Terahertz radiation

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The terahertz radiation , also submillimeter called, is a electromagnetic wave , and is located in the electromagnetic spectrum between the infrared radiation and microwaves .

Classification of terahertz radiation in the electromagnetic spectrum between infrared and microwaves

If the wavelength is less than 1 mm (= 1000 µm), its frequency range is accordingly above 300 GHz. The limits are not uniformly defined and are 0.3 THz to 6 THz, 10 THz and 30 THz.

The range of terahertz radiation is sometimes assigned to the far infrared . Terahertz radiation is in the range that heterodyne receivers almost no longer cover but optical sensors do not yet, and has therefore become the subject of intensive application developments.

properties

Since the terahertz radiation could not be used for a long time or only to a very limited extent, one also spoke of the terahertz gap in the electromagnetic spectrum. This gap is between the frequency range that was classically opened up by microwave technology and the infrared frequency range. The main problem with using the terahertz frequency range is the manufacture of transmitters and receivers. Compact and inexpensive transmitters with sufficient output power are not yet available today. The receiver technology also requires further development in order to be able to detect even weaker signals with more sensitive receivers. The detection of broadband, pulsed terahertz radiation is carried out, for example, in a pump-probe structure with photoconductive antennas or by using the electro-optical Pockels effect . Continuous terahertz radiation is detected with bolometers or with Golay cells .

Terahertz radiation penetrates many dielectric materials, for example paper, clothing or plastic as well as organic tissue, but has a  non- ionizing effect due to the low photon energy  - in the range of a few milli- electron volts . There are many molecular rotations in this energy range, which makes terahertz radiation very interesting for spectroscopy in order to identify specific substances. Water and other polar substances absorb the rays and can heat up as a result. Terahertz radiation is strongly weakened by water and reflected by metal. The absorption coefficient of water at 1 THz is 230 cm −1 .

technology

Continuous terahertz radiation

Every body emits thermal radiation , including in the terahertz range. Since this radiation is incoherent , such a transmitter must be regarded as a source of noise . In order to be able to detect the very low noise power that bodies emit according to Planck's law of radiation , highly sensitive radiometric measuring devices are used. Radiometers can be set up both uncooled and cooled (mostly at 4  K ). With cooled radiometers, superconducting mixer elements such as bolometers or SIS mixers are usually used . In uncooled radiometers also can GaAs - Schottky diodes are used.

A wide variety of transmitters are used to generate coherent terahertz radiation. In addition to the generation of terahertz radiation through frequency multiplication (mostly with the help of GaAs Schottky diodes) or differential frequency formation of two laser signals (e.g. from distributed feedback lasers ) on non-linear components, there are quantum cascade lasers , molecular gas lasers, free-electron lasers , optical parametric oscillators and Backward wave oscillators . If a high frequency tuning range is required, photo mixers ( low-temperature-grown GaAs , uni-traveling-carrier photodiodes , ni-pn-ip superlattice photodiodes) are often used, which convert the difference frequency of two lasers into alternating current, which ultimately is emitted by a suitable antenna.

Pulsed terahertz radiation

Ultrashort laser pulses with a duration of a few femtoseconds (1 fs = 10 −15  s) can  generate terahertz pulses in the picosecond range (1 ps = 10 −12 s) in semiconductors or nonlinear optical materials . These terahertz pulses consist of only one or two cycles of electromagnetic oscillation - they can also be measured time-resolved using electro-optical methods.

Applications

Spectroscopy

Terahertz spectroscopy examines substances with weak bonds, for example hydrogen bonds, or bonds with heavy binding partners, for example collective excitation of atomic groups, these are phonons in crystals .

nondestructive material test

Since many materials such as paper , plastics or ceramics are permeable to terahertz radiation, while others such as metals or water are not, terahertz imaging supplements other methods such as optical or X-ray images . It is also possible to obtain spatially resolved spectroscopic information. This makes it possible to visualize and measure defects inside a body without having to destroy it. Such methods, which are called non-invasive or anti - destructive in the medical field, have the advantage that terahertz radiation does not cause any genetic damage, which is in principle unavoidable with ionizing X - rays , when using terahertz radiation .

communication

Wireless communication (see radio network ) typically works at carrier frequencies in the microwave range . WLANs or cellular networks ( LTE-Advanced ) achieve transmission rates of several 100 Mbit / s - in principle approx. 10 Gbit / s are possible. The frequency spectrum up to 275 GHz is heavily regulated and does not offer enough unused bandwidth to meet the increasing demand (doubling every 18 months) in the future.

