Because terahertz radiation begins at a wavelength of around one millimeter and proceeds into shorter wavelengths, it is sometimes known as the submillimeter band , and its radiation as submillimeter waves , especially in astronomy. This band of electromagnetic radiation can be regarded either as microwave radio waves or far infrared.
Terahertz radiation is strongly absorbed by the gases of the atmosphere, and in air is attenuated to zero within a few meters, so it is not usable for terrestrial radio communication. It can penetrate thin layers of materials but is blocked by thicker objects. THz beams transmitted through materials can be used for material characterization , layer inspection, and as an alternative to X-rays for producing high resolution images of the interior of solid objects.
The generation and modulation of electromagnetic waves in this frequency range ceases to be possible by the conventional electronic devices used to generate radio waves and microwaves, requiring the development of new devices and techniques.
Terahertz radiation falls in between infrared radiation and microwave radiation in the electromagnetic spectrum , and it shares some properties with each of these.
Terahertz radiation travels in a line of sight and is non-ionizing. Like microwaves, terahertz radiation can penetrate a wide variety of non-conducting materials ; clothing, paper, cardboard , wood, masonry , plastic and ceramics. The penetration depth is typically less than that of microwave radiation. Like infrared, terahertz radiation has limited penetration through fog and clouds and cannot penetrate liquid water or metal. Due to its longer wavelength, images made using terahertz waves have lower resolution than X-rays and need to be enhanced see figure at right.
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The earth's atmosphere is a strong absorber of terahertz radiation, so the range of terahertz radiation in air is limited to tens of meters, making it unsuitable for long-distance communications. In addition, producing and detecting coherent terahertz radiation remains technically challenging, though inexpensive commercial sources now exist in the 0.
The terahertz band, covering the wavelength range between 0. 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.
The opacity of the Earth's atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, or to space. There have also been solid-state sources of millimeter and submillimeter waves for many years.
Nowadays, most time-domain work is done via ultrafast lasers. In mid, scientists at the U. Department of Energy's Argonne National Laboratory , along with collaborators in Turkey and Japan, announced the creation of a compact device that could lead to portable, battery-operated terahertz radiation sources.
These crystals comprise stacks of Josephson junctions , which exhibit a property known as the Josephson effect —when external voltage is applied, alternating current flows across the junctions at a frequency proportional to the voltage.
This alternating current induces an electromagnetic field. A small voltage around two millivolts per junction can induce frequencies in the terahertz range. In engineers at Harvard University achieved room temperature emission of several hundred nanowatts of coherent terahertz radiation using a semiconductor source.
THz radiation was generated by nonlinear mixing of two modes in a mid-infrared quantum cascade laser. Previous sources had required cryogenic cooling, which greatly limited their use in everyday applications.
The mechanism of its creation is tribocharging of the adhesive tape and subsequent discharge; this was hypothesized to involve bremsstrahlung with absorption or energy density focusing during dielectric breakdown of a gas. Such an antenna could be used to emit radio waves in the terahertz frequency range. Unlike X-rays , terahertz radiation is not ionizing radiation and its low photon energies in general do not damage living tissues and DNA.
Some frequencies of terahertz radiation can penetrate several millimeters of tissue with low water content e. Terahertz radiation can also detect differences in water content and density of a tissue. Such methods could allow effective detection of epithelial cancer with an imaging system that is safe, non-invasive, and painless. The first images generated using terahertz radiation date from the s; however, in images generated using terahertz time-domain spectroscopy generated a great deal of interest.
Some frequencies of terahertz radiation can be used for 3D imaging of teeth and may be more accurate than conventional X-ray imaging in dentistry.
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 have unique spectral "fingerprints" in the terahertz range.
This offers the possibility to combine spectral identification with imaging. The prototype system successfully imaged guns and explosives concealed under clothing.
In January , the NYPD announced plans to experiment with the new technology to detect concealed weapons ,  prompting Miami blogger and privacy activist Jonathan Corbett to file a lawsuit against the department in Manhattan federal court that same month, challenging such use: "For thousands of years, humans have used clothing to protect their modesty and have quite reasonably held the expectation of privacy for anything inside of their clothing, since no human is able to see through them.
