Frequency range | 0.1THz to 30THz |
|---|---|
Wavelength range | 3mm to 30μm |
| Radio bands | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ITU | ||||||||||||
| ||||||||||||
| EU / NATO / US ECM | ||||||||||||
| IEEE | ||||||||||||
| Other TV and radio | ||||||||||||
Terahertz radiation – also known assubmillimeter radiation,terahertz waves,tremendously high frequency[1] (THF),T-rays,T-waves,T-light,T-lux orTHz – consists ofelectromagnetic waves within theInternational Telecommunication Union-designated band offrequencies from 0.1 to 10 terahertz (THz),[2] (from 0.3 to 3 terahertz (THz) in older texts,[3] which is now called "decimillimetric waves"[4]), although the upper boundary is somewhat arbitrary and has been considered by some sources to be 30 THz.[5]
One terahertz is 1012 Hz or 1,000 GHz. Wavelengths of radiation in the decimillimeter band correspondingly range 1 mm to 0.1 mm = 100 μm and those in the terahertz band 3 mm = 3000 μm to 30 μm. Because terahertz radiation begins at a wavelength of around 1 millimeter and proceeds into shorter wavelengths, it is sometimes known as thesubmillimeter band, and its radiation assubmillimeter waves, especially inastronomy. This band of electromagnetic radiation lies within the transition region betweenmicrowave andfar infrared, and can be regarded as either.
Compared to lower radio frequencies, terahertz radiation is stronglyabsorbed by thegases of theatmosphere, and in air most of the energy isattenuated within a few meters,[6][7][8] so it is not practical for long distance terrestrialradio communication. It can penetrate thin layers of materials but is blocked by thicker objects. THz beams transmitted through materials can be used formaterial characterization, layer inspection, relief measurement,[9] and as a lower-energy alternative toX-rays for producing high resolution images of the interior of solid objects.[10]
Terahertz radiation occupies a middle ground where the ranges ofmicrowaves andinfrared light waves overlap, known as the "terahertz gap"; it is called a "gap" because the technology for its generation and manipulation is still in its infancy. The generation andmodulation 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 betweeninfrared radiation andmicrowave radiation in theelectromagnetic spectrum, and it shares some properties with each of these. Terahertz radiation travels in aline of sight and isnon-ionizing. Like microwaves, terahertz radiation can penetrate a wide variety ofnon-conducting materials; clothing, paper,cardboard, wood,masonry, plastic andceramics. The penetration depth is typically less than that of microwave radiation. Like infrared, terahertz radiation has limited penetration throughfog andclouds and cannot penetrate liquid water or metal.[12] Terahertz radiation can penetrate some distance through body tissue like x-rays, but unlike them isnon-ionizing, so it is of interest as a replacement for medical X-rays. 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).[11]
Theearth'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. However, at distances of ~10 meters the band may still allow many useful applications in imaging and construction of high bandwidthwireless networking systems, especially indoor systems. In addition, producing and detectingcoherent 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), includinggyrotrons,backward wave oscillators, andresonant-tunneling diodes.[citation needed] Due to the small energy of THz photons, current THz devices require low temperature during operation to suppress environmental noise. Tremendous efforts thus have been put into THz research to improve the operation temperature, using different strategies such as optomechanical meta-devices.[13][14]
Terahertz radiation is emitted as part of theblack-body radiation from anything with a temperature greater than about 2 kelvin. While this thermal emission is very weak,observations at these frequencies are important for characterizing cold 10–20 Kcosmic dust ininterstellar clouds in the Milky Way galaxy, and in distantstarburst galaxies.[citation needed]
Telescopes operating in this band include theJames Clerk Maxwell Telescope, theCaltech Submillimeter Observatory and theSubmillimeter Array at theMauna Kea Observatory in Hawaii, theBLAST balloon borne telescope, theHerschel Space Observatory, theHeinrich Hertz Submillimeter Telescope at theMount Graham International Observatory in Arizona, and at theAtacama Large Millimeter Array. Due to Earth's atmospheric absorption spectrum, the opacity of the atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, or to space.[15][16]
_THz_Source.jpg)
