View source for Terahertz radiation

{{short description|Range 300-3000 GHz of the electromagnetic spectrum}}
{{redirect|T-ray}}
{{redirect|T-light|the candle|tealight}}
{{Use dmy dates|date=March 2019}}
{{MWband
| name = Terahertz band
| freq = 0.1 [[Hertz|THz]] to 30 [[Hertz|THz]]
| wave = 3 [[Meter|mm]] to 30 [[Meter|μm]]
}}
[[File:Spectre Terahertz.svg|thumb|Terahertz waves lie mostly at the far end of the infrared band, the longest ones in the microwave band.]]

'''Terahertz radiation''' &ndash; also known as '''submillimeter radiation''', '''terahertz waves''', '''tremendously high frequency'''<ref name="Jones">
{{cite book
 |author1=Jones, Graham A.
 |author2=Layer, David H.
 |author3=Osenkowsky, Thomas G.
 |year=2007
 |title=National Association of Broadcasters Engineering Handbook
 |page=7
 |publisher=Taylor and Francis
 |isbn=978-1-136-03410-7
 |url=https://books.google.com/books?id=K9N1TVhf82YC&pg=PA7
}}
</ref> ('''THF'''), '''T-rays''', '''T-waves''', '''T-light''', '''T-lux''' or '''THz'''&nbsp;– consists of [[electromagnetic wave]]s within the [[International Telecommunication Union]]-designated band of [[Frequency|frequencies]] from 0.1 to 10&nbsp;[[Hertz#SI prefixed forms of hertz|terahertz]] (THz),<ref>{{cite web |url=https://www.etsi.org/committee/thz |title=Industry Specification Group (ISG) TeraHertz (THz) |date=2025 |publisher=ETSI}}</ref> (from 0.3 to 3&nbsp;[[Hertz#SI prefixed forms of hertz|terahertz]] (THz) in older texts,<ref name="ITU2016">
{{cite book
  | chapter = Article 2.1: Frequency and wavelength bands
  | year = 2017
  | title = Radio Regulations
  | edition = 2016
  | publisher = [[International Telecommunication Union]]
  | type = zipped PDF
  | chapter-url = https://www.itu.int/dms_pub/itu-r/opb/reg/R-REG-RR-2016-ZPF-E.zip
  | access-date = 9 November 2019
}}
</ref> which is now called "[[decimillimetric waves]]"<ref name="ITU2024">
{{cite book
  | chapter = Article 2.1: Frequency and wavelength bands
  | year = 2024
  | title = Radio Regulations
  | edition = 2024
  | publisher = [[International Telecommunication Union]]
  | type = zipped PDF
  | chapter-url = https://www.itu.int/hub/publication/r-reg-rr-2024/
  | access-date = 2 February 2025
}}
</ref>), although the upper boundary is somewhat arbitrary and has been considered by some sources to be 30&nbsp;THz.<ref>
{{cite journal
  |author1 =Dhillon, S.S.         |author2 =Vitiello, M.S. 
  |author3 =Linfield, E.H.        |author4 =Davies, A.G. 
  |author5 =Hoffmann, Matthias C. |author6 =Booske, John 
  |author7 =Paoloni, Claudio         |author8 =Gensch, M. 
  |author9 =Weightman, P.         |author10=Williams, G.P. 
  |author11=Castro-Camus, E.      |author12=Cumming, D.R.S. 
  |author13=Simoens, F.           |author14=Escorcia-Carranza, I. 
  |author15=Grant, J.             |author16=Lucyszyn, Stepan 
  |author17=Kuwata-Gonokami, Makoto  |author18=Konishi, Kuniaki 
  |author19=Koch, Martin             |author20=Schmuttenmaer, Charles A. 
  |author21=Cocker, Tyler L.      |author22=Huber, Rupert 
  |author23=Markelz, A.G.         |author24=Taylor, Z.D. 
  |author25=Wallace, Vincent P.   |author26=Zeitler, J. Axel 
  |author27=Sibik, Juraj             |author28=Korter, Timothy M. 
  |author29=Ellison, B.           |author30=Rea, S. 
  |author31=Goldsmith, P.         |author32=Cooper, Ken B. 
  |author33=Appleby, Roger           |author34=Pardo, D. 
  |author35=Huggard, P.G.         |author36=Krozer, V. 
  |author37=Shams, Haymen            |author38=Fice, Martyn 
  |author39=Renaud, Cyril            |author40=Seeds, Alwyn 
  |author41=Stöhr, Andreas           |author42=Naftaly, Mira 
  |author43=Ridler, Nick             |author44=Clarke, Roland 
  |author45=Cunningham, John E.   |author46=Johnston, Michael B. 
  |display-authors=6
  |year=2017
  |title=The 2017 terahertz science and technology roadmap
  |journal=Journal of Physics D: Applied Physics
  |volume=50  |issue=4   |page=2
  |doi=10.1088/1361-6463/50/4/043001  |bibcode =2017JPhD...50d3001D 
 |doi-access=free
|hdl=10044/1/43481|hdl-access=free}}
</ref>

One terahertz is 10<sup>12</sup>&nbsp;[[Hertz|Hz]] or 1,000&nbsp;GHz. Wavelengths of radiation in the decimillimeter band correspondingly range 1&nbsp;mm to 0.1&nbsp;mm = 100&nbsp;μm and those in the terahertz band 3&nbsp;mm = 3000&nbsp;μm to 30&nbsp;μm. Because terahertz radiation begins at a wavelength of around 1&nbsp;millimeter and proceeds into shorter wavelengths, it is sometimes known as the ''submillimeter band'', and its radiation as ''submillimeter waves'', especially in [[submillimeter astronomy|astronomy]]. This band of electromagnetic radiation lies within the transition region between [[microwave]] and [[far infrared]], and can be regarded as either.

Compared to lower radio frequencies, terahertz radiation is strongly [[Absorption (electromagnetic radiation)|absorbed]] by the [[Gas|gases]] of the [[Atmosphere of Earth|atmosphere]], and in air most of the energy is [[Attenuation|attenuated]] within a few meters,<ref name="Coutaz">
{{cite book
 | last1 = Coutaz  | first1 = Jean-Louis
 | last2 = Garet   | first2 = Frederic
 | last3 = Wallace | first3 = Vincent P.
 | year  = 2018
 | title = Principles of Terahertz Time-Domain Spectroscopy: An introductory textbook
 | page  = 18
 | publisher = CRC Press 
 | isbn  = 978-1-351-35636-7
 | url   = https://books.google.com/books?id=zah8DwAAQBAJ&pg=PA18
 | via   = Google Books
}}
</ref><ref name="Siegel">{{cite web
  | last = Siegel
  | first = Peter
  | year = 2002
  | title = Studying the Energy of the Universe
  | series = Education materials
  | website = NASA
  | publisher = U.S. [[National Aeronautics and Space Administration]]
  | url = https://www.nasa.gov/audience/foreducators/k-4/features/Peter_Siegel.html
  | access-date = 19 May 2021
  | archive-date = 20 June 2021
  | archive-url = https://web.archive.org/web/20210620092047/https://www.nasa.gov/audience/foreducators/k-4/features/Peter_Siegel.html
  }}</ref><ref name="Gosling">{{cite book
 |last1        = Gosling
 |first1       = William
 |title        = Radio Spectrum Conservation: Radio Engineering Fundamentals
 |publisher    = Newnes
 |date         = 2000
 |pages        = 11–14
 |url          = https://books.google.com/books?id=MvbZ2eK7luMC&pg=PA11
 |isbn         = 978-0-7506-3740-4
 |access-date  = 2019-11-25
 |archive-date = 2022-04-07
 |archive-url  = https://web.archive.org/web/20220407192824/https://books.google.com/books?id=MvbZ2eK7luMC&pg=PA11
 |url-status   = live
}}</ref> so it is not practical for long distance terrestrial [[Radio|radio communication]]. It can penetrate thin layers of materials but is blocked by thicker objects. THz beams transmitted through materials can be used for [[characterization (materials science)|material characterization]], layer inspection, relief measurement,<ref>
{{cite journal
 |last = Petrov 
 |first = Nikolay V.
 |author2 = Maxim S. Kulya 
 |author3 = Anton N. Tsypkin           
 |author4 = Victor G. Bespalov
 |author5 = Andrei Gorodetsky
 |date=5 April 2016 
 |title=Application of Terahertz Pulse Time-Domain Holography for Phase Imaging
 |journal=IEEE Transactions on Terahertz Science and Technology
 |volume=6  
 |issue=3
 |pages=464–472
 |doi=10.1109/TTHZ.2016.2530938
 |url= https://www.researchgate.net/publication/301240194
 |bibcode = 2016ITTST...6..464P
 |s2cid = 20563289
}}
</ref> and as a lower-energy alternative to [[X-ray]]s for producing high resolution images of the interior of solid objects.<ref>
{{cite conference
 |last1=Ahi   |first1=Kiarash
 |last2=Anwar |first2=Mehdi F.
 |date=26 May 2016
 |title=Advanced terahertz techniques for quality control and counterfeit detection
 |conference=SPIE Commercial + Scientific Sensing and Imaging
 |place=Baltimore, MD
 |editor1-last=Anwar  |editor1-first=Mehdi F. 
 |editor2-last=Crowe  |editor2-first=Thomas W.
 |editor3-last=Manzur |editor3-first=Tariq    
 |book-title=Proceedings SPIE Volume 9856, Terahertz Physics, Devices, and Systems X: Advanced Applications in Industry and Defense
 |id=98560G
 |publisher=SPIE: The International Society for Optics and Photonics
 |doi=10.1117/12.2228684
 |bibcode=2016SPIE.9856E..0GA  |s2cid=138587594
 |url=https://www.researchgate.net/publication/303563438
 |access-date=26 May 2016  |via=researchgate.net
}}
</ref>

