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 backTerahertz radiation – Terahertz waves lie at the far end of the infrared band, just before the start of the microwave band.
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 variedAuston switch – Typical time response of a photocurrent generated with an Auston switch using a femtosecond laser pulse.
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 beamBackward-wave oscillator – Miniature O-type backward-wave oscillator tube produced by Varian in 1956. It could be voltage-tuned over an 8.2 - 12.4 GHz range and required a supply voltage of 600V.
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 microbunchingFree-electron laser – The free-electron laser FELIX at the FOM Institute for Plasma Physics Rijnhuizen (nl), Nieuwegein, The Netherlands.
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 strengthGunn diode – A Russian-made Gunn diode
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 ReportGyrotron – High-power 84–118 GHz gyrotron tubes for plasma heating in the TCV tokamak fusion reactor, Switzerland. The colored cylinders are the tops of the gyrotron tubes.
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 convertersHeterojunction bipolar transistor – Bands in graded heterojunction npn bipolar transistor. Barriers indicated for electrons to move from emitter to base, and for holes to be injected backward from base to emitter; Also, grading of bandgap in base assists electron transport in base region; Light colors indicate depleted regions.
8. High-electron-mobility transistor – A commonly used material combination is GaAs with AlGaAs, though there is wide variation, dependent on the application of the device. Devices incorporating more indium generally show better performance, while in recent years. Like other FETs, HEMTs are used in integrated circuits as digital on-off switches, FETs can also be used as amplifiers for large amounts of current using a small voltage as a control signal. Both of these uses are possible by the FET’s unique current-voltage characteristics. They are widely used in receivers, in low power amplifiers. The invention of the HEMT is usually attributed to Takashi Mimura, in America, Ray Dingle and Horst Störmer also played an important role in the invention of the HEMT. Daniel Delagebeaudeuf and Trong Linh Nuyen from Thomson-CSF filed for a patent of this device in March 1979 HEMTs are heterojunctions and this means that the semiconductors used have dissimilar band gaps. For instance, silicon has a gap of 1.1 electron volts. When a heterojunction is formed, the band and valence band throughout the material must bend in order to form a continuous level. HEMTs exceptional carrier mobility and switching speed come from the following conditions, the wide band element is doped with donor atoms, thus it has excess electrons in its conduction band. These electrons will diffuse to the adjacent narrow band material’s conduction band due to the availability of states with lower energy, the movement of electrons will cause a change in potential and thus an electric field between the materials. The electric field will push back to the wide band element’s conduction band. The diffusion process continues until electron diffusion and electron drift balance each other, note that the undoped narrow band gap material now has excess majority charge carriers. An important aspect of HEMTs is that the band discontinuities across the conduction and this allows the type of carriers in and out of the device to be controlled. This diffusion of carriers leads to the accumulation of electrons along the boundary of the two regions inside the band gap material. The accumulation of electrons leads to a high current in these devices. The accumulated electrons are known as 2DEG or two-dimensional electron gas. The term modulation doping refers to the fact that the dopants are spatially in a different region from the current carrying electrons and this technique was invented by Horst Störmer at Bell LabsHigh-electron-mobility transistor – Band diagram of GaAs/AlGaAs heterojunction -based HEMT, at equilibrium.
9. Optical rectification – For typical intensities, optical rectification is a second-order phenomenon which is based on the inverse process of the electro-optic effect. It was reported for the first time in 1962, when radiation from a laser was transmitted through potassium dihydrogen phosphate. If the latter is represented by a wave, then an average DC polarization will be generated. Optical rectification is analogous to the electric rectification effect produced by diodes, however, it is not the same thing. A diode can turn a sinusoidal electric field into a DC current, while optical rectification can turn a sinusoidal electric field into a DC polarization, on the other hand, a changing polarization is a kind of current. But again, if the intensity is constant, optical rectification cannot cause a DC current. When the applied field is delivered by a femtosecond-pulse-width laser. The mixing of different frequency components produces a beating polarization, which results in the emission of electromagnetic waves in the terahertz region, a popular material for generating radiation in the 0. 5–3 THz range is zinc telluride. Optical rectification also occurs on surfaces by similar effect as surface second harmonic generation. The effect is however influenced e. g. by nonequilibrium electron excitation, similar to other nonlinear optical processes, optical rectification is also reported to become enhanced when surface plasmons are excited on a metal surface. Together with carrier acceleration in semiconductors and polymers, optical rectification is one of the mechanisms for the generation of terahertz radiation using lasers. This is different from other processes of generation such as polaritonics where a polar lattice vibration is thought to generate the terahertz radiationOptical rectification – A schematic of an ionic crystal with no applied electric field (top), and with a sinusoidal electric field caused by a light wave (bottom). The blurryness indicates the sinusoidal oscillation of the ions. The red arrow indicates optical rectification: The oscillating electric field causes a shift of the ions' average positions, which in turn changes the crystal's DC polarization.
