Terahertz radiation

Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency[1] (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of frequencies from 0.1 to 30 terahertz (THz)[2]. One terahertz is 1012 Hz or 1000 GHz. Wavelengths of radiation in the terahertz band correspondingly range from 1 mm to 0.1 mm (or 100 μm). Because terahertz radiation begins at a wavelength of one millimeter and proceeds into shorter wavelengths, it is sometimes known as the submillimeter band, and its radiation as submillimeter waves, especially in astronomy.

Terahertz radiation can penetrate thin layers of materials but is blocked by thicker objects. THz beams transmitted through materials can be used for material characterization, layer inspection, and as an alternative to X-rays for producing high resolution images of the interior of solid objects.[3]

Terahertz radiation occupies a middle ground between microwaves and infrared light waves known as the “terahertz gap”, where technology for its generation and manipulation is in its infancy. It represents the region in the electromagnetic spectrum where the frequency of electromagnetic radiation becomes too high to be measured digitally via electronic counters, so must be measured by proxy using the properties of wavelength and energy. Similarly, the generation and modulation of coherent electromagnetic signals 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.

Tremendously high frequency
Frequency range
0.1 THz to 30 THz
Wavelength range
1 mm to 100 μm
Spectre Terahertz
Terahertz waves lie at the far end of the infrared band, just before the start of the microwave band.

Introduction

Resolution Enhancement
In THz-TDS systems, since the time-domain version of the THz signal is available, the distortion effects of the diffraction can be suppressed.[4]

Terahertz radiation falls in between infrared radiation and microwave radiation in the electromagnetic spectrum, and it shares some properties with each of these. Like infrared and microwave radiation, terahertz radiation travels in a line of sight and is non-ionizing. Like microwave radiation, terahertz radiation can penetrate a wide variety of non-conducting materials. Terahertz radiation can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. The penetration depth is typically less than that of microwave radiation. Terahertz radiation has limited penetration through fog and clouds and cannot penetrate liquid water or metal.[5] Terahertz radiation is not ionizing yet can penetrate some distance through body tissue, 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).[4]

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 meters the band may still allow many useful applications in imaging and construction of high bandwidth wireless networking systems, especially indoor systems. In addition, producing and detecting coherent terahertz radiation remains technically challenging, though inexpensive commercial sources now exist in the 0.3–1.0 THz range (the lower part of the spectrum), including gyrotrons, backward wave oscillators, and resonant-tunneling diodes.

Terahertz versus submillimeter waves

The terahertz band, covering the wavelength range between 0.1–1 mm, is identical to the submillimeter wavelength band. However, typically, the term "terahertz" is used more often in marketing in relation to generation and detection with pulsed lasers, as in terahertz time domain spectroscopy, while the term "submillimeter" is used for generation and detection with microwave technology, such as harmonic multiplication.

Sources

Natural

Terahertz radiation is emitted as part of the black-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 K cosmic dust in interstellar clouds in the Milky Way galaxy, and in distant starburst galaxies.

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 balloon borne telescope, the Herschel Space Observatory, the Heinrich Hertz Submillimeter Telescope at the Mount Graham International Observatory in Arizona, and at the recently built Atacama Large Millimeter Array. The opacity of the Earth's atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, or to space.

Artificial

As of 2012, viable sources of terahertz radiation are the gyrotron, the backward wave oscillator ("BWO"), the organic gas far infrared laser ("FIR laser"), Schottky diode multipliers,[6] varactor (varicap) multipliers, quantum cascade laser,[7][8][9][10] the free electron laser (FEL), 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,[11] and electronic oscillators based on resonant tunneling diodes have been shown to operate up to 700 GHz.[12]

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 1000 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.[13] The device uses high-temperature superconducting crystals, grown at the University of Tsukuba in Japan. These crystals comprise stacks of Josephson junctions, 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 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.[14]

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 is tribocharging of the adhesive tape and subsequent discharge; this was hypothesized to involve bremsstrahlung with absorption or energy density focusing during dielectric breakdown of a gas. [15]

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 nanometers wide and one micrometer long. Such an antenna could be used to emit radio waves in the terahertz frequency range.[16][17]

Research

Medical imaging

Unlike X-rays, terahertz radiation is not ionizing radiation and its low photon energies in general do not damage living tissues 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.

The first images generated using terahertz radiation date from the 1960s; however, in 1995 images generated using terahertz time-domain spectroscopy generated a great deal of interest.

Some frequencies of terahertz radiation can be used for 3D imaging of teeth and may be more accurate than conventional X-ray imaging in dentistry.