Terahertz radiation occupies a middle ground where the ranges of [[microwave]]s and [[infrared light]] waves overlap, known as the "[[#Terahertz gap|terahertz gap]]"; it is called a "gap" because the technology for its generation and manipulation is still in its infancy. 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.

==Description==
[[File:Resolution Enhancement.gif|alt=|thumb|In THz-TDS systems, since the time-domain version of the THz signal is available, the distortion effects of the diffraction can be suppressed.<ref name=":0">
{{cite journal
 |last=Ahi  |first=Kiarash
 |date=2018
 |title=A method and system for enhancing the resolution of terahertz imaging
 |journal= Measurement
 |volume=138  |pages=614–619
 |doi=10.1016/j.measurement.2018.06.044  |s2cid=116418505
}}
</ref>]]

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 (telecommunications)|line of sight]] and is [[non-ionizing radiation|non-ionizing]]. Like microwaves, terahertz radiation can penetrate a wide variety of [[insulator (electricity)|non-conducting material]]s; clothing, paper, [[paperboard|cardboard]], wood, [[masonry]], plastic and [[ceramic]]s. The penetration depth is typically less than that of microwave radiation. Like infrared, terahertz radiation has limited penetration through [[fog]] and [[cloud]]s and cannot penetrate liquid water or metal.<ref>{{cite news
 |url=http://cerncourier.com/cws/article/cern/28777
 |title=JLab generates high-power terahertz light
 |periodical=CERN Courier
 |date=1 January 2003
 |archive-date=17 November 2010
 |access-date=12 May 2010
 |archive-url=https://web.archive.org/web/20101117174453/http://cerncourier.com/cws/article/cern/28777
 }}</ref> Terahertz radiation can penetrate some distance through body tissue like x-rays, but unlike them is [[non-ionizing radiation|non-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).<ref name=":0"/>

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. However, at distances of ~10&nbsp;meters the band may still allow many useful applications in imaging and construction of high bandwidth [[wireless network]]ing systems, especially indoor systems. In addition, producing and detecting [[coherence (physics)|coherent]] terahertz radiation remains technically challenging, though inexpensive commercial sources now exist in the 0.3&ndash;1.0&nbsp;THz range (the lower part of the spectrum), including [[gyrotron]]s, [[backward wave oscillator]]s, and [[resonant-tunneling diode]]s.{{citation needed|date=February 2021}} 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.<ref>{{cite journal |last1=Liu |first1=Jiawen |last2=Chomet |first2=Baptiste |last3=Beoletto |first3=Paolo |last4=Gacemi |first4=Djamal |last5=Pantzas |first5=Konstantinos |last6=Beaudoin |first6=Grégoire |last7=Sagnes |first7=Isabelle |last8=Vasanelli |first8=Angela |last9=Sirtori |first9=Carlo |last10=Todorov |first10=Yanko |date=18 May 2022 |title=Ultrafast Detection of TeraHertz Radiation with Miniaturized Optomechanical Resonator Driven by Dielectric Driving Force |journal=ACS Photonics |volume=9 |issue=5 |pages=1541–1546 |doi=10.1021/acsphotonics.2c00227|bibcode=2022ACSP....9.1541L |s2cid=247959476 |url=https://hal.science/hal-03829642 }}</ref><ref>{{cite journal |last1=Liu |first1=Jiawen |last2=Gacemi |first2=Djamal |last3=Pantzas |first3=Konstantinos |last4=Beaudoin |first4=Grégoire |last5=Sagnes |first5=Isabelle |last6=Vasanelli |first6=Angela |last7=Sirtori |first7=Carlo |last8=Todorov |first8=Yanko |date=February 2023 |title=Nonlinear Oscillation States of Optomechanical Resonator for Reconfigurable Light-Compatible Logic Functions |journal=Advanced Optical Materials |volume=11 |issue=4 |article-number=2202133 |doi=10.1002/adom.202202133|s2cid=254776067 }}</ref>

==Sources==

===Natural===
Terahertz radiation is emitted as part of the [[black-body radiation]] from anything with a temperature greater than about 2&nbsp;[[kelvin]]. While this thermal emission is very weak, [[submillimetre astronomy|observations at these frequencies]] are important for characterizing cold 10&ndash;20&nbsp;[[kelvin|K]] [[cosmic dust]] in [[interstellar cloud]]s in the Milky Way galaxy, and in distant [[starburst galaxy|starburst galaxies]].{{citation needed|date=February 2021}}

Telescopes operating in this band include the [[James Clerk Maxwell Telescope]], the [[Caltech Submillimeter Observatory]] and the [[Submillimeter Array]] at the [[Mauna Kea Observatory]] in Hawaii, the [[BLAST (telescope)|BLAST]] balloon borne telescope, the [[Herschel Space Observatory]], the [[Heinrich Hertz Submillimeter Telescope]] at the [[Mount Graham International Observatory]] in Arizona, and at the [[Atacama 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.<ref>
{{cite web
 |title=Atmospheric Absorption & Transmission
 |website=Humboldt State Geospatial Online Learning Modules
 |publisher=[[Humboldt State University]]
 |url=http://gsp.humboldt.edu/OLM/Courses/GSP_216_Online/lesson2-1/atmosphere.html
 |access-date=19 May 2021 |url-status=live
 |archive-url=https://web.archive.org/web/20201107032343/http://gsp.humboldt.edu/OLM/Courses/GSP_216_Online/lesson2-1/atmosphere.html
 |archive-date=2020-11-07
}}
</ref><ref>
{{cite web
 |title=Absorption Bands and Atmospheric Windows
 |website=The Earth Observatory
 |date=17 September 1999
 |publisher=[[NASA]]
 |url=https://earthobservatory.nasa.gov/features/RemoteSensing/remote_04.php
 |access-date=19 May 2021
}}
</ref>

