In physics

Physics

Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

, terahertz radiation refers to electromagnetic waves propagating at frequencies

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 in the terahertz range. It is synonymously termed submillimeter radiation, terahertz waves, terahertz light, T-rays, T-waves, T-light, T-lux, THz. The term typically applies to electromagnetic radiation with frequencies between high-frequency edge of the microwave

Microwave

Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 band, 300 gigahertz (3×10¹¹ Hz

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

), and the long-wavelength edge of far-infrared light, 3000 GHz (3×10¹² Hz or 3 THz). In wavelengths, this range corresponds to 0.1 mm (or 100 μm) infrared to 1.0 mm microwave. The THz band straddles the region where electromagnetic physics can best be described by its wave-like characteristics (microwave) and its particle-like characteristics (infrared). According to some authors the THz band is also designated as Tremendously high frequency

Tremendously high frequency

Tremendously high frequency is a band designation used by some authors to refer to a decade band in the submillimeter radiation/terahertz radiation range. It follows the ITU EHF band in increasing frequency order....

 or THF.

Introduction

Like infrared radiation or microwaves, these waves usually travel in line of sight. Terahertz radiation is non-ionizing

Ionizing radiation

Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

 submillimeter microwave radiation and shares with microwaves the capability to penetrate a wide variety of non-conducting materials. Terahertz radiation can pass through clothing

Clothing

Clothing refers to any covering for the human body that is worn. The wearing of clothing is exclusively a human characteristic and is a feature of nearly all human societies...

, paper

Paper

Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....

, cardboard

Paperboard

Paperboard is a thick paper based material. While there is no rigid differentiation between paper and paperboard, paperboard is generally thicker than paper. According to ISO standards, paperboard is a paper with a basis weight above 224 g/m2, but there are exceptions. Paperboard can be single...

, wood

Wood

Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

, masonry

Masonry

Masonry is the building of structures from individual units laid in and bound together by mortar; the term masonry can also refer to the units themselves. The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone; concrete block, glass block, stucco, and...

, plastic

Plastic

A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

 and ceramic

Ceramic

A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

s. It can also penetrate fog

Fog

Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...

 and cloud

Cloud

A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

s, but cannot penetrate metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 or water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

.
The Earth's atmosphere

Earth's atmosphere

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 is a strong absorber of terahertz radiation, so the range of terahertz radiation is quite short, limiting its usefulness for communications. In addition, producing and detecting coherent

Coherence (physics)

In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....

 terahertz radiation was technically challenging until the 1990s.

Sources

Terahertz radiation is emitted as part of the black body

Black body

A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...

 radiation from anything with temperatures greater than about 10 kelvin

Kelvin

The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

. While this thermal emission is very weak, observations at these frequencies

Submillimetre astronomy

Submillimetre astronomy or submillimeter astronomy is the branch of observational astronomy that is conducted at submillimetre wavelengths of the electromagnetic spectrum. Astronomers place the submillimetre waveband between the far-infrared and microwave wavebands, typically taken to be between a...

 are important for characterizing the cold 10-20K dust in the interstellar medium

Interstellar medium

In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

 in the Milky Way galaxy, and in distant starburst galaxies

Starburst galaxy

A starburst galaxy is a galaxy in the process of an exceptionally high rate of star formation, compared to the usual star formation rate seen in most galaxies. Galaxies are often observed to have a burst of star formation after a collision or close encounter between two galaxies...

. Telescopes operating in this band include the James Clerk Maxwell Telescope

James Clerk Maxwell Telescope

The James Clerk Maxwell Telescope is a submillimetre-wavelength telescope at Mauna Kea Observatory in Hawaii. Its primary mirror is 15 metres across: it is the largest astronomical telescope that operates in submillimetre wavelengths of the electromagnetic spectrum...

, the Caltech Submillimeter Observatory

Caltech Submillimeter Observatory

The Caltech Submillimeter Observatory is a diameter submillimeter wavelength telescope situated alongside the James Clerk Maxwell Telescope at Mauna Kea Observatory. It is engaged in submillimeter astronomy, of the terahertz radiation band.The CSO and JCMT were combined to form the first...

 and the Submillimeter Array

Submillimeter Array

The Submillimeter Array consists of eight diameter radio telescopes arranged as an interferometer for submillimeter wavelength observations. It is the first purpose-built submillimeter interferometer, constructed after successful interferometry experiments using the pre-existing James Clerk...

 at the Mauna Kea Observatory

Mauna Kea Observatory

The Observatories at Mauna Kea, , are an independent collection of astronomical research facilities located on the summit of Mauna Kea on the Big Island of Hawai'i, USA. The facilities are located in a special land use zone known as the "Astronomy Precinct," which is located in the Mauna Kea...

 in Hawaii, the BLAST

BLAST (telescope)

The Balloon-borne Large Aperture Submillimeter Telescope is a submillimeter telescope that hangs from a high altitude balloon. It has a 2 meter primary mirror that directs light into bolometer arrays operating at 250, 350, and 500 µm. These arrays were developed for the SPIRE instrument on the...

 balloon borne telescope, the Herschel Space Observatory

Herschel Space Observatory

The Herschel Space Observatory is a European Space Agency space observatory sensitive to the far infrared and submillimetre wavebands. It is the largest space telescope ever launched, carrying a single mirror of in diameter....