Security

Terahertz radiation can penetrate fabrics and plastics, so it can be used in surveillance, such as security screening, to uncover concealed weapons on a person, remotely. This is of particular interest because many materials of interest have unique spectral "fingerprints" in the terahertz range. This offers the possibility to combine spectral identification with imaging. In 2002 the European Space Agency (ESA) Star Tiger team,[18] based at the Rutherford Appleton Laboratory (Oxfordshire, UK), produced the first passive terahertz image of a hand.[19] 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.[20] 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.[21][22]

In January 2013, the NYPD announced plans to experiment with the new technology to detect concealed weapons,[23] 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.[24] By early 2017, the department said it had no intention of ever using the sensors given to them by the federal government.[25]

Scientific use and imaging

In addition to its current use in submillimetre astronomy, terahertz radiation spectroscopy could provide new sources of information for chemistry and biochemistry.

Recently developed methods of THz time-domain spectroscopy (THz TDS) and THz tomography have been shown to be able to image samples that are opaque in the visible and near-infrared regions of the spectrum. The utility of THz-TDS is limited when the sample is very thin, or has a low absorbance, since it is very difficult to distinguish changes in the THz pulse caused by the sample from those caused by long-term fluctuations in the driving laser source or experiment. However, THz-TDS produces radiation that is both coherent and spectrally broad, so such images can contain far more information than a conventional image formed with a single-frequency source.

Submillimeter waves are used in physics to study materials in high magnetic fields, since at high fields (over about 11 tesla), the electron spin Larmor frequencies are in the submillimeter band. Many high-magnetic field laboratories perform these high-frequency EPR experiments, such as the National High Magnetic Field Laboratory (NHMFL) in Florida.

Terahertz radiation could let art historians see murals hidden beneath coats of plaster or paint in centuries-old buildings, without harming the artwork.[26]

THz driven dielectric wakefield acceleration

New types of particle accelerators that could achieve multi Giga-electron volts per metre (GeV/m) accelerating gradients are of utmost importance to reduce the size and cost of future generations of high energy colliders as well as provide a widespread availability of compact accelerator technology to smaller laboratories around the world. Gradients in the order of 100 MeV/m have been achieved by conventional techniques and are limited by RF-induced plasma breakdown.[27] Beam driven dielectric wakefield accelerators (DWAs) [28][29] typically operate in the Terahertz frequency range, which pushes the plasma breakdown threshold for surface electric fields into the multi-GV/m range.[30] 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 [31] 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 [32][33] in a dielectric capillary with a variable inner radius. When an electron bunch propagates through the capillary, its self-field interacts with the dielectric material and produces wakefields that propagate inside the material at the Cherenkov angle. The wakefields are slowed down below the speed of light, as the relative dielectric permittivity of the material is larger than 1. The radiation is then reflected from the capillary's metallic boundary and diffracted back into the vacuum region, producing high accelerating fields on the capillary axis with a distinct frequency signature. In presence of a periodic boundary the Smith-Purcell radiation imposes frequency dispersion.

A preliminary study with corrugated capillaries has shown some modification to the spectral content and amplitude of the generated wakefields,[34] but the possibility of using Smith-Purcell effect in DWA is still under consideration.

Communication

In May 2012, a team of researchers from the Tokyo Institute of Technology[35] 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.[36] The team's proof of concept device used a resonant 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.[36][36] It doubled the record for data transmission rate set the previous November.[37] 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.[36] 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.[38]

Potential uses exist in high-altitude telecommunications, above altitudes where water vapor causes signal absorption: aircraft to satellite, or satellite to satellite.

Manufacturing

Many possible uses of terahertz sensing and imaging are proposed in manufacturing, quality control, and process monitoring. These in general exploit the traits of plastics and cardboard being transparent to terahertz radiation, making it possible to inspect packaged goods. The first imaging system based on optoelectronic terahertz time-domain spectroscopy were developed in 1995 by researchers from AT&T Bell Laboratories and was used for producing a transmission image of a packaged electronic chip.[39] 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.[40]

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 an objects. This approach is similar to X-ray transmission imaging, where images are developed based on attenuation of the transmitted beam.[41]

In the second approach, terahertz images are developed based on the time delay of the received pulse. In this approach, thicker parts of the objects are well recognized as the thicker parts cause more time delay of the pulse. Energy of the laser spots are distributed by a Gaussian function. The geometry and behavior of Gaussian beam in the Fraunhofer region imply that the electromagnetic beams diverge more as the frequencies of the beams decrease and thus the resolution decreases.[42] 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.[43] Obviously the resolution of X-ray is higher than terahertz image, but X-ray is ionizing and can be impose harmful effects on certain objects such as semiconductors and live tissues.