===Artificial===
[[File:Dendrimer Dipole Excitation (DDE) THz Source.jpg|thumb|Dendrimer Dipole Excitation (DDE) Mechanism - The Rahman-Tomalia Effect]]
{{As of|2012}}, viable sources of terahertz radiation are the [[gyrotron]], the [[backward wave oscillator]] ("BWO"), the molecule gas [[far-infrared laser]], [[Schottky-diode]] multipliers,<ref>
{{cite web
 |title=Multipliers
 |series=Products
 |publisher=Virginia Diodes
 |url=http://vadiodes.com/index.php/en/products/multipliers
 |archive-url=https://web.archive.org/web/20140315214429/http://vadiodes.com/index.php/en/products/multipliers
 |archive-date=15 March 2014
}}
</ref> varactor ([[varicap]]) multipliers, [[quantum-cascade laser]],<ref name="nature_kohler_2002">
{{Cite journal
 |author1 = Köhler, Rüdeger
 |author2=Tredicucci, Alessandro 
 |author3=Beltram, Fabio 
 |author4=Beere, Harvey E. 
 |author5=Linfield, Edmund H. 
 |author6=Davies, A. Giles 
 |author7=Ritchie, David A. 
 |author8=Iotti, Rita C. 
 |author9=Rossi, Fausto 
 |year = 2002
 | title = Terahertz semiconductor-heterostructure laser
 | journal = Nature
 | volume = 417  | issue = 6885  | pages = 156–159
 | pmid=12000955  |bibcode = 2002Natur.417..156K
 | doi = 10.1038/417156a  | s2cid = 4422664
}}
</ref><ref name="lpr_scalari_2009">
{{cite journal
 |author1=Scalari, G.
 |author2=Walther, C. 
 |author3=Fischer, M. 
 |author4=Terazzi, R. 
 |author5=Beere, H. 
 |author6=Ritchie, D. 
 |author7=Faist, J. 
 |year = 2009
 |title = THz and sub-THz quantum-cascade lasers
 |journal = Laser & Photonics Reviews
 |volume = 3   | issue = 1–2   | pages = 45–66
 |doi = 10.1002/lpor.200810030
 |bibcode=2009LPRv....3...45S
|s2cid=121538269 
 }}
</ref><ref name="apl_lee_2006">
{{Cite journal
 |author1=Lee, Alan W.M.
 |author2=Qin, Qi 
 |author3=Kumar, Sushil 
 |author4=Williams, Benjamin S. 
 |author5=Hu, Qing 
 |author6=Reno, John L. 
 |date = 2006
 |title = Real-time terahertz imaging over a standoff distance (>25&nbsp;meters)
 |journal = Appl. Phys. Lett.
 |volume = 89  | issue = 14  | page = 141125
 |doi = 10.1063/1.2360210
 |bibcode = 2006ApPhL..89n1125L | s2cid = 122942520
 }}
</ref><ref name="oe_fathololoumi_2012">
{{Cite journal
 | first1 = S.  | last1 = Fathololoumi     | first2 = E. | last2 = Dupont
 | first3 = C.W.I. | last3 = Chan             | first4 = Z.R. | last4 = Wasilewski
 | first5 = S.R. | last5 = Laframboise      | first6 = D. | last6 = Ban
 | first7 = A.  | last7 = Matyas           | first8 = C. | last8 = Jirauschek
 | first9 = Q.  | last9 = Hu               |first10 = H.C. |last10 = Liu
 | display-authors=6
 | date = 13 February 2012
 | title = Terahertz quantum-cascade lasers operating up to ~200&nbsp;K with optimized oscillator strength and improved injection tunneling
 | journal = Optics Express
 | volume = 20 | issue = 4 | pages = 3866–3876
 | doi = 10.1364/OE.20.003866
 | bibcode = 2012OExpr..20.3866F | pmid = 22418143
 | hdl = 1721.1/86343 | s2cid = 9383885 | hdl-access = free
}}
</ref> the [[free-electron laser]], [[synchrotron light]] sources, [[photomixing]] sources, single-cycle or pulsed sources used in [[terahertz time-domain spectroscopy]] such as photoconductive, surface field, [[photo-Dember]] and [[optical rectification]] emitters,<ref name="rama">{{cite book|last=Ramakrishnan|first=Gopakumar|title=Enhanced terahertz emission from thin film semiconductor/metal interfaces|date=2012|isbn=978-94-6191-5641|publisher=Delft University of Technology, The Netherlands|url=http://repository.tudelft.nl/view/ir/uuid:d69d7778-c5fc-4d2c-9b17-f3aaf2ee5f82/|archive-date=4 March 2016|access-date=29 November 2013|archive-url=https://web.archive.org/web/20160304085002/http://repository.tudelft.nl/view/ir/uuid:d69d7778-c5fc-4d2c-9b17-f3aaf2ee5f82/}}</ref> and electronic oscillators based on [[resonant tunneling diode]]s have been shown to operate up to 1.98&nbsp;THz.<ref>{{Cite book|last1=Izumi|first1=R.|last2=Suzuki|first2=S.|last3=Asada|first3=M.|title=2017 42nd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THZ) |chapter=1.98 THZ resonant-tunneling-diode oscillator with reduced conduction loss by thick antenna electrode |year=2017 |pages=1–2 |doi=10.1109/IRMMW-THz.2017.8066877|isbn=978-1-5090-6050-4 }}</ref> To the right, image of Dendrimer Dipole Excitation (DDE) Mechanism for broadband 30THz emitter used for sub-nanometer 3D Imaging and Spectroscopy.<ref>{{Cite journal |last1=Rahman |first1=Anis |last2=Rahman |first2=A. K. |last3=Tomalia |first3=Donald A. |date=2017 |title=Engineering dendrimers to produce dendrimer dipole excitation based terahertz radiation sources suitable for spectrometry, molecular and biomedical imaging |url=https://xlink.rsc.org/?DOI=C7NH00010C |journal=Nanoscale Horizons |language=en |volume=2 |issue=3 |pages=127–134 |doi=10.1039/C7NH00010C |pmid=32260656 |bibcode=2017NanoH...2..127R |issn=2055-6756|url-access=subscription }}</ref>

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&nbsp;GHz to 1,000&nbsp;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'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.<ref>Science News: [https://www.sciencedaily.com/releases/2007/11/071126121732.htm New T-ray Source Could Improve Airport Security, Cancer Detection], ''ScienceDaily'' (27 November 2007).</ref> The device uses high-temperature superconducting crystals, grown at the [[University of Tsukuba]] in Japan. These crystals comprise stacks of [[Josephson junction]]s, 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 [[Electromagnetic induction|induces]] an [[electromagnetic 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 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.<ref>[http://www.physorg.com/news130385859.html Engineers demonstrate first room-temperature semiconductor source of coherent terahertz radiation] Physorg.com. 19 May 2008. Retrieved May 2008</ref>

In 2009, it was discovered that the act of unpeeling adhesive tape generates non-polarized terahertz radiation, with a narrow peak at 2&nbsp;THz and a broader peak at 18&nbsp;THz. The mechanism of its creation is [[triboelectric effect|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.<!-- that was basically the entire abstract --><ref>{{Cite journal | doi=10.1364/OL.34.002195| title=Peeling adhesive tape emits electromagnetic radiation at terahertz frequencies| year=2009| last1=Horvat| first1=J.| last2=Lewis| first2=R. A.| journal=Optics Letters| volume=34| issue=14| pages=2195–7| pmid=19823546| bibcode=2009OptL...34.2195H| url=https://ro.uow.edu.au/cgi/viewcontent.cgi?article=1507&context=engpapers}}</ref>

In 2013, researchers at [[Georgia Institute of Technology]]'s Broadband Wireless Networking Laboratory and the [[Polytechnic University of Catalonia]] developed a method to create a [[graphene antenna]]: an antenna that would be shaped into graphene strips from 10 to 100&nbsp;nanometers wide and one micrometer long. Such an antenna could be used to emit radio waves in the terahertz frequency range.<ref>{{cite news|url=http://www.extremetech.com/extreme/149172-samsung-funds-graphene-antenna-project-for-wireless-ultra-fast-intra-chip-links|title=Samsung funds graphene antenna project for wireless, ultra-fast intra-chip links|first=John|last=Hewitt|work=[[ExtremeTech]]|date=25 February 2013|access-date=8 March 2013}}</ref><ref>{{cite news|url=http://www.technologyreview.com/news/511726/graphene-antennas-would-enable-terabit-wireless-downloads/|title=Graphene Antennas Would Enable Terabit Wireless Downloads|work=[[MIT Technology Review]]|first=David|last=Talbot|author-link=David Talbot|date=5 March 2013|access-date=8 March 2013}}</ref>

