Pages in category "Terahertz technology"
The following 24 pages are in this category, out of 24 total. This list may not reflect recent changes (learn more).
The following 24 pages are in this category, out of 24 total. This list may not reflect recent changes (learn more).
1. Terahertz radiation – Photon energy in THz regime is less than band-gap of nonmetallic materials and thus THz beam can traverse through such materials. The transmitted THz beam is used for material characterization, layer inspection, 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, 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, THz 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 wavelength, images made using THz are low resolution. However, at distances of ~10 meters the band may still allow many useful applications in imaging and construction of high bandwidth wireless networking systems, terahertz radiation is emitted as part of the black-body radiation from anything with temperatures greater than about 10 kelvin. The opacity of the Earths atmosphere to submillimeter radiation restricts these observatories to very high altitude sites, and electronic oscillators based on resonant tunneling diodes have been shown to operate up to 700 GHz. 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. The group was led by Ulrich Welp of Argonnes Materials Science Division, the device uses high-temperature superconducting crystals, grown at the University of Tsukuba in Japan. This alternating current induces an electromagnetic field, even a small voltage can induce frequencies in the terahertz range, according to Welp. 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 limited their use in everyday applications. In 2009, it was discovered that the act of unpeeling adhesive tape generates non-polarized terahertz radiation, with a peak at 2 THz. In 2011, Japanese electronic parts maker Rohm and a team at Osaka University produced a chip capable of transmitting 1.5 Gbit/s using terahertz radiation. Such an antenna would broadcast in the frequency range. Unlike X-rays, terahertz radiation is not ionizing radiation and its low photon energies in general do not damage tissues, some frequencies of terahertz radiation can penetrate several millimeters of tissue with low water content and reflect back
2. Auston switch – An Auston 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, an Auston switch consist of a transmission line antenna with a gap that is bridged by a semiconductor. For terahertz generation, a DC bias voltage is applied across the antenna, instead, the incident terahertz pulse itself provides the bias field for the charge carriers during the interval when the switch is activated by the laser pulse. The induced photocurrent can then be amplified and measured, to map the entire span of the terahertz pulse, the time delay between the femtosecond pulses at generation and detection can be varied
3. 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 and it is an oscillator with a wide electronic tuning range. An electron gun generates a beam that is interacting with a slow-wave structure. It sustains the oscillations by propagating a traveling wave backwards against the beam, the generated electromagnetic wave power has its group velocity directed oppositely to the direction of motion of the electrons. The output power is coupled out near the electron gun and it has two main subtypes, the M-type, the most powerful, and the O-type. The output power of the O-type is typically in the range of 1 mW at 1000 GHz to 50 mW at 200 GHz, carcinotrons are used as powerful and stable microwave sources. Due to the good quality wavefront they produce, they use as illuminators in terahertz imaging. The backward wave oscillators were demonstrated in 1951, M-type by Bernard Epsztein, the M-type BWO is a voltage-controlled non-resonant extrapolation of magnetron interaction, both types are tunable over a wide range of frequencies by varying the accelerating voltage. They can be swept through the band fast enough to be appearing to radiate all the band at once. Carcinotrons allowed airborne radar jammers to be highly effective, however, frequency-agile radars can hop frequencies fast enough to force the jammer to use barrage jamming, diluting its output power over a wide band and significantly impairing its efficiency. Carcinotrons are used in research, civilian and military applications, for example, the Czechoslovak air defense detection systems Kopac passive sensor and Ramona passive sensor employed carcinotrons in their receiver systems. Due to the periodicity of the geometry, the fields are identical from cell to cell except for a constant phase shift Φ and this phase shift, a purely real number in a passband of a lossless structure, varies with frequency. As the magnitude of the space harmonics decreases rapidly when the value of n is large, strong interaction occurs when the phase velocity of one space harmonic of the wave is equal to the electron velocity. Both Ez and Ey components of the RF field are involved in the interaction, electrons which are in a decelerating Ez electric field of the slow-wave, lose the potential energy they have in the static electric field E and reach the circuit. The sole electrode is more negative than the cathode, in order to avoid collecting those electrons having gained energy while interacting with the slow-wave space harmonic. The O-type carcinotron, or O-type backward wave oscillator, uses an electron beam focused by a magnetic field. A collector collects the beam at the end of the tube, the BWO is a voltage tunable oscillator, whose voltage tuning rate is directly related to the propagation characteristics of the circuit. The oscillation starts at a frequency where the wave propagating on the circuit is synchronous with the space charge wave of the beam
