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Terahertz Radiation

 
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In physics
Physics

Physics is the natural science which examines basic concepts such as energy, force, and spacetime and all that derives from these, such as mass, charge, matter and its Motion ....
, terahertz radiation refers to electromagnetic waves sent 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
Hertz

The hertz is a measure of frequency per unit of time, or the number of list of cycles per second. It is the SI base unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts....
 range. It is also referred to as submillimeter radiation, terahertz waves, terahertz light, T-rays, T-light, T-lux and THz. The term is normally used for the region of the electromagnetic spectrum
Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible electromagnetic radiation frequencies. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation from that particular object....
 between 300 gigahertz (3x1011 Hz
Hertz

The hertz is a measure of frequency per unit of time, or the number of list of cycles per second. It is the SI base unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts....
) and 3 terahertz (3x1012 Hz), corresponding to the submillimeter wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
 range between 1 millimeter (high-frequency edge of the microwave
Microwave

Microwaves are electromagnetic radiation with wavelengths ranging from 1 mm to 1 m, or frequency between 0.3 hertz and 300 GHz....
 band) and 100 micrometer
Micrometre

A micrometre or micron is one Micro- of a metre, or equivalently one thousandth of a millimetre. It is also commonly known as a micron....
 (long-wavelength edge of far-infrared light
Light

Light, or visible light, is electromagnetic radiation of a wavelength that is Visible spectrum to the human eye , or up to 380?750 nm. In the broader field of physics, light is sometimes used to refer to electromagnetic radiation of all wavelengths, whether visible or not....
).

Introduction
Like infrared radiation or microwaves, these waves usually travel in line of sight.






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Encyclopedia


In physics
Physics

Physics is the natural science which examines basic concepts such as energy, force, and spacetime and all that derives from these, such as mass, charge, matter and its Motion ....
, terahertz radiation refers to electromagnetic waves sent 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
Hertz

The hertz is a measure of frequency per unit of time, or the number of list of cycles per second. It is the SI base unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts....
 range. It is also referred to as submillimeter radiation, terahertz waves, terahertz light, T-rays, T-light, T-lux and THz. The term is normally used for the region of the electromagnetic spectrum
Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible electromagnetic radiation frequencies. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation from that particular object....
 between 300 gigahertz (3x1011 Hz
Hertz

The hertz is a measure of frequency per unit of time, or the number of list of cycles per second. It is the SI base unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts....
) and 3 terahertz (3x1012 Hz), corresponding to the submillimeter wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
 range between 1 millimeter (high-frequency edge of the microwave
Microwave

Microwaves are electromagnetic radiation with wavelengths ranging from 1 mm to 1 m, or frequency between 0.3 hertz and 300 GHz....
 band) and 100 micrometer
Micrometre

A micrometre or micron is one Micro- of a metre, or equivalently one thousandth of a millimetre. It is also commonly known as a micron....
 (long-wavelength edge of far-infrared light
Light

Light, or visible light, is electromagnetic radiation of a wavelength that is Visible spectrum to the human eye , or up to 380?750 nm. In the broader field of physics, light is sometimes used to refer to electromagnetic radiation of all wavelengths, whether visible or not....
).

Introduction


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

Ionizing radiation consists of subatomic particle radiation or electromagnetic radiation that are energetic enough to detach electrons from atoms or molecules, ionize them....
 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

A feature of all human societies, except perhaps the most primitive, is the wearing of clothing or clothes, especially in public. The primary purpose of clothing is functional, as a protection from the weather....
, paper
Paper

Paper is thin material mainly used for writing upon, printing upon or 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
Cardboard

Corrugated fiberboard is a paper-based construction material consisting of a fluted corrugated sheet and one or two flat linerboards. It is widely used in the manufacture of corrugated boxes and shipping containers....
, wood
Wood

Wood is an organic material; in the strict sense wood is produced as secondary xylem in the stems of woody plants, notably trees but also shrubs, etc....
, masonry
Masonry