, and the Heinrich Hertz Submillimeter Telescope

Heinrich Hertz Submillimeter Telescope

The Submillimeter Telescope , formerly known as the Heinrich Hertz Submillimeter Telescope, is a submillimeter wavelength radio telescope located on Mount Graham, Arizona. It is a 10-meter-wide parabolic dish inside a building to protect it from bad weather. The building front doors and roof are...

 at the Mount Graham International Observatory

Mount Graham International Observatory

Mount Graham International Observatory is a division of Steward Observatory the research arm for the Department of Astronomy at The University of Arizona. It is located in southeast Arizona's Pinaleno Mountains near Mount Graham....

 in Arizona. The Atacama Large Millimeter Array

Atacama Large Millimeter Array

The Atacama Large Millimeter/sub-millimeter Array is an array of radio telescopes in the Atacama desert of northern Chile. Since a high and dry site is crucial to millimeter wavelength operations, the array is being constructed on the Chajnantor plateau at 5000 metres altitude...

, under construction, will operate in the submillimeter range. The opacity of the Earth's atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, or to space.

the only viable sources of terahertz radiation were:

• the gyrotron
  Gyrotron
  Gyrotrons are high powered vacuum tubes which emit millimeter-wave beams by bunching electrons with cyclotron motion in a strong magnetic field. Output frequencies range from about 20 to 250 GHz, covering wavelengths from microwave to the edge of the terahertz gap. Typical output powers range from...
  
  
• the backward wave oscillator
  Backward wave oscillator
  A backward wave oscillator , also called carcinotron or backward wave tube, is a vacuum tube that is used to generate microwaves up to the terahertz range. It belongs to the traveling-wave tube family...
  
   ("BWO")
• the far infrared laser
  Far infrared laser
  Far infrared laser is a laser with output wavelength in far infrared part of the electromagnetic spectrum, between 30-1000 µm . It is one of the possible sources of terahertz radiation....
  
   ("FIR laser")
• quantum cascade laser
  Quantum cascade laser
  Quantum cascade lasers are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jerome Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, and Alfred Cho at Bell Laboratories in 1994.Unlike typical...
  
  
• the free electron laser
  Free electron laser
  A free-electron laser, or FEL, is a laser that shares the same optical properties as conventional lasers such as emitting a beam consisting of coherent electromagnetic radiation which can reach high power, but which uses some very different operating principles to form the beam...
  
   (FEL)
• synchrotron light
  Synchrotron light
  A synchrotron light source is a source of electromagnetic radiation produced by a synchrotron, which is artificially produced for scientific and technical purposes by specialized particle accelerators, typically accelerating electrons...
  
   sources
• photomixing
  Photomixing
  Photomixing is the generation of continuous wave terahertz radiation from two lasers. The beams are mixed together and focussed onto a photomixer device which generates the terahertz radiation....
  
   sources
• single-cycle sources used in terahertz time domain spectroscopy
  Terahertz time domain spectroscopy
  In physics, terahertz time-domain spectroscopy is a spectroscopic technique in which the properties of a material are probed with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the sample material's effect on both the amplitude and the phase of the...
  
   such as photoconductive, surface field, photo-Dember
  Photo-dember
  In semiconductor physics, the photo-Dember effect consists in the formation of a charge dipole in the vicinity of a semiconductor surface after ultra-fast photo-generation of charge carriers...
  
   and optical rectification emitters.

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, and sparked a rapid growth in the field of terahertz science and technology. This excitement, along with the associated coining of the term "T-rays", even showed up in a contemporary novel by Tom Clancy

Tom Clancy

Thomas Leo "Tom" Clancy, Jr. is an American author, best known for his technically detailed espionage, military science, and techno thriller storylines set during and in the aftermath of the Cold War, along with video games on which he did not work, but which bear his name for licensing and...

.