To overcome low resolution of the terahertz systems near-field terahertz imaging systems are under development.[44][45] 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.[46] 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.

Amateur radio

In the ITU Table of Frequency Allocations, no formal allocation to any radio service is present above 275 GHz, although the regulations themselves cover up to 3000 GHz (3 THz) and include footnote RR5.565 concerning this range. However, a number of administrations permit amateur radio experimentation within the 275–3000 GHz range on a national basis, under licence conditions that are usually based on RR5.565.

Safety

The terahertz region is between the radio frequency region and the optical region generally associated with lasers. Both the IEEE RF safety standard[47] and the ANSI Laser safety standard[48] have limits into the terahertz region, but both safety limits are based on extrapolation. It is expected that effects on tissues are thermal in nature and, therefore, predictable by conventional thermal models. Research is underway to collect data to populate this region of the spectrum and validate safety limits.

A study published in 2010 and conducted by Boian S. Alexandrov and colleagues at the Center for Nonlinear Studies at Los Alamos National Laboratory in New Mexico[49] created mathematical models predicting how terahertz radiation would interact with double-stranded DNA, showing that, even though involved forces seem to be tiny, nonlinear resonances (although much less likely to form than less-powerful common resonances) could allow terahertz waves to "unzip double-stranded DNA, creating bubbles in the double strand that could significantly interfere with processes such as gene expression and DNA replication".[50] Experimental verification of this simulation was not done. A recent analysis of this work concludes that the DNA bubbles do not occur under reasonable physical assumptions or if the effects of temperature are taken into account.[51] It should also be noted that the T-ray intensity drops to less than 1% in the first 500 μm of skin.[52]