THz radiation is ideal because frequencies between 300 GHz and 1 THz have not yet been subject to any regulation and higher carrier frequencies can work with large bandwidths (10 ... 100 GHz) and thus enable transmission rates of more than 100 Gbit / s. Data rates of 24 Gbit / s at 300 GHz and 100 Gbit / s at 237.5 GHz (on 4 channels) have already been demonstrated. The superimposition reception technology enables the use of different carrier frequencies below 1 THz and could be of medium-term interest for commercial radio link connections (these systems are currently still too large and too expensive for private use). However, the water vapor in the atmosphere absorbs the THz rays and limits their spread. Below 1 THz there are only three frequency windows with an attenuation of less than 60 dB / km, which could be used for telecommunications. Beyond 1 THz, the absorption (of water vapor and other atmospheric gases) in the atmosphere increases too much to use this range, let alone to implement systems with high data rates. This restriction defines the possible areas of application. The attenuation in the atmosphere does not play a major role in indoor data communication and the need for higher bandwidths (including HD videos, streaming) is constantly increasing. Outside, households can be connected to the Internet (last mile) or backhaul links in the mobile area are conceivable. Another possibility is communication between satellites or a satellite-based Internet connection for aircraft. The limited range and the low distribution of receivers could make the technology interesting for military purposes with regard to interception.

In addition to the lack of compact, powerful and inexpensive sources and receivers that were previously lacking (2014), the special properties of terahertz radiation must be taken into account for a broader application. In buildings, reflections on surfaces and multilayer systems as well as scattering play a greater role than with currently used wavelengths. The strong directivity that is possible with small antennas at the same time can have advantages or disadvantages.

Security technology

The security checks at airports have been tightened after incidents in recent years and the use of terahertz wave-based body scanners promises to speed up checks and make them more reliable. Terahertz radiation appears to be very promising for these purposes: the radiation penetrates clothing and is reflected by the skin. Weapons made of metal, ceramic or plastic hidden under clothing are therefore easy to recognize. The resolution is high enough to localize the objects on the body.

When searching for explosives or drugs, unknown substances on the body or in containers could be identified, as they have characteristic absorption spectra above 500 GHz. So far, however, measurements have often only been successful under (idealized) laboratory conditions: Absorption measurements were carried out in transmission (good signal-to-noise ratio), on pure material samples or at low temperatures (sharper spectra). The challenges of a possible implementation are as follows: From 500 GHz, the atmosphere absorbs significantly more, clothing is largely transparent, but reflections occur at the interfaces and scattering occurs in the materials . If there are several layers of clothing, the signal will be very weak. In the case of mixtures of substances, the absorption spectra overlap and identification is made more difficult. The surface structure also influences the reflection behavior. That is why many scientists are extremely critical of a simple implementation.

In addition to body scanners, there are other applications in the security industry whose implementation seems realistic. Mail items could be checked for dangerous or prohibited substances, additives in explosives could provide information about the manufacturing process and help to determine their origin. Medicines could be checked through the packaging for authenticity and changes during storage.

The biggest obstacle currently (2014) is the lack of inexpensive, compact and tunable THz sources.

Biology and medicine

The large refractive index of organic tissue in the THz spectrum allows very high-contrast images and can complement conventional imaging techniques. The radiation is non-ionizing and can be used safely for medical and biological applications. Full-body scanners (analogous to CT or MRT) are not possible because the radiation is already absorbed by the skin and does not penetrate the body. When used non-invasively, the technology for diagnosis is limited to the external organs, but internal organs can be examined using endoscopic probes.

Initial studies show the potential for early detection of cancer on the surface of the skin or with probes in colon or cervical cancer. During surgical interventions to remove tumor cells, the boundary between tumor cells and healthy tissue can be made visible. Cancer cells differ from healthy body cells in their water content.