==Terahertz gap==
Until the 2008 manufacture of an EO (electro-optic) Dipole Dendrimer Excitation (DDE<ref name=":1" />) emitter, no practical technologies existed for generating and detecting radiation in a [[frequency band]] in the THz region, known as the "'''terahertz gap"'''. This gap has previously been defined as 0.1 to 10&nbsp;THz ([[wavelength]]s of 3&nbsp;mm to 30&nbsp;μm) although the upper boundary is considered by some sources as 30&nbsp;THz (a [[wavelength]] of 10&nbsp;μm).<ref>{{cite journal |last1=Dhillon |first1=S S |display-authors=etal |title=The 2017 terahertz science and technology roadmap |journal=Journal of Physics D: Applied Physics |date=2017 |volume=50 |issue=4 |page=2 |doi=10.1088/1361-6463/50/4/043001 |bibcode=2017JPhD...50d3001D |doi-access=free |hdl=10044/1/43481 |hdl-access=free }}</ref> Until the 2008 DDE<ref name=":1" /> 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 at [[room temperature]] (at which energy [[kT (energy)|''kT'']] is equal to the [[photon energy|energy of a photon]] with a frequency of 6.2&nbsp;THz) are mostly impractical. This leaves a gap between mature [[microwave]] technologies in the highest frequencies of the [[radio spectrum]] and the well-developed [[optical engineering]] of [[infrared detector]]s in their lowest frequencies. This radiation is mostly used in small-scale, specialized applications such as [[submillimetre astronomy]]. [[Research]] that attempts to resolve this issue has been conducted since the late 20th&nbsp;century.<ref name="springer">{{cite book |last1=Gharavi |first1=Sam |last2=Heydari |first2=Babak |date=2011-09-25 |title=Ultra High-Speed CMOS Circuits: Beyond 100&nbsp;GHz |publisher=Springer Science+Business Media |edition=1st |location=New York |pages=1–5 (Introduction) and 100 |url=https://books.google.com/books?id=iJZIcUwmyfYC&pg=PA1 |doi=10.1007/978-1-4614-0305-0 |isbn=978-1-4614-0305-0}}</ref><ref name="sirt">{{cite journal |last1=Sirtori |first1=Carlo |year=2002 |title=Bridge for the terahertz gap |series=Applied physics |journal=Nature |volume=417 |issue=6885 |pages=132–133 |doi=10.1038/417132b |url=http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/Nature/nature417-132.pdf |format=Free PDF download |pmid=12000945 |bibcode=2002Natur.417..132S |s2cid=4429711 |doi-access=free }}{{Dead link|date=October 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref name="borak">{{cite journal |last1=Borak |first1=A. |year=2005 |title=Toward bridging the terahertz gap with silicon-based lasers |series=Applied physics |journal=Science |volume=308 |issue=5722 |pages=638–639 |pmid=15860612 |doi=10.1126/science.1109831 |s2cid=38628024 |url=http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/science/vol308/308-638.pdf |format=Free PDF download }}{{Dead link|date=October 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref>{{cite journal |last1=Karpowicz |first1=Nicholas |last2=Dai |first2=Jianming |last3=Lu |first3=Xiaofei |last4=Chen |first4=Yunqing |last5=Yamaguchi |first5=Masashi |last6=Zhao |first6=Hongwei |last7=Zhang |first7=X.-C. |last8=Zhang |first8=Liangliang |last9=Zhang |first9=Cunlin |last10=Price-Gallagher |first10=Matthew |last11=Fletcher |first11=Clark |last12=Mamer |first12=Orval |last13=Lesimple |first13=Alain |last14=Johnson |first14=Keith |display-authors=6 |year=2008 |title=Coherent heterodyne time-domain spectrometry covering the entire ''terahertz gap'' |journal=Applied Physics Letters |volume=92 |issue=1 |page=011131 |bibcode=2008ApPhL..92a1131K |doi=10.1063/1.2828709 |type=Abstract|doi-access=free }}</ref><ref name="klnnr">{{cite journal |last1=Kleiner |first1=R. |year=2007 |title=Filling the terahertz gap |journal=Science |volume=318 |issue=5854 |pages=1254–1255 |doi=10.1126/science.1151373 |pmid=18033873 |s2cid=137020083 |type=Abstract}}</ref>

In 2024, an experiment was published by German researchers<ref>{{Cite journal |last1=Wubs |first1=Jente R. |last2=Macherius |first2=Uwe |last3=Lü |first3=Xiang |last4=Schrottke |first4=Lutz |last5=Budden |first5=Matthias |last6=Kunsch |first6=Johannes |last7=Weltmann |first7=Klaus-Dieter |last8=van Helden |first8=Jean-Pierre H. |date=January 2024 |title=Performance of a High-Speed Pyroelectric Receiver as Cryogen-Free Detector for Terahertz Absorption Spectroscopy Measurements |journal=Applied Sciences |language=en |volume=14 |issue=10 |page=3967 |doi=10.3390/app14103967 |doi-access=free |issn=2076-3417}}</ref> 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.

===Closure of the terahertz gap===
See DDE<ref name=":1" /> as exception to, "Most vacuum electronic devices that are used for microwave generation can be modified to operate at terahertz frequencies, including the magnetron,<ref>{{cite web |last1=Larraza |first1=Andres |last2=Wolfe |first2=David M. |last3=Catterlin |first3=Jeffrey K. |date=2013-05-21 |title=Terahertz (THZ) reverse magnetron |id=US Patent 8,446,096 B1 |department=Dudley Knox Library |publisher=Naval Postgraduate School |place=Monterey, California |url=https://calhoun.nps.edu/handle/10945/33987}}{{full citation needed |date=March 2020|df=dmy-all}}</ref> gyrotron,<ref>{{cite journal |first1=Mikhail |last1=Glyavin |first2=Grigory |last2=Denisov |first3=V.E. |last3=Zapevalov |first4=A.N. |last4=Kuftin |date=August 2014 |title=Terahertz gyrotrons: State of the art and prospects |journal=Journal of Communications Technology and Electronics |volume=59 |issue=8 |pages=792–797 |doi=10.1134/S1064226914080075 |s2cid=110854631 |url=https://www.researchgate.net/publication/271745395 |via=researchgate.net |access-date=2020-03-18 }}</ref> synchrotron,<ref>{{cite journal |first1=C. |last1=Evain |first2=C. |last2=Szwaj |first3=E. |last3=Roussel |first4=J. |last4=Rodriguez |first5=M. |last5=Le Parquier |first6=M.-A. |last6=Tordeux |first7=F. |last7=Ribeiro |first8=M. |last8=Labat |first9=N. |last9=Hubert |first10=J.-B. |last10=Brubach |first11=P. |last11=Roy |first12=S. |last12=Bielawski |title=Stable coherent terahertz synchrotron radiation from controlled relativistic electron bunches |journal=Nature Physics |volume=15 |pages=635–639 |date=8 April 2019 |issue=7 |doi=10.1038/s41567-019-0488-6 |arxiv=1810.11805 |bibcode=2019NatPh..15..635E |s2cid=53606555 }}</ref> and free-electron laser.<ref>{{cite web |title=UCSB free-electron laser source |series=Terahertz facility |website=www.mrl.ucsb.edu |publisher=University of California – Santa Barbara |url=http://www.mrl.ucsb.edu/terahertz-facility/instruments/ucsb-free-electron-laser-source}}{{full citation needed |date=March 2020|df=dmy-all}}</ref> " Similarly, microwave detectors such as the [[tunnel diode]] have been re-engineered to detect at terahertz<ref>{{cite journal | doi=10.1149/04901.0093ecst | title=Enhanced Terahertz Detection in Resonant Tunnel Diode-Gated HEMTs | year=2012 | last1=Sensale-Rodríguez | first1=B. | last2=Fay | first2=P. | last3=Liu | first3=L. | last4=Jena | first4=D. | last5=Xing | first5=H. G. | journal=ECS Transactions | volume=49 | issue=1 | pages=93–102 | bibcode=2012ECSTr..49a..93S }}</ref> and infrared<ref>{{cite conference |last=Davids |first=Paul |date=2016-07-01 |title=Tunneling rectification in an infrared nanoantenna coupled MOS diode
|conference=Meta 16 |place=Malaga, Spain |website=osti.gov |url=https://www.osti.gov/servlets/purl/1371628 |publisher=U.S. Department of Energy |department=Office of Scientific and Technical Information}}{{full citation needed |date=March 2020|df=dmy-all}}</ref> 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.