Masonry is the building of structures from individual units laid in and bound together by mortar , and the term "masonry" can also refer to the units themselves....
, plastic
Plastic

Plastic is the general common term for a wide range of synthetic or semisynthetic organic chemistry solid materials suitable for the manufacture of industrial products....
 and ceramic
Ceramic

File:Bridge from dental porcelain.jpgFile:Qing vase p1070256.jpgA ceramic is an inorganic, nonmetal solid prepared by the action of heat and subsequent cooling....
s. It can also penetrate fog
Fog

Fog is a cloud bank that is in contact with the ground. A cloud may be considered partly fog; for example, the part of a cloud that is suspended in the air above the ground is not considered fog, whereas the part of the cloud that comes in contact with higher ground is considered fog....
 and cloud
Cloud

A cloud is a visible mass of Drop or frozen crystals floating in the Celestial body atmosphere above the surface of the Earth or another planetary body....
s, but cannot penetrate metal
Metal

In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions , and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms....
 or water
Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
.

The Earth's atmosphere
Earth's atmosphere

The Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by the Earth's gravity. Dry air contains roughly 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% Carbon dioxide in the Earth's atmosphere, and trace amounts of other gases....
 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 correlation properties 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

In physics, a black body is an Physical body that absorbs all electromagnetic radiation that falls on it. No electromagnetic radiation passes through it and none is Reflection ....
 radiation from anything with temperatures greater than about 10 kelvin
Kelvin

The kelvin is a Units of measurement of temperature and is one of the seven SI base units. The Kelvin scale is a Thermodynamic temperature scale where absolute zero, the theoretical absence of all thermal energy, is zero ....
. 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 terahertz radiation of the electromagnetic spectrum....
 are important for characterizing the cold 10-20K dust in the interstellar medium
Interstellar medium

In astronomy, the interstellar medium is the gas and cosmic dust that pervade interstellar space: the matter that exists between the stars within a galaxy....
 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....
. Telescopes operating in this band include the James Clerk Maxwell Telescope
James Clerk Maxwell Telescope

The James Clerk Maxwell Telescope is a 15-metre Terahertz radiation telescope at Mauna Kea Observatory in Hawaii. It is the largest astronomical telescope in the world designed specifically to operate in the submillimetre astronomy regime ....
, 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....
 and the Submillimeter Array
Submillimeter Array

The Submillimeter Array consists of eight 6 m diameter radio telescopes arranged as an interferometer for submillimetre astronomy wavelength observations....
 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 Hawaii , United States....
 in Hawaii, the BLAST
BLAST (telescope)

The Balloon-borne Large Aperture Submillimeter Telescope was a Submillimetre astronomy telescope that hung from a high altitude balloon. It had a 2 meter primary mirror that directs light into bolometer arrays operating at 250, 350, and 500 micrometre....
 balloon borne telescope, and the Heinrich Hertz Submillimeter Telescope
Heinrich Hertz Submillimeter Telescope

The Submillimeter Telescope , formerly known as the Heinrich Hertz Submillimeter Telescope, is a submillimetre astronomy radio telescope located on Mount Graham, Arizona....
 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....
 in Arizona. Planned telescopes operating in the submillimeter include the Atacama Large Millimeter Array
Atacama Large Millimeter Array

The Atacama Large Millimeter/submillimeter Array is an international astronomy project that consists of an astronomical interferometer formed from an array of radio telescopes, located at Llano de Chajnantor Observatory in the Atacama desert in northern Chile....
 and the Herschel Space Observatory
Herschel Space Observatory

The Herschel Space Observatory is a European Space Agency mission originally proposed in 1982 by a consortium of European scientists that included Thijs de Graauw , Gisbert Winnewisser , Michael Rowan-Robinson , Glenn White , and Malcolm Longair ....
. 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 Light beams by bunching electrons with cyclotron motion in a strong magnetic field Field ....
, 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 radiation range....
 ("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 ?m . 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....
, the free electron laser
Free electron laser