4. Free-electron laser – A free-electron laser, is a kind of laser whose lasing medium consists of very-high-speed electrons moving freely through a magnetic structure, hence the term free electron. The free-electron laser was invented by John Madey in 1971 at Stanford University, Madey used a 43 MeV electron beam and 5 m long wiggler to amplify a signal. To create a FEL, a beam of electrons is accelerated to almost the speed of light, the beam passes through a periodic arrangement of magnets with alternating poles across the beam path, which creates a side to side magnetic field. The direction of the beam is called the direction, while the direction across the beam path is called transverse. The resulting radiation power scales linearly with the number of electrons, mirrors at each end of the undulator create an optical cavity, causing the radiation to form standing waves, or alternately an external excitation laser is provided. This energy modulation evolves into electron density modulations with a period of one optical wavelength, the electrons are thus longitudinally clumped into microbunches, separated by one optical wavelength along the axis. Whereas an undulator alone would cause the electrons to radiate independently, the emitted by the bunched electrons is in phase. The radiation intensity grows, causing additional microbunching of the electrons and this process continues until the electrons are completely microbunched and the radiation reaches a saturated power several orders of magnitude higher than that of the undulator radiation. The wavelength of the radiation emitted can be tuned by adjusting the energy of the electron beam or the magnetic-field strength of the undulators. This formula can be understood as a combination of two relativistic effects, imagine you are sitting on an electron passing through the undulator. Due to Lorentz contraction the undulator is shortened by a γ factor, however, the radiation emitted at this wavelength is observed in the laboratory frame of reference and the relativistic Doppler effect brings the second γ factor to the above formula. Rigorous derivation from Maxwells equations gives the divisor of 2 and the proportionality constant. In an X-ray FEL the typical wavelength of 1 cm is transformed to X-ray wavelengths on the order of 1 nm by γ ≈2000. In most cases, the theory of classical electromagnetism adequately accounts for the behavior of free electron lasers, for sufficiently short wavelengths, quantum effects of electron recoil and shot noise may have to be considered. Free-electron lasers require the use of an accelerator with its associated shielding. These accelerators are typically powered by klystrons, which require a voltage supply. The electron beam must be maintained in a vacuum which requires the use of vacuum pumps along the beam path. X-ray free electron lasers use long undulators, the underlying principle of the intense pulses from the X-ray laser lies in the principle of self-amplified spontaneous emission, which leads to the microbunching
5. Gunn diode – A Gunn diode, also known as a transferred electron device, is a form of diode, a two-terminal passive semiconductor electronic component, with negative resistance, used in high-frequency electronics. It is based on the Gunn effect discovered in 1962 by physicist J. B and its largest use is in electronic oscillators to generate microwaves, in applications such as radar speed guns, microwave relay data link transmitters, and automatic door openers. Its internal construction is unlike other diodes in that it consists only of N-doped semiconductor material and it therefore does not conduct in only one direction and cannot rectify alternating current like other diodes, which is why some sources do not use the term diode but prefer TED. In the Gunn diode, three regions exist, two of those are heavily N-doped on each terminal, with a layer of lightly n-doped material between. When a voltage is applied to the device, the gradient will be largest across the thin middle layer. This means a Gunn diode has a region of negative resistance in its current-voltage characteristic curve, in which an increase of applied voltage. This property allows it to amplify, functioning as a frequency amplifier, or to become unstable. A microwave oscillator can be created simply by applying a DC voltage to bias the device into its negative resistance region, the oscillation frequency is determined partly by the properties of the middle diode layer, but can be tuned by external factors. In practical oscillators an electronic resonator is usually added to control frequency, in the form of a waveguide, the diode is usually mounted inside the cavity. The diode cancels the loss resistance of the resonator, so it produces oscillations at its resonant frequency, the frequency can be tuned mechanically, by adjusting the size of the cavity, or in case of YIG spheres by changing the magnetic field. Gunn diodes are used to build