==Research==

===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.<ref>{{cite journal 
| last1= Liu | first1=Xi
| last2= Qiao | first2=Zhi 
| last3= Chai | first3=Yuming
| last4= Zhu | first4=Zhi 
| last5= Wu | first5= Kaijie
| last6= Ji | first6=Wenliang
| last7= Daguang | first7= Li
| last8= Xiao | first8=Yujie
| last9= Mao | first9= Lanqun
| last10= Chang | first10=Chao
| last11= Wen | first11=Quan
| last12= Song | first12= Bo
| last13= Shu | first13=Yousheng
| title= Nonthermal and reversible control of neuronal signaling and behavior by midinfrared stimulation | journal= Proceedings of the National Academy of Sciences USA | volume=118 | date=2021 | issue=10| article-number=e2015685118
| doi=10.1073/pnas.2015685118| pmid=33649213| pmc= 7958416 | bibcode=2021PNAS..11815685L
| doi-access=free}}</ref><ref>{{cite journal 
| last1= Zhang | first1=Jun
| last2= Song | first2=Li
| last3= Li | first3=Weidong
| title= Advances of terahertz technology in neuroscience: Current status and a future perspective | journal= iScience| date=2021
| volume=24
| issue=12
| article-number=103548
|doi=10.1016/j.isci.2021.103548| pmid=34977497| pmc=8683584 | bibcode=2021iSci...24j3548Z
| doi-access=free}}</ref>

===Medical imaging===
Unlike [[X-ray]]s, terahertz radiation is not [[ionizing radiation]] and its low [[photon energy|photon energies]] in general do not damage living [[Tissue (biology)|tissue]]s and [[DNA]]. 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 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.<ref>{{cite journal | last1= Sun | first1=Q.| last2= He | first2=Y. | last3= Liu | first3=K.| last4= Fan | first4=S. | last5= Parrott | first5= E. P. J.| last6= Pickwell-MacPherson | first6=E. | title= Recent advances in terahertz technology for biomedical applications | journal= Quantitative Imaging in Medicine and Surgery| volume=7 | date=2017 | issue=3| pages=345–355 | doi=10.21037/qims.2017.06.02| pmid=28812001| pmc= 5537133 | doi-access=free}}</ref> In response to the demand for COVID-19 screening terahertz spectroscopy and imaging has been proposed as a rapid screening tool.<ref>{{Cite web|title=Terahertz spectroscopy opens options in COVID-19 screening|url=https://www.labpulse.com/index.aspx?sec=ser&sub=def&pag=dis&ItemID=801395&wf=2|access-date=2021-06-14|website=LabPulse.com| date=22 June 2020 }}</ref><ref>{{Cite patent|title=Method and System for Enhancing Resolution of Terahertz Imaging and Detection of Symptoms of COVID-19|pubdate=2021-02-11|country=US|number=2021038111|inventor1-last=Ahi|inventor1-first= Kiarash}}</ref>

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.{{Cn|date=February 2021}}

Some frequencies of terahertz radiation can be used for [[3D imaging]] of [[tooth|teeth]] and may be more accurate than conventional X-ray imaging in [[dentistry]].{{Cn|date=February 2021}} 

===Security===
Terahertz radiation can penetrate fabrics and plastics, so it can be used in [[surveillance]], such as [[Airport security|security]] screening, to uncover [[concealment device|concealed]] [[weapon]]s 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,<ref>[http://www.esa.int/spaceinimages/Images/2002/06/Meeting_the_team "Space in Images – 2002 – 06 – Meeting the team"]. ''European Space Agency''. June 2002.</ref> based at the [[Rutherford Appleton Laboratory]] (Oxfordshire, UK), produced the first passive terahertz image of a hand.<ref>[http://www.timeshighereducation.co.uk/news/space-camera-blazes-new-terahertz-trails/174657.article Space camera blazes new terahertz trails]. timeshighereducation.co.uk. 14 February 2003.</ref> 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.<ref>[https://web.archive.org/web/20140315232115/http://www.epsrc.ac.uk/newsevents/news/2004/Pages/rcukbusinessplan.aspx Winner of the 2003/04 Research Councils' Business Plan Competition – 24 February 2004]. epsrc.ac.uk. 27 February 2004</ref> 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.<ref>{{cite news |title=Camera 'looks' through clothing
 |url=http://news.bbc.co.uk/1/hi/technology/7287135.stm |publisher=BBC News 24
 |date=10 March 2008 |access-date=10 March 2008 }}</ref><ref>{{cite web|url=http://www.i4u.com/article15314.html
 |title=ThruVision T5000 T-Ray Camera sees through Clothes |publisher=I4u.com |access-date=17 May 2012}}</ref>

In January 2013, the [[NYPD]] announced plans to experiment with the new technology to detect [[concealed weapon]]s,<ref name=NYPD>{{cite web |url=http://www.nydailynews.com/new-york/nypd-readies-scan-and-frisk-article-1.1245663
 |title=NYPD Commissioner says department will begin testing a new high-tech device that scans for concealed weapons
 |author=Parascandola, Bruno |publisher=NYDailyNews.com |date=23 January 2013 |access-date=10 April 2013}}</ref> 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.<ref name=Corbett>{{cite web |url=http://www.nypost.com/p/news/local/blogger_goes_after_nypd_for_anti_EAKz4HPYWGiH70XU2SMP2J
 |title=Blogger sues NYPD over gun detecting 'terahertz' scanners
 |author1=Golding, Bruce  |author2=Conley, Kirsten  |name-list-style=amp |publisher=NYpost.com
 |date=28 January 2013 |access-date=10 April 2013}}</ref> By early 2017, the department said it had no intention of ever using the sensors given to them by the federal government.<ref>{{cite news|last1=Parascandola|first1=Rocco|title=NYPD's pricey, controversial 'T-Ray' gun sensors sit idle, but that's OK with cops|url=http://www.nydailynews.com/new-york/nypd-t-ray-gun-sensors-sit-idle-cops-article-1.2978581|access-date=22 February 2017|work=New York Daily News|date=22 February 2017}}</ref>

===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]].<ref>{{Cite journal |last=Uddin |first=Jamal |date=2018 |title=Terahertz multispectral imaging for the analysis of gold nanoparticles' size and the number of unit cells in comparison with other techniques |url=https://medcraveonline.com/IJBSBE/terahertz-multispectral-imaging-for-the-analysis-of-gold-nanoparticlesrsquo-size-and-the-number-of-unit-cells-in-comparison-with-other-techniques.html |journal=International Journal of Biosensors & Bioelectronics |volume=4 |issue=3 |doi=10.15406/ijbsbe.2018.04.00118|doi-access=free }}</ref>

Recently developed methods of [[Terahertz time domain spectroscopy|THz time-domain spectroscopy]] (THz TDS) and [[Terahertz tomography|THz tomography]] have been shown to be able to image samples that are opaque in the visible and [[infrared|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.{{Cn|date=February 2021}}

Submillimeter waves are used in physics to study materials in high magnetic fields, since at high fields (over about 11&nbsp;[[Tesla (unit)|tesla]]), the electron spin [[Larmor frequency|Larmor frequencies]] are in the submillimeter band. Many high-magnetic field laboratories perform these high-frequency [[electron paramagnetic resonance|EPR]] experiments, such as the [[National High Magnetic Field Laboratory]] (NHMFL) in Florida.{{Cn|date=February 2021}}

Terahertz radiation could let art historians see murals hidden beneath coats of plaster or paint in centuries-old buildings, without harming the artwork.<ref>[http://newswise.com/articles/view/537448/ Hidden Art Could be Revealed by New Terahertz Device] Newswise, Retrieved 21 September 2008.</ref>

In additional, THz imaging has been done with lens antennas to capture radio image of the object.<ref>{{cite journal |last1=Hillger |first1=Philipp |last2=Grzyb |first2=Janusz |last3=Jain |first3=Ritesh |last4=Pfeiffer |first4=Ullrich R. |title=Terahertz Imaging and Sensing Applications With Silicon-Based Technologies |journal=IEEE Transactions on Terahertz Science and Technology |date=January 2019 |volume=9 |issue=1 |pages=1–19 |doi=10.1109/TTHZ.2018.2884852|bibcode=2019ITTST...9....1H |s2cid=57764017 |doi-access=free }}</ref><ref>{{cite journal |last1=Ghavidel |first1=Ali |last2=Myllymäki |first2=Sami |last3=Kokkonen |first3=Mikko |last4=Tervo |first4=Nuutti |last5=Nelo |first5=Mikko |last6=Jantunen |first6=Heli |title=A Sensing Demonstration of a Sub THz Radio Link Incorporating a Lens Antenna |journal=Progress in Electromagnetics Research Letters |date=2021 |volume=99 |pages=119–126 |doi=10.2528/PIERL21070903|s2cid=237351452 |doi-access=free }}</ref>