See also

References

  1. ^ Jones, Graham A.; Layer, David H.; Osenkowsky, Thomas G. (2007). National Association of Broadcasters Engineering Handbook. Taylor and Francis. p. 7. ISBN 978-1-136-03410-7.
  2. ^ Dhillon, S S; et al. (2017). "The 2017 terahertz science and technology roadmap". Journal of Physics D: Applied Physics. 50 (4): 2. doi:10.1088/1361-6463/50/4/043001. Retrieved 18 April 2019.
  3. ^ Ahi, Kiarash (26 May 2016). "Advanced terahertz techniques for quality control and counterfeit detection". Proc. SPIE 9856, Terahertz Physics, Devices, and Systems X: Advanced Applications in Industry and Defense, 98560G. Terahertz Physics, Devices, and Systems X: Advanced Applications in Industry and Defense. 9856: 98560G. Bibcode:2016SPIE.9856E..0GA. doi:10.1117/12.2228684. Retrieved 26 May 2016.
  4. ^ a b Ahi, Kiarash (2018). "A Method and System for Enhancing the Resolution of Terahertz Imaging". Measurement. doi:10.1016/j.measurement.2018.06.044. ISSN 0263-2241.
  5. ^ JLab generates high-power terahertz light. CERN Courier. 1 January 2003.
  6. ^ Virginia Diodes Virginia Diodes Multipliers Archived 15 March 2014 at the Wayback Machine
  7. ^ Köhler, Rüdeger; Alessandro Tredicucci; Fabio Beltram; Harvey E. Beere; Edmund H. Linfield; A. Giles Davies; David A. Ritchie; Rita C. Iotti; Fausto Rossi (2002). "Terahertz semiconductor-heterostructure laser". Nature. 417 (6885): 156–159. Bibcode:2002Natur.417..156K. doi:10.1038/417156a. PMID 12000955.
  8. ^ Scalari, G.; C. Walther; M. Fischer; R. Terazzi; H. Beere; D. Ritchie; J. Faist (2009). "THz and sub-THz quantum cascade lasers". Laser & Photonics Review. 3 (1–2): 45–66. Bibcode:2009LPRv....3...45S. doi:10.1002/lpor.200810030.
  9. ^ Lee, Alan W. M.; Qi Qin; Sushil Kumar; Benjamin S. Williams; Qing Hu; John L. Reno (2006). "Real-time terahertz imaging over a standoff distance (>25 meters)". Appl. Phys. Lett. 89 (14): 141125. Bibcode:2006ApPhL..89n1125L. doi:10.1063/1.2360210.
  10. ^ Fathololoumi, S.; Dupont, E.; Chan, C. W. I.; Wasilewski, Z. R.; Laframboise, S. R.; Ban, D.; Matyas, A.; Jirauschek, C.; Hu, Q.; Liu, H. C. (13 February 2012). "Terahertz quantum cascade lasers operating up to ~200 K with optimized oscillator strength and improved injection tunneling". Optics Express. 20 (4): 3866–3876. Bibcode:2012OExpr..20.3866F. doi:10.1364/OE.20.003866. PMID 22418143. Retrieved 21 March 2012.
  11. ^ Ramakrishnan, Gopakumar (2012). Enhanced terahertz emission from thin film semiconductor/metal interfaces. Delft University of Technology, The Netherlands. ISBN 978-94-6191-5641.
  12. ^ Brown, E. R.; SöDerström, J. R.; Parker, C. D.; Mahoney, L. J.; Molvar, K. M.; McGill, T. C. (1991). "Oscillations up to 712 GHz in InAs/AlSb resonant-tunneling diodes". Applied Physics Letters. 58 (20): 2291. Bibcode:1991ApPhL..58.2291B. doi:10.1063/1.104902.
  13. ^ Science News: New T-ray Source Could Improve Airport Security, Cancer Detection, ScienceDaily (27 November 2007).
  14. ^ Engineers demonstrate first room-temperature semiconductor source of coherent terahertz radiation Physorg.com. 19 May 2008. Retrieved May 2008
  15. ^ Peeling adhesive tape emits electromagnetic radiation at terahertz frequencies www.opticsinfobase.org 6 August 2009. Retrieved August 2009
  16. ^ Hewitt, John (25 February 2013). "Samsung funds graphene antenna project for wireless, ultra-fast intra-chip links". ExtremeTech. Retrieved 8 March 2013.
  17. ^ Talbot, David (5 March 2013). "Graphene Antennas Would Enable Terabit Wireless Downloads". Technology Review. Massachusetts Institute of Technology. Retrieved 8 March 2013.
  18. ^ "Space in Images – 2002 – 06 – Meeting the team". European Space Agency. June 2002.
  19. ^ Space camera blazes new terahertz trails. timeshighereducation.co.uk. 14 February 2003.
  20. ^ Winner of the 2003/04 Research Councils' Business Plan Competition – 24 February 2004. epsrc.ac.uk. 27 February 2004
  21. ^ "Camera 'looks' through clothing". BBC News 24. 10 March 2008. Retrieved 10 March 2008.
  22. ^ "ThruVision T5000 T-Ray Camera sees through Clothes". I4u.com. Retrieved 17 May 2012.
  23. ^ Parascandola, Bruno (23 January 2013). "NYPD Commissioner says department will begin testing a new high-tech device that scans for concealed weapons". NYDailyNews.com. Retrieved 10 April 2013.
  24. ^ Golding, Bruce & Conley, Kirsten (28 January 2013). "Blogger sues NYPD over gun detecting 'terahertz' scanners". NYpost.com. Retrieved 10 April 2013.
  25. ^ Parascandola, Rocco (22 February 2017). "NYPD's pricey, controversial 'T-Ray' gun sensors sit idle, but that's OK with cops". New York Daily News. Retrieved 22 February 2017.
  26. ^ Hidden Art Could be Revealed by New Terahertz Device Newswise, Retrieved 21 September 2008.