As of 2012[update], viable sources of terahertz radiation are thegyrotron, thebackward wave oscillator ("BWO"), the molecule gasfar-infrared laser,Schottky-diode multipliers,[17] varactor (varicap) multipliers,quantum-cascade laser,[18][19][20][21] thefree-electron laser,synchrotron light sources,photomixing sources, single-cycle or pulsed sources used interahertz time-domain spectroscopy such as photoconductive, surface field,photo-Dember andoptical rectification emitters,[22] and electronic oscillators based onresonant tunneling diodes have been shown to operate up to 1.98 THz.[23] To the right, image of Dendrimer Dipole Excitation (DDE) Mechanism for broadband 30THz emitter used for sub-nanometer 3D Imaging and Spectroscopy.[24]
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 1,000 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'sArgonne 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.[25] The device uses high-temperature superconducting crystals, grown at theUniversity of Tsukuba in Japan. These crystals comprise stacks ofJosephson junctions, which exhibit a property known as theJosephson effect: when external voltage is applied, alternating current flows across the junctions at a frequency proportional to the voltage. This alternating currentinduces anelectromagnetic field. A small voltage (around two millivolts per junction) can induce frequencies in the terahertz range.
In 2008, engineers at Harvard University achieved room temperature emission of several hundred nanowatts of coherent terahertz radiation using a semiconductor source. THz radiation was generated bynonlinear mixing of two modes in a mid-infraredquantum cascade laser. Previous sources had required cryogenic cooling, which greatly limited their use in everyday applications.[26]
In 2009, it was discovered that the act of unpeeling adhesive tape generates non-polarized terahertz radiation, with a narrow peak at 2 THz and a broader peak at 18 THz. The mechanism of its creation istribocharging of the adhesive tape and subsequent discharge; this was hypothesized to involvebremsstrahlung with absorption orenergy density focusing duringdielectric breakdown of a gas.[27]
In 2013, researchers atGeorgia Institute of Technology's Broadband Wireless Networking Laboratory and thePolytechnic University of Catalonia developed a method to create agraphene antenna: an antenna that would be shaped into graphene strips from 10 to 100 nanometers wide and one micrometer long. Such an antenna could be used to emit radio waves in the terahertz frequency range.[28][29]
Until the 2008 manufacture of an EO (electro-optic) Dipole Dendrimer Excitation (DDE[30]) emitter, no practical technologies existed for generating and detecting radiation in afrequency band in the THz region, known as the "terahertz gap". This gap has previously been defined as 0.1 to 10 THz (wavelengths of 3 mm to 30 μm) although the upper boundary is considered by some sources as 30 THz (awavelength of 10 μm).[31] Until the 2008 DDE[30] implementation by Applied Research & Photonics (ARP) Inc., frequencies within the range from 0.1 to 30THz, useful power generation and receiver technologies were inefficient and unfeasible. Since 2008, ARP has commercially manufactured sub-nanometer resolution 3D Imaging & Spectroscopy tools, known as TeraSpectra.
Mass production of devices in this range and operation atroom temperature (at which energykT is equal to theenergy of a photon with a frequency of 6.2 THz) are mostly impractical. This leaves a gap between maturemicrowave technologies in the highest frequencies of theradio spectrum and the well-developedoptical engineering ofinfrared detectors in their lowest frequencies. This radiation is mostly used in small-scale, specialized applications such assubmillimetre astronomy.Research that attempts to resolve this issue has been conducted since the late 20th century.[32][33][34][35][36]
In 2024, an experiment was published by German researchers[37] where a TDLAS experiment at 4.75 THz was performed in "infrared quality" with an uncooled pyroelectric receiver. The THz source was a cw DFB-QC-Laser operating at 43.3 K, with laser currents between 480 mA and 600 mA.
See DDE[30] as exception to, "Most vacuum electronic devices that are used for microwave generation can be modified to operate at terahertz frequencies, including the magnetron,[38] gyrotron,[39] synchrotron,[40] and free-electron laser.[41] " Similarly, microwave detectors such as thetunnel diode have been re-engineered to detect at terahertz[42] and infrared[43] frequencies as well. 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.