===Particle accelerators===
New types of [[particle accelerator]]s 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.<ref>
{{cite journal
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 | last2=Tantawi | first2=Sami
 | last3=Higashi | first3=Yasuo
 | last4=Spataro | first4=Bruno
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 | title=Geometric dependence of radio-frequency breakdown in normal conducting accelerating structures
 | journal=Applied Physics Letters
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</ref> Beam driven dielectric wakefield accelerators (DWAs)<ref>
{{cite journal
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 | last3=Hong | first3=Kyung-Han | last4=Ravi | first4=Koustuban
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</ref><ref>
{{cite journal
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 | year=2016
 | title=Dielectric Wakefield Accelerators
 | journal=Reviews of Accelerator Science and Technology
 | volume=09 | issue=6
 | pages=127–149
 | doi=10.1142/s1793626816300061
| bibcode=2016RvAST...9..127J
 }}
</ref> typically operate in the Terahertz frequency range, which pushes the plasma breakdown threshold for surface electric fields into the multi-GV/m range.<ref>
{{cite journal
 | last1 =Thompson   | first1 =M.C. | last2 =Badakov    | first2 =H.
 | last3 =Cook       | first3 =A.M. | last4 =Rosenzweig | first4 =J.B.
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 | last7 =Blumenfeld | first7 =I.  | last8 =Hogan      | first8 =M.J.
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 | last13=Muggli     | first13=P.  | last14=Scott      | first14=A.
 | last15=Yoder      | first15=R.B.    | display-authors=6
 | date=2008-05-27
 | title=Breakdown limits on gigavolt-per-meter electron-beam-driven wakefields in dielectric structures
 | journal=Physical Review Letters
 | volume=100 | issue=21 | article-number=214801
 | doi=10.1103/physrevlett.100.214801 | pmid=18518609 | bibcode =2008PhRvL.100u4801T | osti=933022 | s2cid =6728675 | url=https://digital.library.unt.edu/ark:/67531/metadc893525/ }}</ref> 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<ref>
{{cite journal
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 | title=Observation of acceleration and deceleration in gigaelectron-volt-per-metre gradient dielectric wakefield accelerators
 | journal=Nature Communications
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 | pmid=27624348 | pmc=5027279
 | doi=10.1038/ncomms12763 | bibcode=2016NatCo...712763O | doi-access=free
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</ref> 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<ref>
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</ref><ref>
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</ref> 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.{{Cn|date=February 2021}}

A preliminary study with corrugated capillaries has shown some modification to the spectral content and amplitude of the generated wakefields,<ref>
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 | last3=Tishchenko | first3=A.A. | last4=Shevelev | first4=M.
 | last5=Lyapin     | first5=A. | last6=Boogert  | first6=S.
 | last7=Karataev   | first7=P. | last8=Terunuma | first8=N.
 | last9=Urakawa    | first9=J.
 | display-authors=6
 | date=2018-05-10
 | title=Driver-witness electron beam acceleration in dielectric mm-scale capillaries
 | journal=Physical Review Accelerators and Beams
 | volume=21 | issue=5 | article-number=051301
 | doi=10.1103/physrevaccelbeams.21.051301 | bibcode=2018PhRvS..21e1301L |doi-access=free
}}
</ref> but the possibility of using Smith-Purcell effect in DWA is still under consideration.{{Cn|date=February 2021}}

===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 (signal processing)|bandwidth]] available in the band (ten times the bandwidth of the [[millimeter wave]] band, 100 times that of the [[superhigh frequency|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.<ref name="Rappaport">{{Cite journal|last1=Rappaport|first1=Theodore S.|last2=Xing|first2=Yunchou|last3=Kanhere|first3=Ojas|last4=Ju|first4=Shihao|last5=Madanayake|first5=Arjuna|last6=Mandal|first6=Soumyajit|last7=Alkhateeb|first7=Ahmed|last8=Trichopoulos|first8=Georgios C.|date=2019|title=Wireless Communications and Applications Above 100 GHz: Opportunities and Challenges for 6G and Beyond|journal=IEEE Access|volume=7|pages=78729–78757|doi=10.1109/ACCESS.2019.2921522|bibcode=2019IEEEA...778729R |issn=2169-3536|doi-access=free}}</ref> One promising application area is the [[6G]] cellphone and wireless standard, which will supersede the current [[5G]] standard around 2030.<ref name="Rappaport" /> In particular, [[6G]] is expected to leverage advanced technologies such as terahertz and [[full duplex]] (FD) communications, combined with dynamic spectrum sharing to meet the growing demand for higher data rates and more efficient spectrum efficiency.<ref>{{Cite journal |last1=Tani |first1=Andrea |last2=Marabissi |first2=Dania |date=2025 |title=Efficient Switched-Beam Detection for Dynamic Spectrum Sharing in 6G Wireless Networks With Full Duplex Technology at the THz Band |journal=IEEE Access |volume=13 |pages=57662–57675 |doi=10.1109/ACCESS.2025.3554606 |bibcode=2025IEEEA..1357662T |issn=2169-3536|doi-access=free |hdl=2158/1417056 |hdl-access=free }}</ref> 

For a given antenna aperture, the [[gain (antenna)|gain]] of [[directional antenna|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.<ref name="Rappaport" /> Small directive antennas a few centimeters in diameter can produce very narrow 'pencil' beams of THz radiation, and [[phased array antenna|phased 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 the [[Tokyo Institute of Technology]]<ref>
{{Cite journal
 | last1 = Ishigaki | first1 = K. | last2 = Shiraishi | first2 = M.
 | last3 = Suzuki | first3 = S. | last4 = Asada | first4 = M.
 | last5 = Nishiyama | first5 = N. | last6 = Arai | first6 = S.
 | year = 2012
 | title = Direct intensity modulation and wireless data transmission characteristics of terahertz-oscillating resonant tunnelling diodes
 | journal = Electronics Letters
 | volume = 48 | issue = 10 | page = 582
 | doi = 10.1049/el.2012.0849
| bibcode = 2012ElL....48..582I }}
</ref> 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.<ref name=BBC/> The team's [[proof of concept]] device used a [[resonant tunneling diode]] (RTD) [[negative resistance#oscillators|negative resistance oscillator]] to produce waves in the terahertz band. With this RTD, the researchers sent a signal at 542&nbsp;GHz, resulting in a data transfer rate of 3 Gigabits per second.<ref name=BBC/> It doubled the record for data transmission rate set in November 2011.<ref>
{{cite web
 |first=Marc |last=Chacksfield
 |date=16 May 2012
 |title=Scientists show off the future of Wi-Fi – smash through 3Gbps barrier
 |website=Tech Radar
 |url=http://www.techradar.com/news/internet/scientists-show-off-the-future-of-wi-fi-smash-through-3gbps-barrier-1080568
 |access-date=16 May 2012
}}
</ref> The study suggested that Wi-Fi using the system would be limited to approximately {{convert|10|m}}, but could allow data transmission at up to 100&nbsp;Gbit/s.<ref name=BBC>
{{cite news
 |title=Milestone for Wi-Fi with 'T-rays'
 |date=16 May 2012
 |website=BBC News
 |url=https://www.bbc.co.uk/news/science-environment-18072618
 |access-date=16 May 2012
}}
</ref>{{clarify| reason = The difference between 100&nbsp;Gbit/s and the 3&nbsp;Gbit/s earlier said to have been achieved is considerable and deserves explanation.|date=May 2012}} 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.<ref>
{{cite news
 |title=New chip enables record-breaking wireless data transmission speed
 |date=22 November 2011
 |website=techcrunch.com
 |url=https://techcrunch.com/2011/11/22/up-to-30-gbps-new-chip-enables-record-breaking-wireless-data-transmission-speed/
 |access-date=30 November 2011
}}
</ref> According to nature journal, researchers reported to transfer two videos error free at the speed of 50 Gbps.<ref>{{Cite web |title=Scientists report first data transmission through terahertz multiplexer {{!}} Brown University |url=https://www.brown.edu/news/2017-08-10/multiplexer |access-date=2025-03-27 |website=www.brown.edu |date=10 August 2017 |language=en}}</ref> Which was way more than the previous record.