10. Quantum cascade laser – The two energy bands are separated by an energy band gap in which there are no permitted states available for electrons to occupy. Conventional semiconductor laser diodes generate light by a photon being emitted when a high energy electron in the conduction band recombines with a hole in the valence band. The energy of the photon and hence the emission wavelength of laser diodes is therefore determined by the gap of the material system used. A QCL however does not use bulk semiconductor materials in its active region. Instead it consists of a series of thin layers of varying material composition forming a superlattice. This is referred to as multiple quantum well confinement and leads to the splitting of the band of permitted energies into a number of discrete electronic subbands. By suitable design of the layer thicknesses it is possible to engineer a population inversion between two subbands in the system which is required in order to achieve laser emission. Additionally, in laser diodes, electrons and holes are annihilated after recombining across the band gap. QCLs are typically based upon a three-level system, each subband contains a number of electrons n i which scatter between levels with a lifetime τ i f, where i and f are the initial and final subband indices. If the total density of carriers N2 D in the system is also known, then the absolute population of carriers in each subband may be determined using. As an approximation, it can be assumed all the carriers in the system are supplied by doping. If the dopant species has a negligible ionisation energy then N2 D is approximately equal to the doping density, the scattering rates are tailored by suitable design of the layer thicknesses in the superlattice which determine the electron wave functions of the subbands. The scattering rate between two subbands is heavily dependent upon the overlap of the functions and energy spacing between the subbands. The figure shows the wave functions in a three quantum well QCL active region and injector, in order to decrease W32, the overlap of the upper and lower laser levels is reduced. This is known as a diagonal transition, a vertical transition is one in which the upper laser level is localised in mainly the central and right-hand wells. This increases the overlap and hence W32 which reduces the population inversion, the first QCL was fabricated in the InGaAs/InAlAs material system lattice-matched to an InP substrate. This particular material system has a conduction band offset of 520 meV and these InP-based devices have reached very high levels of performance across the mid-infrared spectral range, achieving high power, above room-temperature, continuous wave emission. In 1998 GaAs/AlGaAs QCLs were demonstrated by Sirtori et al. proving that the QC concept is not restricted to one material system and this material system has a varying quantum well depth depending on the aluminium fraction in the barriersQuantum cascade laser – End view of QC facet with ridge waveguide. Darker gray: InP, lighter gray: QC layers, black: dielectric, gold: Au coating. Ridge ~ 10 um wide.
11. Resonant-tunneling diode – A resonant-tunneling diode is a diode with a resonant-tunneling structure in which electrons can tunnel through some resonant states at certain energy levels. The current–voltage characteristic often exhibits negative differential resistance regions, all types of tunneling diodes make use of quantum mechanical tunneling. Characteristic to the relationship of a tunneling diode is the presence of one or more negative differential resistance regions. Tunneling diodes can be compact and are also capable of ultra-high-speed operation because the quantum tunneling effect through the very thin layers is a very fast process. One area of research is directed toward building oscillators and switching devices that can operate at terahertz frequencies. One type of RTDs is formed as a quantum well structure surrounded by very thin layer barriers. This structure is called a double barrier structure, carriers such as electrons and holes can only have discrete energy values inside the quantum well. When a voltage is placed across an RTD, a wave is emitted. As voltage is increased, the wave dies out because the energy value in the quantum well is outside the emitter side energy. Another feature seen in RTD structures is the resistance on application of bias as can be seen in the image generated from Nanohub. The forming of negative resistance will be examined in detail in operation section below and this structure can be grown by molecular beam heteroepitaxy. GaAs and AlAs in particular are used to form this structure, alAs/InGaAs or InAlAs/InGaAs can be used. The operation of circuits containing RTDs can be described by a Liénard system of equations. The following process is illustrated from rightside figure. Depending on the number of barriers and number of confined states inside the well, for low bias, as the bias increase, the 1st confined state between the potential barriers is getting closer to the source Fermi level, so the current it carries increases. As bias increases further, the 1st confined state becomes lower in energy and gradually goes into the range of bandgap. At this time, the 2nd confined state is too high above in energy to conduct significant current. Similar to the first region, as the 2nd confined state becomes closer and closer to source Fermi level, it more currentResonant-tunneling diode – Typical structure of a Si/SiGe resonant interband tunneling diode