  27. ^ Geometric dependence of radio-frequency breakdown in normal conducting accelerating structures Applied Physics Letters 97, 171501 (2010).
  28. ^ Terahertz-driven linear electron acceleration Nature Communications DOI:10.1038/ncomms9486 (2015).
  29. ^ Dielectric Wakefield Accelerators Review of Accelerator Science and Technology 9, 127 (2016).
  30. ^ Breakdown limits on gigavolt-per-meter electron-beam-driven wakefields in dielectric structures Physical Review Letters 100, 214801 (2008).
  31. ^ Observation of acceleration and deceleration in gigaelectron-volt-per-metre gradient dielectric wakefield accelerators Nature Communications DOI:10.1038/ncomms12763 (2016).
  32. ^ Terahertz radiation from electrons moving through a waveguide with variable radius, based on Smith–Purcell and Cherenkov mechanisms Nuclear Instruments and Methods in Physics Research Section B 309, 223 (2013).
  33. ^ Sub-THz radiation from dielectric capillaries with reflectors Nuclear Instruments and Methods in Physics Research Section B 402, 148 (2017).
  34. ^ Driver-witness electron beam acceleration in dielectric mm-scale capillaries Phys. Rev. Accel. Beams 21, 051301 (2018).
  35. ^ Ishigaki, K.; Shiraishi, M.; Suzuki, S.; Asada, M.; Nishiyama, N.; Arai, S. (2012). "Direct intensity modulation and wireless data transmission characteristics of terahertz-oscillating resonant tunnelling diodes". Electronics Letters. 48 (10): 582. doi:10.1049/el.2012.0849.
  36. ^ a b c d "Milestone for wi-fi with 'T-rays'". BBC News. 16 May 2012. Retrieved 16 May 2012.
  37. ^ Chacksfield, Marc (16 May 2012). "Scientists show off the future of Wi-Fi – smash through 3Gbps barrier". Tech Radar. Retrieved 16 May 2012.
  38. ^ New Chip Enables Record-Breaking Wireless Data Transmission Speed www.techcrunch.com 22 November 2011. Retrieved November 2011
  39. ^ Hu, B. B.; Nuss, M. C. (15 August 1995). "Imaging with terahertz waves". Optics Letters. 20 (16): 1716. Bibcode:1995OptL...20.1716H. doi:10.1364/OL.20.001716.
  40. ^ Chan, Wai Lam; Deibel, Jason; Mittleman, Daniel M (1 August 2007). "Imaging with terahertz radiation". Reports on Progress in Physics. 70 (8): 1325–1379. Bibcode:2007RPPh...70.1325C. doi:10.1088/0034-4885/70/8/R02.
  41. ^ Prince, Jerry L. Jr; Links, Jonathan M. (2006). Medical imaging signals and systems. Upper Saddle River, N.J.: Pearson Prentice Hall. ISBN 978-0130653536.
  42. ^ Marshall, Gerald F.; Stutz, Glenn E., eds. (2012). Handbook of optical and laser scanning (2nd ed.). Boca Raton, FL: CRC Press. ISBN 978-1439808795.
  43. ^ Ahi, Kiarash (13 May 2015). "Terahertz characterization of electronic components and comparison of terahertz imaging with X-ray imaging techniques". SPIE Sensing Technology+ Applications. Terahertz Physics, Devices, and Systems IX: Advanced Applications in Industry and Defense. 9483: 94830K–94830K–15. Bibcode:2015SPIE.9483E..0KA. doi:10.1117/12.2183128.
  44. ^ Mueckstein, Raimund; Mitrofanov, Oleg (3 February 2011). "Imaging of terahertz surface plasmon waves excited on a gold surface by a focused beam". Optics Express. 19 (4): 3212–7. Bibcode:2011OExpr..19.3212M. doi:10.1364/OE.19.003212. PMID 21369143.
  45. ^ Adam, Aurele; Brok, Janne; Seo, Min Ah; Ahn, Kwang Jun; Kim, Dai Sik; Kang, Ji-Hun; Park, Q-Han; Nagel, M.; Nagel, Paul C. M. (19 May 2008). "Advanced terahertz electric near-field measurements at sub-wavelength diameter metallic apertures: erratum". Optics Express. 16 (11): 8054. Bibcode:2008OExpr..16.8054A. doi:10.1364/OE.16.008054.
  46. ^ Kiwa, Toshihiko; Tonouchi, Masayoshi; Yamashita, Masatsugu; Kawase, Kodo (1 November 2003). "Laser terahertz-emission microscope for inspecting electrical faults in integrated circuits". Optics Letters. 28 (21): 2058. Bibcode:2003OptL...28.2058K. doi:10.1364/OL.28.002058.
  47. ^ IEEE C95.1–2005, IEEE Standard for Safety Levels With Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz
  48. ^ ANSI Z136.1–2007, American National Standard for Safe Use of Lasers
  49. ^ Alexandrov, B. S.; Gelev, V.; Bishop, A. R.; Usheva, A.; Rasmussen, K. O. (2010). "DNA Breathing Dynamics in the Presence of a Terahertz Field". Physics Letters A. 374 (10): 1214–1217. arXiv:0910.5294. Bibcode:2010PhLA..374.1214A. doi:10.1016/j.physleta.2009.12.077. PMC 2822276. PMID 20174451.
  50. ^ "How Terahertz Waves Tear Apart DNA". Technology Review. 30 October 2010. Retrieved 27 December 2010.
  51. ^ Swanson, Eric S. (2010). "Modelling DNA Response to THz Radiation". Physical Review E. 83 (4): 040901. arXiv:1012.4153. Bibcode:2011PhRvE..83d0901S. doi:10.1103/PhysRevE.83.040901. PMID 21599106.
  52. ^ A.J. Fitzgerald et al. Catalogue of Human Tissue Optical Properties at Terahertz Frequencies. Journal of Biological Physics 129: 123–128, 2003.