Potential uses exist in high-altitude telecommunications, above altitudes where water vapor causes signal absorption: aircraft to [[satellite]], or satellite to satellite.{{citation needed|date=June 2013}}

====Amateur radio====
{{main|Submillimeter amateur radio}}
A number of administrations permit [[amateur radio]] experimentation within the 275–3,000&nbsp;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 {{convert|1.42|km|mi}} on 403&nbsp;GHz using CW (Morse code) on 21&nbsp;December 2004. In [[Australia]], at 30&nbsp;THz a distance of {{convert|60|m|ft}} was achieved by stations VK3CV and VK3LN on 8&nbsp;November 2020.<ref name="ARRL-DXrecords">
{{cite report
 |last=Clausell |first=A.
 |date=11 September 2020
 |title=Distance records
 |series=World above 50&nbsp;MHz standings
 |publisher=[[American Radio Relay League]]
 |website=ARRL.org
 |url=http://www.arrl.org/files/file/WA50-Standings/Distance-Records-11-September-2020.pdf
 |access-date=19 November 2020
}}
</ref><ref name=UK-MW-DX>
{{cite report
 |first1 = Peter |last1 = Day    
 |first2 = John  |last2 = Qaurmby
 |date = 9 May 2019
 |title = Microwave distance records
 |publisher  = UK Microwave Group
 |url        = https://www.microwavers.org/index.htm?records.htm
 |access-date= August 2, 2019
}}
</ref>
<ref name=VK-DX-Records>
{{cite report
 |title      = Australian VHF-UHF records
 |date       = 5 January 2021
 |publisher  = [[Wireless Institute of Australia]]
 |url        = https://www.wia.org.au/members/records/data/documents/VHF%20Records%20210105.pdf
 |access-date= January 5, 2021
}}
</ref>

===Manufacturing===
Many possible uses of terahertz sensing and imaging are proposed in [[manufacturing]], [[quality control]], and [[manufacturing process management|process monitoring]]. These in general exploit the traits of plastics and [[paperboard|cardboard]] being transparent to terahertz radiation, making it possible to inspect [[packaging and labelling|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.<ref>
{{cite journal
 |last1=Hu   |first1=B.B.
 |last2=Nuss |first2=M.C.
 |date=15 August 1995
 |title=Imaging with terahertz waves
 |journal=Optics Letters
 |volume=20 |issue=16 |page=1716
 |doi=10.1364/OL.20.001716 |pmid=19862134 |bibcode = 1995OptL...20.1716H
 |s2cid=11593500
 }}
</ref> 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.<ref name="Imaging with terahertz radiation">
{{cite journal
 |last1=Chan      |first1=Wai Lam
 |last2=Deibel    |first2=Jason
 |last3=Mittleman |first3=Daniel M.
 |date=1 August 2007
 |title=Imaging with terahertz radiation
 |journal=Reports on Progress in Physics
 |volume=70  |issue=8  |pages=1325–1379
 |doi=10.1088/0034-4885/70/8/R02
 |bibcode = 2007RPPh...70.1325C  |s2cid=17397271
 }}
</ref>

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.<ref>
{{cite book
 |last1=Prince |first1=Jerry L. Jr.
 |last2=Links  |first2=Jonathan M.
 |year=2006
 |title=Medical imaging signals and systems
 |publisher=Pearson Prentice Hall
 |location=Upper Saddle River, N.J.
 |isbn=978-0-13-065353-6
}}
</ref>

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 diffraction|Fraunhofer region]] imply that the electromagnetic beams diverge more as the frequencies of the beams decrease and thus the resolution decreases.<ref>
{{cite book
 |editor-last1=Marshall |editor-first1=Gerald F.
 |editor-last2=Stutz    |editor-first2=Glenn E.
 |year=2012
 |title=Handbook of optical and laser scanning
 |edition=2nd
 |publisher=CRC Press
 |location=Boca Raton, FL
 |isbn=978-1-4398-0879-5
}}
</ref> 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.<ref name="Terahertz characterization of electronic components and comparison of terahertz imaging with X-ray imaging techniques">
{{cite conference
 |author1=Ahi, Kiarash
 |author2=Shahbazmohamadi, Sina
 |author3=Tehranipoor, Mark
 |author4=Anwar, Mehdi 
 |date=2015-05-13
 |title=Terahertz characterization of electronic components and comparison of terahertz imaging with X-ray imaging techniques
 |id=94830K
 |conference=SPIE Sensing Technology + Applications
 |place=Baltimore, MD
 |editor1-last=Anwar  |editor1-first=Mehdi F. 
 |editor2-last=Crowe  |editor2-first=Thomas W.
 |editor3-last=Manzur |editor3-first=Tariq    
 |book-title=Proceedings Volume 9483, Terahertz Physics, Devices, and Systems IX: Advanced Applications in Industry and Defense
 |bibcode=2015SPIE.9483E..0KA  |s2cid=118178651  |doi=10.1117/12.2183128
 |url=https://www.researchgate.net/publication/278034592
}}
</ref> 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.{{Cn|date=February 2021}}

To overcome low resolution of the terahertz systems near-field terahertz imaging systems are under development.<ref>
{{cite journal
 |last1=Mueckstein |first1=Raimund
 |last2=Mitrofanov |first2=Oleg
 |date=3 February 2011
 |title=Imaging of terahertz surface plasmon waves excited on a gold surface by a focused beam
 |journal=Optics Express
 |volume=19  |issue=4  |pages=3212–3217
 |doi=10.1364/OE.19.003212  |doi-access=free
 |pmid=21369143  |bibcode = 2011OExpr..19.3212M  |s2cid=21438398
 }}
</ref><ref>
{{cite journal
 |last1=Adam  |first1=Aurele       |last2=Brok  |first2=Janne
 |last3=Seo   |first3=Min Ah       |last4=Ahn   |first4=Kwang Jun
 |last5=Kim   |first5=Dai Sik      |last6=Kang  |first6=Ji-Hun
 |last7=Park  |first7=Q-Han        |last8=Nagel |first8=M.
 |last9=Nagel |first9=Paul C.M.
 |date=19 May 2008
 |title=Advanced terahertz electric near-field measurements at sub-wavelength diameter metallic apertures: erratum|journal=Optics Express
 |volume=16  |issue=11  |page=8054
 |doi=10.1364/OE.16.008054  |doi-access=free
 |bibcode = 2008OExpr..16.8054A
 |url=http://resolver.tudelft.nl/uuid:0d005f8c-f2b8-466a-8c59-ff520f66db62
}}
</ref> 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.<ref>
{{cite journal
 |last1=Kiwa      |first1=Toshihiko       |last2=Tonouchi |first2=Masayoshi
 |last3=Yamashita |first3=Masatsugu       |last4=Kawase   |first4=Kodo
 |date=1 November 2003
 |title=Laser terahertz-emission microscope for inspecting electrical faults in integrated circuits
 |journal=Optics Letters
 |volume=28  |issue=21  |pages=2058–60
 |doi=10.1364/OL.28.002058  |pmid=14587814  |bibcode = 2003OptL...28.2058K
}}
</ref> 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.{{Cn|date=February 2021}}