A free-electron laser, or FEL, is a laser that shares the same optics properties as conventional lasers such as emitting a beam consisting of Coherence Electromagnetic radiation 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 synchrotron radiation, 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, and single-cycle sources used in Terahertz time domain spectroscopy
Terahertz time domain spectroscopy

In physics, terahertz time-domain spectroscopy is a spectroscopy technique where a special generation and detection scheme is used to probe material properties with short pulses of terahertz radiation....
. 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 Clancy Jr. is an United States author, best known for his technically detailed espionage and military science storylines set during and in the aftermath of the Cold War....
.

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 a 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 current flow across two weakly coupled superconductors, separated by a very thin insulating barrier. This arrangement?two superconductors linked by a non-conducting barrier?is known as a Josephson junction; the current that crosses the barrier is the Josephson current....
. 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 announced they had built a room temperature semiconductor source of coherent Terahertz radiation. Until then sources had required cryogenic cooling, greatly limiting their use in everyday applications.

Theoretical and technological uses under development


  • Medical imaging:
    • Terahertz radiation is non-ionizing, and thus is not expected to damage tissues and DNA
      DNA

      Deoxyribonucleic acid is a nucleic acid that contains the genetics instructions used in the development and functioning of all known living organisms and some viruses....
      , unlike X-rays. 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 density of a material is defined as its mass per unit volume. The symbol of density is ....
       of a tissue. Such methods could allow effective detection of epithelial cancer
      Cancer

      Cancer is a class of diseases in which a group of cell display uncontrolled growth , invasion , and sometimes metastasis . These three malignant properties of cancers differentiate them from benign tumors, which are self-limited, do not invade or metastasize....
       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 whitish structures found in the jaws of many vertebrates that are used to tear, scrape, and chew food. Some animals, particularly carnivores, also use teeth for hunting or defense....
       and may be more accurate and safer than conventional X-ray imaging in dentistry
      Dentistry

      Dentistry is the known evaluation, diagnosis, prevention, and treatment of diseases, disorders and conditions of the mouth, maxillofacial area and the adjacent and associated structures and their impact on the human body....
      .
  • Security:
    • Terahertz radiation can penetrate fabrics and plastics, so it can be used in surveillance
      Surveillance

      Surveillance is the monitoring of behavior. Systems surveillance is the process of monitoring the behavior of people, objects or processes within systems for conformity to expected or desired Norm in trusted systems for security or social control....
      , 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....
       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 object such as a book, a candle, a can, or something as small as a coin....
       weapon
      Weapon

      A weapon is a tool used to apply or threaten to apply force for the purpose of hunting, attack or defense in combat, subduing enemy personnel, or to destroy enemy weapons, equipment and defensive structures....
      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 was originally the study of the interaction between radiation and matter as a function of wavelength . In fact, historically, spectroscopy referred to the use of visible light dispersed according to its wavelength, e.g....
       in terahertz radiation could provide novel information in chemistry
      Chemistry

      Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
       and biochemistry
      Biochemistry

      Biochemistry is the study of the chemistry processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules....
      .
    • Recently developed methods of THz time-domain spectroscopy
      Terahertz time domain spectroscopy

      In physics, terahertz time-domain spectroscopy is a spectroscopy technique where a special generation and detection scheme is used to probe material properties with short pulses of terahertz radiation....
       (THz TDS) and THz tomography
      Tomography

      Tomography is imaging by sections or sectioning. A device used in tomography is called a tomograph, while the image produced is a tomogram....
       have been shown to be able to perform measurements on, and obtain images of, samples which are opaque in the visible and near-infrared
      Infrared

      Infrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves ....
       regions of the spectrum. The utility of THz-TDS is limited when the sample is very thin, or has a low absorbance
      Absorbance