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.[44][45]
UnlikeX-rays, terahertz radiation is notionizing radiation and its lowphoton energies in general do not damage livingtissues andDNA. 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 anddensity of a tissue. Such methods could allow effective detection ofepithelial cancer with an imaging system that is safe, non-invasive, and painless.[46] In response to the demand for COVID-19 screening terahertz spectroscopy and imaging has been proposed as a rapid screening tool.[47][48]
The first images generated using terahertz radiation date from the 1960s; however, in 1995 images generated usingterahertz time-domain spectroscopy generated a great deal of interest.[citation needed]
Some frequencies of terahertz radiation can be used for3D imaging ofteeth and may be more accurate than conventional X-ray imaging indentistry.[citation needed]
Terahertz radiation can penetrate fabrics and plastics, so it can be used insurveillance, such assecurity screening, to uncoverconcealedweapons 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, theEuropean Space Agency (ESA) Star Tiger team,[49] based at theRutherford Appleton Laboratory (Oxfordshire, UK), produced the first passive terahertz image of a hand.[50] By 2004, ThruVision Ltd, a spin-out from theCouncil 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.[51] 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.[52][53]
In January 2013, theNYPD announced plans to experiment with the new technology to detectconcealed weapons,[54] 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.[55] By early 2017, the department said it had no intention of ever using the sensors given to them by the federal government.[56]
In addition to its current use insubmillimetre astronomy, terahertz radiationspectroscopy could provide new sources of information forchemistry andbiochemistry.[57]
Recently developed methods ofTHz time-domain spectroscopy (THz TDS) andTHz tomography have been shown to be able to image samples that are opaque in the visible andnear-infrared regions of the spectrum. The utility of THz-TDS is limited when the sample is very thin, or has a lowabsorbance, 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 drivinglaser 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.[citation needed]
Submillimeter waves are used in physics to study materials in high magnetic fields, since at high fields (over about 11 tesla), the electron spinLarmor frequencies are in the submillimeter band. Many high-magnetic field laboratories perform these high-frequencyEPR experiments, such as theNational High Magnetic Field Laboratory (NHMFL) in Florida.[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.[58]
In additional, THz imaging has been done with lens antennas to capture radio image of the object.[59][60]
New types ofparticle 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.[61] Beam driven dielectric wakefield accelerators (DWAs)[62][63] typically operate in the Terahertz frequency range, which pushes the plasma breakdown threshold for surface electric fields into the multi-GV/m range.[64] 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[65] 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[66][67] 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.[citation needed]
A preliminary study with corrugated capillaries has shown some modification to the spectral content and amplitude of the generated wakefields,[68] 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. However, the huge unallocatedbandwidth available in the band (ten times the bandwidth of themillimeter wave band, 100 times that of theSHF 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.[69] One promising application area is the6G cellphone and wireless standard, which will supersede the current5G standard around 2030.[69] In particular,6G is expected to leverage advanced technologies such as terahertz andfull duplex (FD) communications, combined with dynamic spectrum sharing to meet the growing demand for higher data rates and more efficient spectrum efficiency.[70]
For a given antenna aperture, thegain ofdirective antennas scales with the square of frequency, while for low power transmitters the power efficiency is independent of bandwidth. So theconsumption 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.[69] Small directive antennas a few centimeters in diameter can produce very narrow 'pencil' beams of THz radiation, andphased arrays of multiple antennas could concentrate virtually all the power output on the receiving antenna, allowing communication at longer distances.