In addition to its current use in submillimetre astronomy , terahertz radiation spectroscopy could provide new sources of information for chemistry and biochemistry.
Recently developed methods of THz time-domain spectroscopy THz TDS and THz tomography have been shown to be able to image samples that 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 spectrally broad, so such images can contain far more information than a conventional image formed with a single-frequency source. Terahertz radiation could let art historians see murals hidden beneath coats of plaster or paint in centuries-old buildings, without harming the artwork.
To date 0. When an electron bunch propagates through the capillary, its self-field interacts with the dielectric material and produces wakefields that propagate inside the material at the Cherenkov angle. The wakefields are slowed down below the speed of light, as the relative dielectric permittivity of the material is larger than 1.
The radiation is then reflected from the capillary's metallic boundary and diffracted back into the vacuum region, producing high accelerating fields on the capillary axis with a distinct frequency signature. In presence of a periodic boundary the Smith-Purcell radiation imposes frequency dispersion.
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A preliminary study with corrugated capillaries has shown some modification to the spectral content and amplitude of the generated wakefields,  but the possibility of using Smith-Purcell effect in DWA is still under consideration. In May , a team of researchers from the Tokyo Institute of Technology  published in Electronics Letters that it had set a new record for wireless data transmission by using T-rays and proposed they be used as bandwidth for data transmission in the future.
Potential uses exist in high-altitude telecommunications, above altitudes where water vapor causes signal absorption: aircraft to satellite , or satellite to satellite. Amateur radio operators utilizing submillimeter frequencies often attempt to set two-way communication distance records, and they have achieved distances up to 1.
Many possible uses of terahertz sensing and imaging are proposed in manufacturing , quality control , and process monitoring. These in general exploit the traits of plastics and cardboard being transparent to terahertz radiation, making it possible to inspect packaged goods.
The image can be developed based on either the attenuation or phase delay of the transmitted terahertz pulse. Since the beam is scattered more at the edges and also different materials have different absorption coefficients, the images based on attenuation indicates edges and different materials inside of an objects.
This approach is similar to X-ray transmission imaging, where images are developed based on attenuation of the transmitted beam. In the second approach, terahertz images are developed based on the time delay of the received pulse. In this approach, thicker parts of the objects are well recognized as the thicker parts cause more time delay of the pulse. Energy of the laser spots are distributed by a Gaussian function.
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The geometry and behavior of Gaussian beam in the Fraunhofer region imply that the electromagnetic beams diverge more as the frequencies of the beams decrease and thus the resolution decreases. Although terahertz can be used for inspection of packaged objects, it suffers from low resolution for fine inspections.
X-ray image and terahertz images of an electronic chip are brought in the figure on the right. To overcome low resolution of the terahertz systems near-field terahertz imaging systems are under development.
In another attempt to increase the resolution, laser beams with frequencies higher than terahertz are used to excite the p-n junctions in semiconductor objects, the excited junctions generate terahertz radiation as a result as long as their contacts are unbroken and in this way damaged devices can be detected.
Consequently, a tradeoff between the achievable resolution and the thickness of the penetration of the beam in the packaging material should be considered. The terahertz region is between the radio frequency region and the optical region generally associated with lasers. It is expected that effects on tissues are thermal in nature and, therefore, predictable by conventional thermal models [ citation needed ].
Research is underway to collect data to populate this region of the spectrum and validate safety limits. A study published in and conducted by Boian S. Alexandrov and colleagues at the Center for Nonlinear Studies at Los Alamos National Laboratory in New Mexico  created mathematical models predicting how terahertz radiation would interact with double-stranded DNA , showing that, even though involved forces seem to be tiny, nonlinear resonances although much less likely to form than less-powerful common resonances could allow terahertz waves to "unzip double-stranded DNA, creating bubbles in the double strand that could significantly interfere with processes such as gene expression and DNA replication".
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A recent analysis of this work concludes that the DNA bubbles do not occur under reasonable physical assumptions or if the effects of temperature are taken into account. From Wikipedia, the free encyclopedia. Redirected from Terahertz waves.
For other uses, see T-ray disambiguation. Main article: Submillimeter amateur radio. National Association of Broadcasters Engineering Handbook. Taylor and Francis.
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Terahertz waves: The missing electromagnetic waves
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