Terahertz radiation
Like microwaves, terahertz radiation can penetrate a wide variety of non-conducting materials; clothing, paper, cardboard, wood, masonry, plastic and ceramics.
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).
In addition, producing and detecting coherent terahertz radiation remains technically challenging, though inexpensive commercial sources now exist in the 0.3–1.0 THz range (the lower part of the spectrum), including gyrotrons, backward wave oscillators, and resonant-tunneling diodes.
Tremendous efforts thus have been put into THz research to improve the operation temperature, using different strategies such as optomechanical meta-devices.
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 could lead to portable, battery-operated terahertz radiation sources.
In 2008, engineers at Harvard University achieved room temperature emission of several hundred nanowatts of coherent terahertz radiation using a semiconductor source.
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.
[27][28] In engineering, the terahertz gap is a frequency band in the THz region for which practical technologies for generating and detecting the radiation do not exist.
Most vacuum electronic devices that are used for microwave generation can be modified to operate at terahertz frequencies, including the magnetron,[36] gyrotron,[37] synchrotron,[38] and free-electron laser.
However, many of these devices are in prototype form, are not compact, or exist at university or government research labs, without the benefit of cost savings due to mass production.
Modulation of biological and also neurological function is therefore possible using radiation in the range hundreds of GHz up to a few THz at relatively low energies (without significant heating or ionisation) achieving either beneficial or harmful effects.
[citation needed] 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.
In 2002, the European Space Agency (ESA) Star Tiger team,[47] based at the Rutherford Appleton Laboratory (Oxfordshire, UK), produced the first passive terahertz image of a hand.
[50][51] In January 2013, the NYPD announced plans to experiment with the new technology to detect concealed weapons,[52] 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."
[54] In addition to its current use in submillimetre astronomy, terahertz radiation spectroscopy could provide new sources of information for chemistry and biochemistry.
[citation needed] Terahertz radiation could let art historians see murals hidden beneath coats of plaster or paint in centuries-old buildings, without harming the artwork.
An accelerating gradient larger than 1 GeV/m, can potentially be produced by the Cherenkov Smith-Purcell radiative mechanism[63][64] in a dielectric capillary with a variable inner radius.
[citation needed] A preliminary study with corrugated capillaries has shown some modification to the spectral content and amplitude of the generated wakefields,[65] but the possibility of using Smith-Purcell effect in DWA is still under consideration.
[citation needed] The high atmospheric absorption of terahertz waves limits the range of communication using existing transmitters and antennas to tens of meters.
There are tremendous difficulties to extending the range of THz communication through the atmosphere, but the world telecommunications industry is funding much research into overcoming those limitations.
So the consumption factor theory of communication links indicates that, contrary to conventional engineering wisdom, for a fixed aperture it is more efficient in bits per second per watt to use higher frequencies in the millimeter wave and terahertz range.
[68] The team's proof of concept device used a resonant tunneling diode (RTD) negative resistance oscillator to produce waves in the terahertz band.
[68][clarification needed] In 2011, Japanese electronic parts maker Rohm and a research team at Osaka University produced a chip capable of transmitting 1.5 Gbit/s using terahertz radiation.
These in general exploit the traits of plastics and cardboard being transparent to terahertz radiation, making it possible to inspect packaged goods.
However, drawbacks remain that include the substantial size of emitters, incompatible frequency ranges, and undesirable operating temperatures, as well as component, device, and detector requirements that are somewhere between solid state electronics and photonic technologies.
[citation needed] A theoretical study published in 2010 and conducted by Alexandrov et al at the Center for Nonlinear Studies at Los Alamos National Laboratory in New Mexico[87] 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".
Swanson's 2010 theoretical treatment of the Alexandrov study concludes that the DNA bubbles do not occur under reasonable physical assumptions or if the effects of temperature are taken into account.
[89] A bibliographical study published in 2003 reported that T-ray intensity drops to less than 1% in the first 500 μm of skin but stressed that "there is currently very little information about the optical properties of human tissue at terahertz frequencies".