With THz rays, the extent of a burn disease can be determined much better than with current methods of burn diagnosis.

Further properties of THz rays are of interest for medical and biological applications: The coherent measurement of terahertz pulses enables the thickness of a sample to be determined by measuring the time delay when passing through the sample. The THz spectrum is in the range of many vibration and rotation transitions of organic molecules and is therefore suitable for investigating intermolecular bonds of molecular structures in vivo. The three-dimensional molecular structure is of great importance for many biochemical processes. Initial studies have already been carried out on the risks of terahertz radiation, and no changes in the genetic material could be determined. Due to their strong absorption in water, local warming can occur. An influence on enzymatic processes could be observed in cell cultures, but this cannot be directly transferred to humans.

astronomy

Terahertz radiation also opens up new possibilities in astronomy . The detection of simple chemical compounds such as carbon monoxide, water, hydrogen cyanide and many others is possible by measuring the emissions that arise during rotational transitions of the molecules in the terahertz range. Such instruments (e.g. German Receiver for Astronomy at Terahertz Frequencies , Great) are to be built into the SOFIA flying telescope . The Herschel space telescope is also equipped with appropriate instruments, but is no longer in operation.

Time-resolved measurements

Using laser excitation (femtosecond pulses of n) from semiconductors, terahertz pulses in the sub-picosecond range can be generated. They are therefore suitable for measuring physical or chemical processes on this time scale. One example is the so-called pump-probe measurement for investigating the dynamics of charge carriers in semiconductors . The change in the transmission of the terahertz pulse is measured as a function of the time that has elapsed since the excitation.

literature

  • Kiyomi Sakai: Terahertz optoelectronics . Springer, Berlin 2005, ISBN 3-540-20013-4 .
  • Daniel Mittleman: Sensing with Terahertz radiation . Springer, Berlin 2003, ISBN 3-540-43110-1 .
  • George H. Rieke: Detection of Light: From the Ultraviolet to the Submillimeter . 2nd Edition. Cambridge University Press, Cambridge 2002, ISBN 0-521-81636-X .

