Terahertz radiation: applications and sources] by Eric Mueller
*[http://www.livescience.com/technology/060728_t-rays.html Shadowy T-rays: Hunting Tumors and Exploring the Universe]
*[http://www.physik.uni-kl.de/768.html?L=1 Introduction to THz time-domain-spectroscopy and imaging]
*[http://physorg.com/news130385859.html Engineers demonstrate first room-temperature semiconductor source of coherent Terahertz radiation]

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[[Category:Electromagnetic spectrum]]
[[Category:Terahertz technology]]

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