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 can lead to portable, battery-operated sources of T-rays, or terahertz radiation. The group was led by Ulrich Welp of Argonne's Materials Science Division. This new T-ray source uses high-temperature superconducting crystals grown at the University of Tsukuba, Japan. These crystals comprise stacks of Josephson junctions that exhibit a unique electrical property: When an external voltage is applied, an alternating current will flow back and forth across the junctions at a frequency proportional to the strength of the voltage; this phenomenon is known as the Josephson effect

Josephson effect

The Josephson effect is the phenomenon of supercurrent across two superconductors coupled by a weak link...

. These alternating currents then produce electromagnetic fields whose frequency is tuned by the applied voltage. Even a small voltage – around two millivolts per junction – can induce frequencies in the terahertz range, according to Welp.

In 2008, engineers at Harvard University demonstrated that room temperature emission of several hundred nanowatts of coherent terahertz radiation could be achieved with a semiconductor source. THz radiation was generated by nonlinear mixing of two modes in a mid-infrared quantum cascade laser. Until then, sources had required cryogenic cooling, greatly limiting their use in everyday applications.

In 2009, it was shown that T-waves are produced when unpeeling adhesive tape. The observed spectrum of this terahertz radiation exhibits a peak at 2 THz and a broader peak at 18 THz. The radiation is not polarized. The mechanism of terahertz radiation is tribocharging

Triboelectric effect

The triboelectric effect is a type of contact electrification in which certain materials become electrically charged after they come into contact with another different material and are then separated...

 of the adhesive tape and subsequent discharge.

In 2011, Japanese electronic parts maker Rohm and a research team at Osaka University produced a chip capable of transmitting 1.5 Gbps using terahertz radiation.

Research

• Medical imaging:
  • Contrary to X-ray
    X-ray
    X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
    
    s, terahertz radiation has a relatively low photon energy for damaging tissue
    Tissue (biology)
    Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
    
    s and DNA
    DNA
    Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
    
    . 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
    Density
    The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
    
     of a tissue. Such methods could allow effective detection of epithelial cancer
    Cancer
    Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
    
     with a safer and less invasive or painful system using imaging.
  • Some frequencies of terahertz radiation can be used for 3D imaging of teeth
    Tooth
    Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...
    
     and may be more accurate than conventional X-ray imaging in dentistry
    Dentistry
    Dentistry is the branch of medicine that is involved in the study, diagnosis, prevention, and treatment of diseases, disorders and conditions of the oral cavity, maxillofacial area and the adjacent and associated structures and their impact on the human body. Dentistry is widely considered...
    
    .
• Security:
  • Terahertz radiation can penetrate fabrics and plastics, so it can be used in surveillance
    Surveillance
    Surveillance is the monitoring of the behavior, activities, or other changing information, usually of people. It is sometimes done in a surreptitious manner...
    
    , such as security
    Airport security
    Airport security refers to the techniques and methods used in protecting airports and aircraft from crime.Large numbers of people pass through airports. This presents potential targets for terrorism and other forms of crime due to the number of people located in a particular location...
    
     screening, to uncover concealed
    Concealment device
    Concealment devices or diversion safes are used to hide things for the purpose of secrecy or security. They are made from an ordinary household object such as a book, a soda can, a candle, a can, or something as small as a coin...
    
     weapon
    Weapon
    A weapon, arm, or armament is a tool or instrument used with the aim of causing damage or harm to living beings or artificial structures or systems...
    
    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. 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.
• Scientific use and imaging:
  • Spectroscopy
    Spectroscopy
    Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
    
     in terahertz radiation could provide novel information in chemistry
    Chemistry
    Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
    
     and biochemistry
    Biochemistry
    Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
    
    .
  • Recently developed methods of THz time-domain spectroscopy
    Terahertz time domain spectroscopy
    In physics, terahertz time-domain spectroscopy is a spectroscopic technique in which the properties of a material are probed with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the sample material's effect on both the amplitude and the phase of the...
    
     (THz TDS) and THz tomography
    Tomography
    Tomography refers to imaging by sections or sectioning, through the use of any kind of penetrating wave. A device used in tomography is called a tomograph, while the image produced is a tomogram. The method is used in radiology, archaeology, biology, geophysics, oceanography, materials science,...
    
     have been shown to be able to perform measurements on, and obtain images of, samples that are opaque in the visible and near-infrared
    Infrared
    Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
    
     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
    Laser
    A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
    
     source or experiment
    Experiment
    An experiment is a methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results...
    
    . 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.
  • A primary use of submillimeter waves in physics is the study of condensed matter in high magnetic fields, since at high fields (over about 15 teslas
    Tesla (unit)
    The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
    
    ), the Larmor frequencies are in the submillimeter band. Many high-magnetic field laboratories perform this work, such as the National High Magnetic Field Laboratory (NHMFL) in Florida.
  • Submillimetre astronomy
    Submillimetre astronomy
    Submillimetre astronomy or submillimeter astronomy is the branch of observational astronomy that is conducted at submillimetre wavelengths of the electromagnetic spectrum. Astronomers place the submillimetre waveband between the far-infrared and microwave wavebands, typically taken to be between a...
    