External links

Auston switch

An Auston switch (also known as a photoconductive switch) is an optically gated antenna that is commonly used in the generation and detection of pulsed terahertz radiation. It is named after the physicist David H. Auston who first developed the technology at Bell Labs in the 1960s.

Caltech Submillimeter Observatory

The Caltech Submillimeter Observatory (CSO) was a 10.4-meter (34 ft) diameter submillimeter wavelength telescope situated alongside the 15-meter (49 ft) James Clerk Maxwell Telescope (JCMT) at Mauna Kea Observatories. It was engaged in submillimetre astronomy, of the terahertz radiation band. The telescope closed on September 18, 2015. As of April 2019, the telescope is set to be dismantled and its site remediated in the near future as part of the Mauna Kea Comprehensive Management Plan.

Comb generator

A comb generator is a signal generator that produces multiple harmonics of its input signal. The appearance of the output at the spectrum analyzer screen, resembling teeth of a comb, gave the device its name.

Comb generators find wide range of uses in microwave technology. E.g., synchronous signals in wide frequency bandwidth can be produced by a comb generator. The most common use is in broadband frequency synthesizers, where the high frequency signals act as stable references correlated to the lower energy references; the outputs can be used directly, or to synchronize phase-locked loop oscillators. It may be also used to generate a complete set of substitution channels for testing, each of which carries the same baseband audio and video signal.

Comb generators are also used in RFI testing of consumer electronics, where their output is used as a simulated RF emissions, as it is a stable broadband noise source with repeatable output. It is also used during compliance testing to various government requirements for products such as medical devices (FDA), military electronics (MIL-STD-461), commercial avionics (Federal Aviation Administration), digital electronics (Federal Communications Commission), in the USA.

An optical comb generator can be used as generators of terahertz radiation. Internally, it is a resonant electro-optic modulator, with the capability of generating hundreds of sidebands with total span of at least 3 terahertz (limited by the optical dispersion of the lithium niobate crystal) and frequency spacing of 17 GHz. Other construction can be based on erbium-doped fiber laser or Ti-sapphire laser often in combination with carrier envelope offset control.

Electromagnetic spectrum

The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.

The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from below one hertz to above 1025 hertz, corresponding to wavelengths from thousands of kilometers down to a fraction of the size of an atomic nucleus. This frequency range is divided into separate bands, and the electromagnetic waves within each frequency band are called by different names; beginning at the low frequency (long wavelength) end of the spectrum these are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays at the high-frequency (short wavelength) end. The electromagnetic waves in each of these bands have different characteristics, such as how they are produced, how they interact with matter, and their practical applications. The limit for long wavelengths is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length. Gamma rays, X-rays, and high ultraviolet are classified as ionizing radiation as their photons have enough energy to ionize atoms, causing chemical reactions. Exposure to these rays can be a health hazard, causing radiation sickness, DNA damage and cancer. Radiation of visible light wavelengths and lower are called nonionizing radiation as they cannot cause these effects.

In most of the frequency bands above, a technique called spectroscopy can be used to physically separate waves of different frequencies, producing a spectrum showing the constituent frequencies. Spectroscopy is used to study the interactions of electromagnetic waves with matter. Other technological uses are described under electromagnetic radiation.

Indium antimonide

Indium antimonide (InSb) is a crystalline compound made from the elements indium (In) and antimony (Sb). It is a narrow-gap semiconductor material from the III-V group used in infrared detectors, including thermal imaging cameras, FLIR systems, infrared homing missile guidance systems, and in infrared astronomy. The indium antimonide detectors are sensitive between 1–5 µm wavelengths.

Indium antimonide was a very common detector in the old, single-detector mechanically-scanned thermal imaging systems. Another application is as a terahertz radiation source as it is a strong photo-Dember emitter.

Indium arsenide

Indium arsenide, InAs, or indium monoarsenide, is a semiconductor composed of indium and arsenic. It has the appearance of grey cubic crystals with a melting point of 942 °C.Indium arsenide is used for construction of infrared detectors, for the wavelength range of 1–3.8 µm. The detectors are usually photovoltaic photodiodes. Cryogenically cooled detectors have lower noise, but InAs detectors can be used in higher-power applications at room temperature as well. Indium arsenide is also used for making of diode lasers.