===THz gap research===
Ongoing investigation has resulted in [[Semiconductor laser theory|improved emitters]] (sources) and [[sensor|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.<ref name=ferguson>{{cite journal |last1=Ferguson |first1=Bradley |last2=Zhang |first2=Xi-Cheng |year=2002 |title=Materials for terahertz science and technology |journal=Nature Materials |volume=1 |issue=1 |pages=26–33 |url=http://www.eleceng.adelaide.edu.au/groups/thz/publications/ferguson_2002_npg.pdf |format=free PDF download |doi=10.1038/nmat708 |pmid=12618844 |bibcode=2002NatMa...1...26F|s2cid=24003436 }}</ref><ref name=tonom>{{cite journal |last1=Tonouchi |first1=Masayoshi |year=2007 |title=Cutting-edge terahertz technology |journal=Nature Photonics |volume=1 |issue=2 |pages=97–105 |id=200902219783121992 |url=http://www.ile.osaka-u.ac.jp/research/THP/pdf/nphoton144.pdf |format=free PDF download |doi=10.1038/nphoton.2007.3 |bibcode=2007NaPho...1...97T}}</ref><ref>{{cite journal |last1=Chen |first1=Hou-Tong |last2=Padilla |first2=Willie J. |last3=Cich |first3=Michael J. |last4=Azad |first4=Abul K. |last5=Averitt |first5=Richard D. |last6=Taylor |first6=Antoinette J. |author6-link=Antoinette Taylor |year=2009 |title=A metamaterial solid-state terahertz phase modulator |journal=Nature Photonics |volume=3 |issue=3 |page=148 |url=http://nanoscience.bu.edu/papers/Averitt%20-%20Nature%20Photonics%20(2009).pdf |format=free PDF download |doi=10.1038/nphoton.2009.3 |bibcode=2009NaPho...3..148C |osti=960853 |citeseerx=10.1.1.423.5531 |access-date=25 August 2022 |archive-date=29 June 2010 |archive-url=https://web.archive.org/web/20100629143138/http://nanoscience.bu.edu/papers/Averitt%20-%20Nature%20Photonics%20(2009).pdf  }}</ref>

[[Free-electron laser]]s can generate a wide range of [[Laser|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|wireless communications]]). Other [[#Sources|sources of terahertz radiation]] which are actively being researched include solid state oscillators (through [[Frequency multiplier|frequency multiplication]]), [[Backward-wave oscillator|backward wave oscillators]] (BWOs), [[quantum cascade laser]]s, and [[gyrotron]]s.

==Safety==
The terahertz region is between the radio frequency region and the laser optical region. Both the IEEE C95.1–2005 RF safety standard<ref name=c95.1>
{{cite report
 |id=IEEE C95.1–2005
 |title=IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3&nbsp;kHz to 300&nbsp;GHz
 |publisher=[[Institute of Electrical and Electronics Engineers]]
 |year=2005
}}
</ref> and the ANSI Z136.1–2007 Laser safety standard<ref name=z136.1>
{{cite report
 |id=ANSI Z136.1–2007
 |title=American National Standard for Safe Use of Lasers
 |publisher=[[American National Standards Institute]]
 |year=2007
}}
</ref> 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|date=January 2013}}. Research is underway to collect data to populate this region of the spectrum and validate safety limits. {{Citation needed|date=January 2013}}

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<ref name=Alexandrov-etal-2010>
{{cite journal
 |author1=Alexandrov, B.S. |author2=Gelev, V.
 |author3=Bishop, A. R. |author4=Usheva, A.
 |author5=Rasmussen, K.O.
 |year= 2010
 |title= DNA breathing dynamics in the presence of a terahertz field
 |journal= [[Physics Letters A]]
 |volume= 374 |issue= 10 |pages= 1214–1217
 |doi= 10.1016/j.physleta.2009.12.077 |pmid= 20174451
 |pmc= 2822276 |arxiv= 0910.5294 |bibcode = 2010PhLA..374.1214A
}}
</ref> created mathematical models predicting how terahertz radiation would interact with double-stranded [[DNA]], showing that, even though involved forces seem to be tiny, [[nonlinear resonance]]s (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]]".{{refn|
{{cite news
 |title = How terahertz waves tear apart DNA
 |date = 30 October 2010
 |magazine = [[MIT Technology Review]]
 |series = Emerging Technology from the arXiv
 |url = https://www.technologyreview.com/2009/10/30/208491/how-terahertz-waves-tear-apart-dna/
 |access-date=5 June 2021   |postscript=;
}}<br/>''MIT Tech. Rev.'' article cites Alexandrov ''et al.'' (2010)<ref name=Alexandrov-etal-2010/> as source.
}} 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.<ref name=swanson10>
{{cite journal
 |author= Swanson, Eric S.
 |title= Modelling DNA Response to THz Radiation
 |year = 2010
 |doi= 10.1103/PhysRevE.83.040901 |pmid= 21599106
 |journal= Physical Review E
 |volume= 83 |issue= 4 |article-number= 040901
 |arxiv= 1012.4153  |bibcode = 2011PhRvE..83d0901S  |s2cid= 23117276
}}
</ref> 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".<ref name=fitz03>
{{cite journal
 |first1 = A.J. |last1 = Fitzgerald     |first2 = E. |last2 = Berry
 |first3 = N.N. |last3 = Zinov'Ev       |first4 = S. |last4 = Homer-Vanniasinkam
 |first5 = R.E. |last5 = Miles          |first6 = J.M. |last6 = Chamberlain
 |first7 = M.A. |last7 = Smith     
 |year = 2003
 |title = Catalogue of human tissue optical properties at terahertz frequencies
 |journal = [[Journal of Biological Physics]]
 |volume = 29 |issue = 2–3 |pages = 123–128
 |pmid = 23345827 |pmc = 3456431
 |doi = 10.1023/A:1024428406218
}}
</ref>

==See also==
*[[Far-infrared laser]]
*[[Full body scanner]]
*[[Heterojunction bipolar transistor]]
*[[High-electron-mobility transistor]] (HEMT)
*[[Picarin]]
*[[Terahertz time-domain spectroscopy]]
*[[Microwave analog signal processing]]

==References==
{{reflist|25em}}

==Further reading==
*{{cite conference
 |editor1-last=Miles
 |editor1-first=Robert E
 |editor2-last=Harrison
 |editor2-first=Paul
 |editor3-last=Lippens
 |editor3-first=D. 
 |title=Terahertz Sources and Systems
 |conference=NATO Advanced Research Workshop
 |series=NATO Science Series II 
 |volume=27
 |publication-date=2001
 |location=Château de Bonas, France
 |date=June 2000
 |url=https://books.google.com/books?id=xYwPHQ6bHkwC |via=Google Books 
 |lccn=2001038180
 |isbn=978-0-7923-7096-3
 |oclc=248547276
 }}

*{{cite journal
 | last1 = Güven
 | first1 = Eray
 | last2 = Karabulut-Kurt
 | first2 = Güneş
 | title = On the Mutuality Between Localization and Channel Modeling in Sub-THz
 | journal = IEEE Wireless Communications
 | year = 2024
 | volume = 31
 | issue = 1
 | pages = 26–32
 | doi = 10.1109/MWC.001.2300307
| arxiv = 2401.01504
 | bibcode = 2024IWC....31a..26G
 }}

==External links==
{{Wiktionary|terahertz radiation|T-ray}}

* {{cite journal
 |first=Eric |last=Mueller
 |date=August–September 2003
 |title=Terahertz radiation: Applications and sources
 |journal=AIP: The Industrial Physicist
 |volume=9  |issue=4   |page=27
 |url=http://www.aip.org/tip/INPHFA/vol-9/iss-4/p27.html
   |access-date=2021-06-05
 |archive-url=https://web.archive.org/web/20031204032750/http://www.aip.org/tip/INPHFA/vol-9/iss-4/p27.html
 |archive-date=2003-12-04
}}
*{{cite web |last=Williams |first=G. |url=http://casa.jlab.org/seminars/2003/slides/williams_031114.pdf |title=Filling the THz gap |series=CASA Seminar |year=2003 |website=jlab.org}}
*{{cite news
 |last=Cooke
 |first=Mike
 |title=Filling the THz gap with new applications
 |volume=2
 |pages=39–43
 |issue=1
 |magazine=Semiconductor Today
 |year=2007
 |url=http://www.semiconductor-today.com/features/PDF/Semiconductor%20Today%20-%20Filling%20the%20THz%20gap.pdf
 |access-date=2019-07-30 }}

*{{cite press release
 |last=Janet
 |first=Rae-Dupree
 |title=New life for old electrons in biological imaging, sensing technologies
 |publisher=Stanford University
 |place=Palo Alto, California
 |department=SLAC National Accelerator Laboratory
 |date=8 November 2011
 |url=https://www6.slac.stanford.edu/news/2011-11-08-new-life-for-old-electrons-in-biological-imaging.aspx
 |quote=...&nbsp;researchers have successfully generated intense pulses of light in a largely untapped part of the electromagnetic spectrum – the so-called ''terahertz gap''.}}

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[[Category:Electromagnetic spectrum]]
[[Category:Terahertz technology]]