    .
  • Terahertz radiation could let art historians see murals hidden beneath coats of plaster or paint in centuries-old buildings, without harming the artwork.
• Communication:
  • Potential uses exist in high-altitude telecommunication
    Telecommunication
    Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
    
    s, above altitudes where water vapor causes signal absorption: aircraft
    Aircraft
    An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
    
     to satellite
    Satellite
    In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
    
    , or satellite to satellite.
• Manufacturing:
  • Many possible uses of terahertz sensing and imaging are proposed in manufacturing
    Manufacturing
    Manufacturing is the use of machines, tools and labor to produce goods for use or sale. The term may refer to a range of human activity, from handicraft to high tech, but is most commonly applied to industrial production, in which raw materials are transformed into finished goods on a large scale...
    
    , quality control
    Quality control
    Quality control, or QC for short, is a process by which entities review the quality of all factors involved in production. This approach places an emphasis on three aspects:...
    
    , and process monitoring
    Manufacturing Process Management
    Manufacturing process management is a collection of technologies and methods used to define how products are to be manufactured. MPM differs from ERP/MRP which is used to plan the ordering of materials and other resources, set manufacturing schedules, and compile cost data.A cornerstone of MPM is...
    
    . These in general exploit the traits of plastic
    Plastic
    A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
    
    s and cardboard
    Paperboard
    Paperboard is a thick paper based material. While there is no rigid differentiation between paper and paperboard, paperboard is generally thicker than paper. According to ISO standards, paperboard is a paper with a basis weight above 224 g/m2, but there are exceptions. Paperboard can be single...
    
     being transparent to terahertz radiation, making it possible to inspect packaged
    Packaging and labelling
    Packaging is the science, art, and technology of enclosing or protecting products for distribution, storage, sale, and use. Packaging also refers to the process of design, evaluation, and production of packages. Packaging can be described as a coordinated system of preparing goods for transport,...
    
     goods.

Terahertz versus submillimeter waves

The terahertz band, covering the wavelength range between 0.1 and 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

Terahertz time domain spectroscopy

In physics, terahertz time-domain spectroscopy is a spectroscopic technique in which the properties of a material are probed with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the sample material's effect on both the amplitude and the phase of the...

, while the term "submillimeter" is used for generation and detection with microwave technology, such as harmonic multiplication.

Safety

The terahertz region is between the radio frequency region and the optical region generally associated with lasers. Both the IEEE RF safety standard and the ANSI Laser safety standard 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 performed mathematical models how terahertz radiation interact with double-stranded DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

, showing that, even though involved forces seem to be tiny, nonlinear resonance

Nonlinear resonance

In physics, nonlinear resonance is the occurrence of resonance in a nonlinear system. In nonlinear resonance the system behaviour – resonance frequencies and modes – depends on the amplitude of the oscillations, while for linear systems this is independent of amplitude.-Description:Generically two...

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

Gene expression

Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...

 and DNA replication". 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.

Books on millimeter and submillimeter waves and RF optics

• Quasioptical systems: Gaussian beam quasioptical propagation and applications, Paul F. Goldsmith, IEEE Press
• Millimeter wave spectroscopy of solids, edited by G. Grüner, Springer
• Detection of light: from the ultraviolet to the submillimeter, George Rieke, Cambridge
• Modern millimeter-wave technologies, Tasuku Teshirogi and Tsukasa Yoneyama, eds, IOS press
• Optoelectronic techniques for microwave and millimeter-wave engineering William Robertson, Artech
• Principles of Terahertz Science and Technology, Yun-Shik Lee, Springer

See also

• Full body scanner
• Terahertz time domain spectroscopy
  Terahertz time domain spectroscopy
  In physics, terahertz time-domain spectroscopy is a spectroscopic technique in which the properties of a material are probed with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the sample material's effect on both the amplitude and the phase of the...
  
  
• Heterojunction bipolar transistor
  Heterojunction bipolar transistor
  The heterojunction bipolar transistor is a type of bipolar junction transistor which uses differing semiconductor materials for the emitter and base regions, creating a heterojunction. The HBT improves on the BJT in that it that can handle signals of very high frequencies, up to several hundred...
  
  
• High electron mobility transistor (HEMT)
• Picarin lens
  Picarin lens
  Picarin is a plastic used to make optics such as lenses for terahertz radiation . Picarin is useful for this purpose because it is highly transparent in both the THz and visible spectral ranges. The refractive index of Picarin is almost the same for THz and visible light...
  
  

External links

• Terahertz radiation: applications and sources by Eric Mueller