Indium arsenide is similar to gallium arsenide and is a direct bandgap material.

Indium arsenide is sometimes used together with indium phosphide. Alloyed with gallium arsenide it forms indium gallium arsenide - a material with band gap dependent on In/Ga ratio, a method principally similar to alloying indium nitride with gallium nitride to yield indium gallium nitride.

InAs is well known for its high electron mobility and narrow energy bandgap. It is widely used as terahertz radiation source as it is a strong photo-Dember emitter.

Quantum dots can be formed in a monolayer of indium arsenide on indium phosphide or gallium arsenide. The mismatches of lattice constants of the materials create tensions in the surface layer, which in turn leads to formation of the quantum dots. Quantum dots can also be formed in indium gallium arsenide, as indium arsenide dots sitting in the gallium arsenide matrix.

National Institute of Information and Communications Technology

The National Institute of Information and Communications Technology (情報通信研究機構, Jōhō Tsūshin Kenkyū Kikō, NICT) is Japan's primary national research institute for information and communications. It is located at 4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan.

NICT was established as an Independent Administrative Institution in 2004 when Japan's Communications Research Laboratory (established 1896) merged with the Telecommunications Advancement Organization. Today NICT's mission is to carry out research and development in the field of information and communications technology. It has a range of responsibilities including generating and disseminating Japan's national frequency and time standards; conducting type approval tests of radio equipment for the Global Maritime Distress Safety System (GMDSS) and marine radar based on Japan's Radio Law; and providing regular observations of the ionosphere and space weather. It also operates the JJY, a low frequency time signal.

In late August 2015, it was announced that a terahertz radiation scanner developed by the institute would be one of the instruments carried by the ESA's Jupiter Icy Moons Explorer, currently due for launch in 2022.

Optical rectification

Electro-optic rectification (EOR), also referred to as optical rectification, is a non-linear optical process that consists of the generation of a quasi-DC polarization in a non-linear medium at the passage of an intense optical beam. For typical intensities, optical rectification is a second-order phenomenon which is based on the inverse process of the electro-optic effect. It was reported for the first time in 1962, when radiation from a ruby laser was transmitted through potassium dihydrogen phosphate (KDP) and potassium dideuterium phosphate (KDdP) crystals.

Photomixing

Photomixing is the generation of continuous wave terahertz radiation from two lasers. The beams are mixed together and focused onto a photomixer device which generates the terahertz radiation.

It is technologically significant because there are few sources capable of providing radiation in this waveband, others include frequency multiplied electronic/microwave sources, quantum cascade laser and ultrashort pulsed lasers with photoconductive switches as used in terahertz time-domain spectroscopy. The advantages of this technique are that it is continuously tunable over the frequency range from 300 GHz to 3 THz (10 cm−1 to 100 cm−1) (1 mm to 0.1 mm), and spectral resolutions in the order of 1 MHz can be achieved. However, the achievable power is on the order of 10−8 W.

Picarin

Picarin (Tsurupica) is a plastic used to make optics such as lenses for terahertz radiation.

Sombrero Galaxy

The Sombrero Galaxy (also known as Messier Object 104, M104 or NGC 4594) is a lenticular galaxy in the constellation Virgo found 9.55 Mpc (31,100,000 ly) from Earth. The galaxy has a diameter of approximately 15kpc (50,000 light-years), 30% the size of the Milky Way. It has a bright nucleus, an unusually large central bulge, and a prominent dust lane in its inclined disk. The dark dust lane and the bulge give this galaxy the appearance of a sombrero hat. Astronomers initially thought that the halo was small and light, indicative of a spiral galaxy, but the Spitzer Space Telescope found that the dust ring around the Sombrero Galaxy is larger and more massive than previously thought, indicative of a giant elliptical galaxy. The galaxy has an apparent magnitude of +8.0, making it easily visible with amateur telescopes, and it is considered by some authors to be the galaxy with the highest absolute magnitude within a radius of 10 megaparsecs of the Milky Way. Its large bulge, its central supermassive black hole, and its dust lane all attract the attention of professional astronomers.

Submillimetre astronomy

Submillimetre astronomy or submillimeter astronomy (see spelling differences) is the branch of observational astronomy that is conducted at submillimetre wavelengths (i.e., terahertz radiation) of the electromagnetic spectrum. Astronomers place the submillimetre waveband between the far-infrared and microwave wavebands, typically taken to be between a few hundred micrometres and a millimetre. It is still common in submillimetre astronomy to quote wavelengths in 'microns', the old name for micrometre.