Web links

Individual evidence

  1. ↑ LS von Chrzanowski, J. Beckmann, B. Marchetti, U. Ewert, U. Schade: Terahertz radiation - possibilities for non-destructive testing of liquids . In: DGZfP Annual Conference 2010 - Di.3.B.2 . 2010 ( ndt.net [PDF]).
  2. ↑ H.-W. Hübers: Terahertz waves. In: World of Physics of the German Physical Society. Accessed on March 24, 2018 .
  3. ↑ https://www.mpg.de/10557881/terahertz-strahl-quelle Tobias Kampfrath: "Terahertz radiation: A source for safe food", in Research / News on the website of the Max Planck Society .
  4. ↑ Ashish Y. Pawar, Deepak D. Sonawane, Kiran B. Erande, Deelip V. Derle: Terahertz technology and its applications . In: Drug Invention Today . tape 5 , no. 2 , June 1, 2013, p. 157–163 , doi : 10.1016 / j.dit.2013.03.009 ( sciencedirect.com [accessed October 18, 2016]).
  5. ↑ Introduction to THz Wave Photonics | Xi-Cheng Zhang | Jumper . ( springer.com [accessed October 18, 2016]).
  6. ↑ Adrian Dobroiu, Chiko Otani, Kodo Kawase: terahertz wave sources and imaging applications . In: Measurement Science and Technology . tape 17 , no. 11 , September 28, 2006, p. R161-R174 , doi : 10.1088 / 0957-0233 / 17/11 / r01 ( iop.org ).
  7. ^ A b c Ho-Jin Song: Present and Future of Terahertz Communications . In: IEEE Transaction on Terahertz Science and Technology, Vol. 1, No. 1 . September 2011, p. 256–263 , doi : 10.1109 / TTHZ.2011.2159552 .
  8. ^ S. Cherry: Edholm's Law of Bandwidth . In: IEEE Spectrum, Vol. 41, No. 7 . 2004, p. 58-60 . link .
  9. ↑ a b c d Radoslaw Piesiewicz et al: Short-Range Ultra-Broadband Terahertz Communications: Concepts and Perspectives . In: IEEE Antennas and Propagation Magazine, Vol. 49, No. 6 . December 2007, p. 24-39 , doi : 10.1109 / MAP.2007.4455844 .
  10. ↑ H.-J. Song, K. Ajito, Y. Muramoto, A. Wakatsuki, T. Nagatsuma, N. Kukutsu: 24 Gbit / s data transmission in 300 GHz band for future terahertz communications . In: Electronic Letters, Vol. 48, No. 15 . July 2012, p. 953-954 , doi : 10.1049 / el.2012.1708 .
  11. Jump up ↑ S. Koenig, D. Lopez-Diaz, J. Antes, F. Boes, R. Henneberger, A. Leuther, A. Tessmann, R. Schmogrow, D. Hillerkuss, R. Palmer, T. Zwick, C. Koos , W. Freude1, O. Ambacher, J. Leuthold, I. Kallfass: Wireless sub-THz communication system with high data rate . In: Nature Photonics . October 13, 2012, doi : 10.1038 / nphoton.2013.275 .
  12. ↑ Michael J. Fitch, Robert Osiander: Terahertz Waves for Communications and Sensing . In: Johns Hopkins APL Technical Digest, Vol. 25, No. 4 . 2004, p. 348-355 . Link ( Memento from November 11, 2013 in the Internet Archive ) (PDF; 782 kB)
  13. ↑ Martin Koch: Terahertz Communications: A 2020 vision . In: Terahertz Frequency Detection and Identification of Materials and Objects . 2007, p. 325-338 . doi : 10.1007 / 978-1-4020-6503-3_18
  14. ↑ Roger Appleby: Standoff Detection of Weapons and Contraband in the 100 GHz to 1 THz Region . November 2007, IEEE Transactions on Antennas and Propagation, Vol. 55, No. 11, p. 2944-2956 , doi : 10.1109 / TAP.2007.908543 .
  15. ↑ a b c A. Giles Davis et al: Terahertz spectroscopy of explosives and drugs . In: Materials Today . Vol. 11, No. 3, March 2007, p. 18–26 ( sciencedirect.com [PDF; 621 kB ; accessed on March 24, 2018]).
  16. ↑ a b Michael C. Kemp: Explosives Detection by Terahertz Spectroscopy - A Bridge Too Far? September 2011, IEEE Transactions on Terahertz Science and Technology, Vol. 1, No. 1, p. 282–292 , doi : 10.1109 / TTHZ.2011.2159647 .
  17. ↑ C. Baker et al.: People screening using terahertz technology, Proc. SPIE, vol. 5790 . 2005, p. 1-10 . Link ( Memento from March 4, 2016 in the Internet Archive ) (PDF file; 567 kB)
  18. ↑ a b c Siegel: Terahertz technology in biology and medicine . In: Microwave Theory and Techniques, IEEE Transactions on, Vol. 52, No. 10 . 204, p. 2438-2447 , doi : 10.1109 / TMTT.2004.835916 .
  19. ↑ Yu et al: The potential of terahertz imaging for cancer diagnosis: A review of investigations to date . In: Quantitative Imaging in Medicine and Surgery, Vol. 2, No. 1 . 2012, p. 33–45 , doi : 10.3978 / j.issn.2223-4292.2012.01.04 .
  20. ↑ Tewari et al: In vivo terahertz imaging of rat skin burns . In: Journal of Biomedical Optics, Vol. 17, No. 4 . April 2012, p. 040503 , doi : 10.1117 / 1.JBO.17.4.040503 .
  21. ↑ Wilmink et al: Invited Review Article: Current State of Research on Biological Effects of Terahertz Radiation . In: Journal of Infrared, Millimeter, and Terahertz Waves, Vol. 32, No. 10 . 2011, p. 1074-1122 , doi : 10.1007 / s10762-011-9794-5 .
  22. ↑ http://www.pro-physik.de/details/articlePdf/1108825/issue.html Harald Gießen: “Snapshot in the semiconductor”, in Physik Journal 1 (2002) No. 1, page 18f