In May 2012, a team of researchers from theTokyo Institute of Technology[71] published inElectronics Letters that it had set a new record forwireless data transmission by using T-rays and proposed they be used as bandwidth for data transmission in the future.[72] The team'sproof of concept device used aresonant tunneling diode (RTD)negative resistance oscillator to produce waves in the terahertz band. With this RTD, the researchers sent a signal at 542 GHz, resulting in a data transfer rate of 3 Gigabits per second.[72] It doubled the record for data transmission rate set in November 2011.[73] The study suggested that Wi-Fi using the system would be limited to approximately 10 metres (33 ft), but could allow data transmission at up to 100 Gbit/s.[72][clarification needed] In 2011, Japanese electronic parts maker Rohm and a research team at Osaka University produced a chip capable of transmitting 1.5Gbit/s using terahertz radiation.[74] According to nature journal, researchers reported to transfer two videos error free at the speed of 50 Gbps.[75] Which was way more than the previous record.
Potential uses exist in high-altitude telecommunications, above altitudes where water vapor causes signal absorption: aircraft tosatellite, or satellite to satellite.[citation needed]
A number of administrations permitamateur 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 theITU Radio Regulations. Amateur radio operators utilizing submillimeter frequencies often attempt to set two-way communication distance records. In theUnited States, WA1ZMS and W4WWQ set a record of 1.42 kilometres (0.88 mi) on 403 GHz using CW (Morse code) on 21 December 2004. InAustralia, at 30 THz a distance of 60 metres (200 ft) was achieved by stations VK3CV and VK3LN on 8 November 2020.[76][77][78]
Many possible uses of terahertz sensing and imaging are proposed inmanufacturing,quality control, andprocess monitoring. These in general exploit the traits of plastics andcardboard being transparent to terahertz radiation, making it possible to inspectpackaged 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.[79] 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.[80]
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 toX-ray transmission imaging, where images are developed based on attenuation of the transmitted beam.[81]
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 aGaussian function. The geometry and behavior ofGaussian beam in theFraunhofer region imply that the electromagnetic beams diverge more as the frequencies of the beams decrease and thus the resolution decreases.[82] This implies that terahertz imaging systems have higher resolution thanscanning acoustic microscope (SAM) but lower resolution thanX-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.[83] Obviously the resolution of X-ray is higher than terahertz image, butX-ray is ionizing and can be impose harmful effects on certain objects such as semiconductors and live tissues.[citation needed]
To overcome low resolution of the terahertz systems near-field terahertz imaging systems are under development.[84][85] 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.[86] 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.[citation needed]
Ongoing investigation has resulted inimproved emitters (sources) anddetectors, 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 betweensolid state electronics andphotonic technologies.[87][88][89]
Free-electron lasers can generate a wide range ofstimulated emission of electromagnetic radiation from microwaves, through terahertz radiation toX-ray. However, they are bulky, expensive and not suitable for applications that require critical timing (such aswireless communications). Othersources of terahertz radiation which are actively being researched include solid state oscillators (throughfrequency multiplication),backward wave oscillators (BWOs),quantum cascade lasers, andgyrotrons.
The terahertz region is between the radio frequency region and the laser optical region. Both the IEEE C95.1–2005 RF safety standard[90] and the ANSI Z136.1–2007 Laser safety standard[91] have limits into the terahertz region, but both safety limits are based on extrapolation. It is expected that effects on biological 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.[citation needed]
A theoretical study published in 2010 and conducted by Alexandrov et al at the Center for Nonlinear Studies atLos Alamos National Laboratory in New Mexico[92] created mathematical models predicting how terahertz radiation would interact with double-strandedDNA, 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 asgene expression andDNA replication".[93] Experimental verification of this simulation was not done. 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.[94] A bibliographical study published in 2003 reported that T-ray intensity drops to less than 1% in the first 500 μm ofskin but stressed that "there is currently very little information about the optical properties of human tissue at terahertz frequencies".[95]
:1 was invoked but never defined (see thehelp page).
... researchers have successfully generated intense pulses of light in a largely untapped part of the electromagnetic spectrum – the so-calledterahertz gap.
Radio spectrum (ITU) | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| |||||||||||||
| |
| Gamma rays | |
| X-rays | |
| Ultraviolet | |
| Visible (optical) | |
| Infrared | |
| Microwaves | |
| Radio | |
| Wavelength types | |
| International | |
|---|---|
| National | |
| Other | |
