Terahertz gap is an engineering term for a frequency band in the terahertz region of the electromagnetic spectrum between radio waves and infrared light for which practical technologies for generating and detecting the radiation do not exist. It is defined as 0.1 to 10 THz (wavelengths of 3 mm to 30 µm). Currently, at frequencies within this range, useful power generation and receiver technologies are inefficient and impractical.
Mass production of devices in this range and operation at room temperature (at which kT is equal to the energy of a photon with a frequency of 6.2 THz) are mostly unfeasible. This leaves a gap between mature microwave technologies in the highest frequencies of the radio spectrum and the well developed optical engineering of infrared detectors 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 century.
Closure of the terahertz gap
Most vacuum electronic devices that are used for microwave generation can be modified to operate at terahertz frequencies, including the magnetron  , gyrotron , synchrotron , and free electron laser . Similarly, microwave detectors such as the tunnel diode have been re-engineered to detect at terahertz  and infrared  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.
Ongoing investigation has resulted in improved emitters (sources) and detectors, and research in this area has intensified. However, drawbacks remain that include the substantial size of emitters, incompatible frequency ranges, and undesirable operating temperatures, as well as component, device, and detector requirements that are somewhere between solid state electronics and photonic technologies.
Free-electron lasers can generate a wide range of stimulated emission of electromagnetic radiation from microwaves, through terahertz radiation to X-ray. However, they are bulky, expensive and not suitable for applications that require critical timing (such as wireless communications). Other sources of terahertz radiation which are actively being researched include solid state oscillators (through frequency multiplication), backward wave oscillators (BWOs), quantum cascade lasers, and gyrotrons.
- Gharavi, Sam; Heydari, Babak (2011-09-25). Ultra High-Speed CMOS Circuits : Beyond 100 GHz (1st ed.). New York: Springer Science+Business Media. pp. 1–5 (Introduction) and 100. doi:10.1007/978-1-4614-0305-0. ISBN 978-1-4614-0305-0.
- Sirtori, Carlo (2002). "Applied physics: Bridge for the terahertz gap" (Free PDF download). Nature. 417 (6885): 132–3. Bibcode:2002Natur.417..132S. doi:10.1038/417132b. PMID 12000945.
- Borak, A. (2005). "Applied physics:: Toward Bridging the Terahertz Gap with Silicon-Based Lasers" (Free PDF download). Science. 308 (5722): 638–9. doi:10.1126/science.1109831. PMID 15860612.
- Karpowicz, Nicholas; Dai, Jianming; Lu, Xiaofei; Chen, Yunqing; Yamaguchi, Masashi; Zhao, Hongwei; Zhang, X.-C.; Zhang, Liangliang; Zhang, Cunlin; Price-Gallagher, Matthew; Fletcher, Clark; Mamer, Orval; Lesimple, Alain; Johnson, Keith (2008). "Coherent heterodyne time-domain spectrometry covering the entire "terahertz gap"". Applied Physics Letters (Abstract). 92 (1): 011131. Bibcode:2008ApPhL..92a1131K. doi:10.1063/1.2828709.
- Kleiner, R. (2007). "Filling the Terahertz Gap". Science (Abstract). 318 (5854): 1254–5. doi:10.1126/science.1151373. PMID 18033873.
- Ferguson, Bradley; Zhang, Xi-Cheng (2002). "Materials for terahertz science and technology" (Free PDF download). Nature Materials. 1 (1): 26–33. Bibcode:2002NatMa...1...26F. doi:10.1038/nmat708. PMID 12618844.
- Tonouchi, Masayoshi (2007). "Cutting-edge terahertz technology" (Free PDF download). Nature Photonics. 1 (2): 97–105. Bibcode:2007NaPho...1...97T. doi:10.1038/nphoton.2007.3. 200902219783121992.
- Chen, Hou-Tong; Padilla, Willie J.; Cich, Michael J.; Azad, Abul K.; Averitt, Richard D.; Taylor, Antoinette J. (2009). "A metamaterial solid-state terahertz phase modulator" (Free PDF download). Nature Photonics. 3 (3): 148. Bibcode:2009NaPho...3..148C. CiteSeerX 10.1.1.423.5531. doi:10.1038/nphoton.2009.3.
- Miles, Robert E; Harrison, Paul; Lippens, D (Eds.) (2001). Terahertz Sources and Systems. Nato Science Series II. 27. June 2000. Château de Bonas, France: Proceedings of the NATO Advanced Research Workshop. ISBN 978-0-7923-7096-3. LCCN 2001038180. OCLC 248547276.CS1 maint: extra text: authors list (link)
- Williams, G. "Filling the THz Gap." CASA Seminar. 2003.
- Cooke, Mike (2007). "Filling the THz Gap with New Applications" (PDF). 2 (1). Semiconductor Today. pp. 39–43. Retrieved 2019-07-30.
- Janet, Rae-Dupree (November 8, 2011). "New Life for Old Electrons in Biological Imaging, Sensing Technologies". SLAC National Accelerator Laboratory.
...researchers have successfully generated intense pulses of light in a largely untapped part of the electromagnetic spectrum – the so-called terahertz gap.