Using submillimetre observations, astronomers examine molecular clouds and dark cloud cores with a goal of clarifying the process of star formation from earliest collapse to stellar birth. Submillimetre observations of these dark clouds can be used to determine chemical abundances and cooling mechanisms for the molecules which comprise them. In addition, submillimetre observations give information on the mechanisms for the formation and evolution of galaxies.

Tera-

Tera is a unit prefix in the metric system denoting multiplication by 1012 or 1000000000000 (one trillion short scale; one billion long scale). It has the symbol T. Tera is derived from Greek word τέρας teras, meaning "monster". The unit prefix was confirmed for use in the International System of Units (SI) in 1960.

Examples of its use:

terahertz radiation: electromagnetic waves within the band of frequencies from 0.3 to 3 THz. Visible light is around 500 THz.

terabit and terabyte, units used in data storage.

teragram: equal to 109 kg. The Great Pyramid of Giza has a mass of about 6 Tg.

terasecond: approximately 31,558 years

teralitre: equal to 109 m3. Lake Zurich contains about 4 TL of water.

terawatt: used to measure total human energy consumption. In 2010 it was 16 TW (TJ/s).

terametre (= 1,000,000,000 km): Light travels 1.079 Tm in one hour.

TeraView

TeraView Limited, or TeraView, is a company that designs terahertz imaging and spectroscopy instruments and equipment for measurement and evaluation of pharmaceutical tablets, nanomaterials, ceramics and composites, integrated circuit chips and more.TeraView was co-founded by Michael Pepper (CSO) and Dr Don Arnone (CEO) as a spin-out of Toshiba Research Europe in April 2001. The company was set up to exploit the intellectual property and expertise developed in sourcing and detecting terahertz radiation (1 THz= 33.3 cm−1), using semiconductor technologies. Leading industry proponents of the technology sit on its Advisory Board, and TeraView maintains close links with the Cavendish Laboratory at the University of Cambridge, which was one of the research universities which had an interest in Terahertz techniques. It is also where Professor Pepper, has held the position of Professor of Physics since 1987.

Terahertz

Terahertz or THz may refer to:

Terahertz (unit), a unit of frequency, defined as one trillion (1012) cycles per second or 1012 hertz

Terahertz radiation, electromagnetic waves within the ITU-designated band of frequencies from 0.3 to 3 terahertz

Intel TeraHertz, a transistor design

Terahertz metamaterial

A terahertz metamaterial is a class of composite metamaterials designed to interact at terahertz (THz) frequencies. The terahertz frequency range used in materials research is usually defined as 0.1 to 10 THz.This bandwidth is also known as the terahertz gap because it is noticeably underutilized. This is because terahertz waves are electromagnetic waves with frequencies higher than microwaves but lower than infrared radiation and visible light. These characteristics mean that it is difficult to influence terahertz radiation with conventional electronic components and devices. Electronics technology controls the flow of electrons, and is well developed for microwaves and radio frequencies. Likewise, the terahertz gap also borders optical or photonic wavelengths; the infrared, visible, and ultraviolet ranges (or spectrums), where well developed lens technologies also exist. However, the terahertz wavelength, or frequency range, appears to be useful for security screening, medical imaging, wireless communications systems, non-destructive evaluation, and chemical identification, as well as submillimeter astronomy. Finally, as a non-ionizing radiation it does not have the risks inherent in X-ray screening.

Terahertz nondestructive evaluation

Terahertz nondestructive evaluation pertains to devices, and techniques of analysis occurring in the terahertz domain of electromagnetic radiation. These devices and techniques evaluate the properties of a material, component or system without causing damage.

Terahertz time-domain spectroscopy

In physics, terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic technique in which the properties of matter are probed with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the sample's effect on both the amplitude and the phase of the terahertz radiation. By measuring in the time-domain, the technique can provide more information than conventional Fourier-transform spectroscopy, which is only sensitive to the amplitude. Since the time-domain, and consequently the frequency-domain, of the THz signal is available, the distorting effect of the diffraction can be mitigated and the resolution of the THz images can be enhanced substantially. This resolution enhancement process is illustrated in the Figure to the right .

Terahertz tomography

Terahertz tomography is a class of tomography where sectional imaging is done by terahertz radiation. Because terahertz radiation can "see" what visible light and IR can not see, unique information can be obtained by terahertz tomography. This is in a sense similar to X-ray except without all the hazardous effects of X-ray.

Visible (optical)
Microwaves
Radio
Wavelength types

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.