Molecular biology Terahertz radiation has comparable frequencies to the motion of biomolecular systems in the course of their function (a frequency 1THz is equivalent to a timescale of 1 picosecond, therefore in particular the range of hundreds of GHz up to low numbers of THz is comparable to biomolecular relaxation timescales of a few ps to a few ns). 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. In response to the demand for COVID-19 screening terahertz spectroscopy and imaging has been proposed as a rapid screening tool. 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. 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.
Security 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. In 2002, the
European Space Agency (ESA) Star Tiger team, based at the
Rutherford Appleton Laboratory (Oxfordshire, UK), produced the first passive terahertz image of a hand. By 2004, ThruVision Ltd, a spin-out from the
Council for the Central Laboratory of the Research Councils (CCLRC) Rutherford Appleton Laboratory, had demonstrated the world's first compact THz camera for security screening applications. The prototype system successfully imaged guns and explosives concealed under clothing. 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. In January 2013, 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." He sought a court order to prohibit using the technology without reasonable suspicion or probable cause. By early 2017, the department said it had no intention of ever using the sensors given to them by the federal government.
Scientific use and imaging 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. Submillimeter waves are used in physics to study materials in high magnetic fields, since at high fields (over about 11
tesla), the electron spin
Larmor frequencies are in the submillimeter band. Many high-magnetic field laboratories perform these high-frequency
EPR experiments, such as the
National High Magnetic Field Laboratory (NHMFL) in Florida. Terahertz radiation could let art historians see murals hidden beneath coats of plaster or paint in centuries-old buildings, without harming the artwork. In additional, THz imaging has been done with lens antennas to capture radio image of the object.
Particle accelerators New types of
particle accelerators that could achieve multi Giga-electron volts per metre (GeV/m) accelerating gradients are of utmost importance to reduce the size and cost of future generations of high energy colliders as well as provide a widespread availability of compact accelerator technology to smaller laboratories around the world. Gradients in the order of 100 MeV/m have been achieved by conventional techniques and are limited by RF-induced plasma breakdown. Beam driven dielectric wakefield accelerators (DWAs) typically operate in the Terahertz frequency range, which pushes the plasma breakdown threshold for surface electric fields into the multi-GV/m range. DWA technique allows to accommodate a significant amount of charge per bunch, and gives an access to conventional fabrication techniques for the accelerating structures. To date 0.3 GeV/m accelerating and 1.3 GeV/m decelerating gradients have been achieved using a dielectric lined waveguide with sub-millimetre transverse aperture. An accelerating gradient larger than 1 GeV/m, can potentially be produced by the Cherenkov Smith-Purcell radiative mechanism in a dielectric capillary with a variable inner radius. 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. 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.
Communication The high atmospheric absorption of terahertz waves limits the range of communication using existing transmitters and antennas to tens of meters. However, the huge unallocated
bandwidth available in the band (ten times the bandwidth of the
millimeter wave band, 100 times that of the
SHF microwave band) makes it very attractive for future data transmission and networking use. 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. One promising application area is the
6G cellphone and wireless standard, which will supersede the current
5G standard around 2030. For a given antenna aperture, the
gain of
directive antennas scales with the square of frequency, while for low power transmitters the power efficiency is independent of bandwidth. 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. 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. The study suggested that Wi-Fi using the system would be limited to approximately , but could allow data transmission at up to 100 Gbit/s. 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. In 2017, researchers at Brown University were able to transfer two videos at a speed of 50 Gbit/s using a terahertz multiplexer, considerably faster than the transfer speed of contemporary cellular data networks. Potential uses exist in high-altitude telecommunications, above altitudes where water vapor causes signal absorption: aircraft to
satellite, or satellite to satellite.
Amateur radio A number of administrations permit
amateur radio experimentation within the 275–3,000 GHz range or at even higher frequencies on a national basis, under license conditions that are usually based on RR5.565 of the
ITU Radio Regulations. Amateur radio operators utilizing submillimeter frequencies often attempt to set two-way communication distance records. In the
United States, WA1ZMS and W4WWQ set a record of on 403 GHz using CW (Morse code) on 21 December 2004. In
Australia, at 30 THz a distance of was achieved by stations VK3CV and VK3LN on 8 November 2020.
Manufacturing 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 first imaging system based on optoelectronic terahertz time-domain spectroscopy were developed in 1995 by researchers from AT&T Bell Laboratories and was used for producing a transmission image of a packaged electronic chip. This system used pulsed laser beams with duration in range of picoseconds. Since then commonly used commercial/ research terahertz imaging systems have used pulsed lasers to generate terahertz images. 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 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. 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. This implies that terahertz imaging systems have higher resolution than
scanning acoustic microscope (SAM) but lower resolution than
X-ray imaging systems. 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. Obviously the resolution of X-ray is higher than terahertz image, but
X-ray is ionizing and can be impose harmful effects on certain objects such as semiconductors and live tissues. To overcome low resolution of the terahertz systems near-field terahertz imaging systems are under development. In nearfield imaging the detector needs to be located very close to the surface of the plane and thus imaging of the thick packaged objects may not be feasible. 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. In this approach, since the absorption increases exponentially with the frequency, again inspection of the thick packaged semiconductors may not be doable. Consequently, a tradeoff between the achievable resolution and the thickness of the penetration of the beam in the packaging material should be considered.
THz gap research Ongoing investigation has resulted in
improved emitters (sources) and
detectors, and research in this area has intensified. 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.
Free-electron lasers can generate a wide range of
stimulated emission of electromagnetic radiation from microwaves, through terahertz radiation to
X-ray. However, they are bulky, expensive and not suitable for applications that require critical timing (such as
wireless communications). Other
sources of terahertz radiation which are actively being researched include solid state oscillators (through
frequency multiplication),
backward wave oscillators (BWOs),
quantum cascade lasers, and
gyrotrons. ==Safety==