12. Terahertz metamaterial – A terahertz metamaterial is a class of composite metamaterials designed to interact at terahertz frequencies. The terahertz frequency range used in research is usually defined as 0.1 to 10 THz. This bandwidth is also known as the gap because it is noticeably underutilized. This is because terahertz waves are waves with frequencies higher than microwaves but lower than infrared radiation. These characteristics mean that it is difficult to influence terahertz radiation with conventional electronic components, electronics technology controls the flow of electrons, and is well developed for microwaves and radio frequencies. Likewise, the gap also borders optical or photonic wavelengths, the infrared, visible, and ultraviolet ranges. Finally, as a non-ionizing radiation it does not have the risks inherent in X-ray screening, currently, a fundamental lack in naturally occurring materials that allow for the desired electromagnetic response has led to constructing new artificial composite materials, termed metamaterials. The metamaterials are based on a structure which mimics crystal structures. However, the structure of this new material consists of rudimentary elements much larger than atoms or single molecules. Yet, the interaction achieved is below the dimensions of the radiation wave. In addition, the results are based on the resonant frequency of fabricated fundamental elements. The appeal and usefulness is derived from a resonant response that can be tailored for specific applications, or the response can be as a passive material. The development of electromagnetic, artificial-lattice structured materials, termed metamaterials, has led to the realization of phenomena that cannot be obtained with natural materials. This is observed, for example, with a glass lens. In other words, light consists of a field and magnetic field. The interaction of a lens, or other natural materials. The magnetic interaction in lens material is essentially nil and this results in common optical limitations such as a diffraction barrier. Moreover, there is a lack of natural materials that strongly interact with lights magnetic fieldTerahertz metamaterial – Section of a terahertz, folded-waveguide traveling-wave tube circuit with hole arrays on walls. Image from NASA Glenn Research Center.
13. 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 materials effect on both the amplitude and the phase of the terahertz radiation, in this respect, the technique can provide more information than conventional Fourier-transform spectroscopy, which is only sensitive to the amplitude. Typically, the pulses are generated by an ultrashort pulsed laser. A single pulse can contain frequency components covering much of the terahertz range, by repeating this procedure and varying the timing of the gating laser pulse, it is possible to scan the THz pulse and reconstruct its electric field as a function of time. Subsequently, a Fourier transform is performed to extract the frequency spectrum from the time-domain data, also, many interesting materials have unique spectral fingerprints in the terahertz range, which means that terahertz radiation can be used to identify them. Since many materials are transparent to THz radiation, these items of interest can be observed through visually opaque intervening layers, though not strictly a spectroscopic technique, the ultrashort width of the THz radiation pulses allows for measurements on difficult to probe materials. The measurement capability shares many similarities to that observed with pulsed ultrasonic systems, reflections from buried interfaces and defects can be found and precisely imaged. There are three widely used techniques for generating terahertz pulses, all based on ultrashort pulses from lasers or mode-locked fiber lasers. When an ultra-short optical pulse illuminates a semiconductor and its wavelength is above the energy band-gap of the material, given that absorption of the pulse is an exponential process, most of the carriers are generated near the surface. Firstly, it generates a band bending, which has the effect of accelerating carriers of different signs in opposite directions, creating a dipole, this effect is known as surface field emission. Secondly, the presence of the surface creates a break of symmetry. In a photoconductive emitter, the laser pulse creates carriers in a semiconductor material. Effectively, the semiconductor changes abruptly from being an insulator into being a conductor and this conduction leads to a sudden electric current across a biased antenna patterned on the semiconductor. This changing current emits terahertz radiation, similar to what happens in the antenna of a radio transmitter, typically the two antenna electrodes are patterned on a low temperature gallium arsenide, semi-insulating gallium arsenide, or other semiconductor substrate. In a commonly used scheme, the electrodes are formed into the shape of a dipole antenna with a gap of a few micrometers and have a bias voltage up to 40 V between them. The ultrafast laser pulse must have a wavelength that is enough to excite electrons across the bandgap of the semiconductor substrate. This scheme is suitable for illumination with a Ti, sapphire laser with pulse energies of about 10 nJ. For use with amplified Ti, sapphire lasers with pulse energies of about 1 mJ, more recent advances towards cost-efficient and compact THz-TDS systems are based on mode-locked fiber lasers sources emitting at a center wavelength of 1550 nmTerahertz time-domain spectroscopy – Typical pulse as measured with THz-TDS.
