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Electromagnetic waves sent at terahertz frequencies, known as terahertz 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).
Additional recommended knowledge
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. 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.
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.
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 the fall of 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. 
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 junctions 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.
Theoretical and technological uses under development
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
Books on millimeter and submillimeter waves and RF optics
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Terahertz_radiation". A list of authors is available in Wikipedia.|