oscillators in the 10 GHz to high frequency range, gallium arsenide Gunn diodes are made for frequencies up to 200 GHz, gallium nitride materials can reach up to 3 terahertz. The Gunn diode is based on the Gunn effect, and both are named for the physicist J. B, Gunn who, at IBM in 1962, discovered the effect because he refused to accept inconsistent experimental results in gallium arsenide as noise, and tracked down the cause. Alan Chynoweth, of Bell Telephone Laboratories, showed in June 1965 that only a transferred-electron mechanism could explain the experimental results, the interpretation refers to the Ridley-Watkins-Hilsum theory, whereby semiconductors display negative resistance, meaning that increasing the applied voltage causes the current to decrease. Several other books that provided the coverage were published in the intervening years. This third band is at a higher energy than the conduction band and is empty until energy is supplied to promote electrons to it. The energy comes from the energy of ballistic electrons, that is, electrons in the conduction band. This creates a region of negative resistance in the voltage/current relationship. It is not possible to balance the population in both bands, so there will always be thin slices of high strength in a general background of low field strength
6. Gyrotron – A gyrotron is a high-power linear-beam vacuum tube which generates millimeter-wave electromagnetic waves by the cyclotron resonance of electrons 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 tens of kilowatts to 1–2 megawatts. Gyrotrons can be designed for pulsed or continuous operation, the gyrotron is a type of free-electron maser which generates high-frequency electromagnetic radiation by stimulated cyclotron resonance of electrons moving through a strong magnetic field. It can produce power at millimeter wavelengths because as a fast-wave device its dimensions can be much larger than the wavelength of the radiation. The field causes the electrons to move helically in tight circles around the field lines as they travel lengthwise through the tube. At the position in the tube where the field reaches its maximum the electrons radiate electromagnetic waves in a transverse direction at their cyclotron resonance frequency. The spent electron beam is absorbed by an electrode at the end of the tube. As in other linear-beam microwave tubes, the energy of the electromagnetic waves comes from the kinetic energy of the electron beam. The orbital velocity of the electrons is 1.5 to 2 times their axial beam velocity. Due to the waves in the resonant cavity, the electrons become bunched. The electron speed in a gyrotron is slightly relativistic and this contrasts to the free-electron laser that work on different principles and whose electrons are highly relativistic. Gyrotrons are used for industrial and high-technology heating applications. Military applications include the Active Denial System, the gyrotron was invented in the Soviet Union. Present makers include Communications & Power Industries, Gycom, Thales Group, CEERI, Toshiba, System developers include Gyrotron Technology, Inc. Electron cyclotron resonance Magnetron Klystron Cyclotron Fusion power Tokamak Terahertz radiation Gyrotron Kupiszezwki, the Gyrotron, A High Frequency Microwave Amplifier. The Deep Space Network Progress Report
7. 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 can handle signals of high frequencies. It is commonly used in modern ultrafast circuits, mostly radio-frequency systems, the idea of employing a heterojunction is as old as the conventional BJT, dating back to a patent from 1951. The principal difference between the BJT and HBT is in the use of differing semiconductor materials for the emitter-base junction and the base-collector junction, creating a heterojunction. The effect is to limit the injection of holes from the base into the emitter region, unlike BJT technology, this allows a high doping density to be used in the base, reducing the base resistance while maintaining gain. Wide-bandgap semiconductors such as gallium nitride and indium gallium nitride are especially promising, in SiGe graded heterostructure transistors, the amount of germanium in the base is graded, making the bandgap narrower at the collector than at the emitter. That tapering of the leads to a field-assisted transport in the base. Due to the need to manufacture HBT devices with extremely high-doped thin base layers, the contact layer underneath the collector, named subcollector, is an active part of the transistor. Other techniques are used depending on the material system, IBM and others use UHV CVD for SiGe, other techniques used include MOVPE for III-V systems. Normally the epitaxial layers are lattice matched, if they are near-lattice-matched the device is pseudomorphic, and if the layers are unmatched it is metamorphic. Besides being record breakers in terms of speed, HBTs made of InP/InGaAs are ideal for monolithic optoelectronic integrated circuits, a PIN-type photo detector is formed by the base-collector-subcollector layers. The bandgap of InGaAs works well for detecting 1550 nm-wavelength infrared laser signals used in communication systems. Biasing the HBT to obtain a device, a photo transistor with high internal gain is obtained. Among other HBT applications are mixed signal circuits such as analog-to-digital and digital-to-analog converters