14. TeraView – TeraView was co-founded by Michael Pepper and Dr Don Arnone as a spin-out of Toshiba Research Europe in April 2001. The company was set up to exploit the property and expertise developed in sourcing and detecting terahertz radiation. It is also where Professor Pepper, has held the position of Professor of Physics since 1987, teraView has developed a number of instruments that harness the properties of terahertz radiation. Terahertz light has some interesting application, many common materials and living tissues are semi-transparent and have ‘terahertz fingerprints’, permitting them to be imaged, identified, and analyzed. Moreover, the properties of terahertz radiation and the relatively low power levels used. TPS Spectra 3000 - spectrometer with modular sample compartment for transmission, attenuated total reflection analysis, cryostats, variable temperature cells, spectral range –0.06 THz to 3.6 THz. CW Spectra 400 - Continuous wave terahertz spectrometer with imaging capabilities that utilizes GaAs-based photomixers, by applying different software analysis packages, the same base technologies can be brought to bear to several applications. The companys primary focus of investigation includes the development of light into a usefulspectroscopic. TeraViews existing instruments generate, detect and manipulate THz light and have been tested in a number of application areas, due in part to its ability to recognize spectral fingerprints, terahertz pulsed imaging can be applied to provide contrast between different types of soft tissue. Also, it is a means of detecting the degree of water content. Attempts have been made to apply Terahertz to image cancers like breast, cancer as well as other diseases in medicine, oral health care, the company announced it has been cleared by the Medicines and Healthcare products Regulatory Agency to trial in-vivo terahertz spectroscopy for bio-medical research. The trials will be held in Guys Hospital in London and aim to determine if the technology can be applied real-time for accurate removal of cancer tissue, Terahertz technology has the potential to safely, noninvasively and quickly image through different types of clothing and other concealment and confusion materials. This has never been proved in a practical sense, the companys technology has been used by the Naval Surface Warfare Command to test the presence of different types of plastic explosives through clothing, including PETN. THz spectroscopy can be used as an analytical method. Terahertz light can be used as technique for analysis in material integrity studies. The time of arrival is measured and then various algorithms complete the picture by developing 3D fine feature images and precise material identifications. Further research by the company and active collaboration with the University of Cambridge is aiming to develop a sensor that can be used to measure the quality of paint coatings on cars. Terahertz technology allows high resolution 3D imaging of semiconductor packages and integrated circuit devices, THz time-domain reflectometry offers significant advantages in imaging resolution compared to existing fault isolation techniques and conventional millimetre wave systemsTeraView – TeraView Ltd
15. Zinc telluride – Zinc telluride is a binary chemical compound with the formula ZnTe. This solid is a material with a direct band gap of 2.26 eV. It is usually a p-type semiconductor and its crystal structure is cubic, like that for sphalerite and diamond. ZnTe has the appearance of grey or brownish-red powder, or ruby-red crystals when refined by sublimation, zinc telluride typically had a cubic crystal structure, but can be also prepared as hexagonal crystals. Irradiated by an optical beam burns in presence of oxygen. Its lattice constant is 0.6101 nm, allowing it to be grown with or on aluminium antimonide, gallium antimonide, indium arsenide, in the wurtzite crystal structure, it has lattice parameters a =0.427 and c =0.699 nm. Zinc telluride can be easily doped, and for this reason it is one of the more common semiconducting materials used in optoelectronics, ZnTe is important for development of various semiconductor devices, including blue LEDs, laser diodes, solar cells, and components of microwave generators. It can be used for cells, for example, as a back-surface field layer. The material can also be used as a component of ternary compounds, such as CdxZnTe. Zinc telluride together with lithium niobate is used for generation of pulsed terahertz radiation in time-domain terahertz spectroscopy. Vanadium-doped zinc telluride, ZnTe, V, is a non-linear optical photorefractive material of use in the protection of sensors at visible wavelengths. ZnTe, V optical limiters are light and compact, without complicated optics of conventional limiters, ZnTe, V can block a high-intensity jamming beam from a laser dazzler, while still passing the lower-intensity image of the observed scene. It can also be used in interferometry, in reconfigurable optical interconnections. It offers superior performance at wavelengths between 600–1300 nm, in comparison with other III-V and II-VI compound semiconductors. By adding manganese as a dopant, its photorefractive yield can be significantly increased. National Compound Semiconductor Roadmap – Accessed April 2006Zinc telluride – STM images of the ZnTe(110) surface, taken at different resolutions and sample rotation, together with its atomic model.