      In spectroscopy, the absorbance A is defined as,where is the intensity of light at a specified wavelength ? that has passed through a sample and is the intensity of the light before it enters the sample or incident light intensity....
      , 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 called stimulated emission. The term laser is an acronym for light amplification by stimulated emission of radiation....
       source or experiment
      Experiment

      In scientific inquiry, an experiment is a method of investigating causal relationships among variables. An experiment is a cornerstone of the empiricism approach to acquiring data about the world and is used in both natural sciences and social sciences....
      . However, THz-TDS produces radiation that is both coherent and broadband
      Broadband

      The term broadband can have different meanings in different contexts. The term's meaning has undergone substantial shifts....
      , 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 flux density B . The tesla is equal to one weber per square metre and was defined in 1960 in honor of inventor, scientist and electrical engineer Nikola Tesla....
      ), the Larmor frequencies are in the submillimeter band. This work is performed at many high-magnetic field laboratories around the world.
    • Submillimetre astronomy
      Submillimetre astronomy

      Submillimetre astronomy or submillimeter astronomy is the branch of observational astronomy that is conducted at terahertz radiation of the electromagnetic spectrum....
      .
    • Terahertz radiation could let art historians see murals hidden beneath coats of plaster or paint in centuries-old building, without harming the artwork.
  • Communication:
    • Potential uses exist in high-altitude telecommunication
      Telecommunication

      Telecommunication is the assisted Transmission of Signal over a distance for the purpose of communication. In earlier times, this may have involved the use of smoke signals, Drum , Semaphore line, flag signals or heliograph....
      s, above altitudes where water vapor causes signal absorption: aircraft
      Aircraft

      An aircraft is a vehicle which is able to flight by being supported by the air, or in general, the atmosphere, of a planet. Examples include balloons, airplanes and helicopters....
       to satellite
      Satellite

      In the context of spaceflight, a satellite is an Physical body which has been placed into orbit by human endeavor. 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 machine, tool and labor to make things 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 Industry production, in which raw material are transformed into finished good on a large scale....
      , quality control
      Quality control

      In engineering and manufacturing, quality control and quality engineering are used in developing systems to ensure product s or Service are designed and produced to meet or exceed customer requirements....
      , and process monitoring
      Manufacturing Process Management

      Manufacturing Process Management is a collection of technology and methods used in the manufacturing and Enterprise resource planning defines when it is to be made; MPM defines how it will be made....
      . These generally exploit the traits of plastic
      Plastic

      Plastic is the general common term for a wide range of synthetic or semisynthetic organic chemistry solid materials suitable for the manufacture of industrial products....
      s and cardboard
      Paperboard

      Paperboard is a paper-like material, usually over ten mils in thickness. Some types of paperboard are used in the construction of Corrugated fiberboard....
       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....
       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 spectroscopy technique where a special generation and detection scheme is used to probe material properties with short pulses of terahertz radiation....
, while the term "submillimeter" is used for generation and detection with microwave technology, such as harmonic multiplication.

Books on millimeter and submillimeter waves and RF optics



See also

  • Terahertz time domain spectroscopy
    Terahertz time domain spectroscopy

    In physics, terahertz time-domain spectroscopy is a spectroscopy technique where a special generation and detection scheme is used to probe material properties with short pulses of terahertz radiation....
  • Heterojunction bipolar transistor
    Heterojunction bipolar transistor

    The heterojunction bipolar transistor is an improvement of the bipolar junction transistor that can handle signals of very high frequencies up to several hundred gigahertz....
  • HEMT
    HEMT

    HEMT stands for High Electron Mobility Transistor, and is also called heterostructure FET or modulation-doped FET . A HEMT is a field effect transistor incorporating a junction between two materials with different band gaps as the channel instead of a doped region, as is generally the case for MOSFETs....
  • Picarin lens
    Picarin lens

    Picarin is a plastic used to make optics such as lens es for terahertz radiation . Picarin is useful for this purpose because it is highly transparency in both the THz and visible light spectrum ranges....


External links

  • by Eric Mueller