Search

A rigorous mathematical model for graphene-based parametric devices based on the Maxwell's equations, where the graphene surface conductivity is determined as the nonlinear function on the electric field intensity, is developed. The projection method is applied to solve the 2D nonlinear diffraction boundary problem. Using the computational algorithm based on autonomous blocks with the Floquet channels, a parametric THz device based on a multilayer graphene-dielectric nanostructure is numerically modeled. The numerical analysis shows that this graphene-based device, simulated here at a 14.15 THz, can demonstrate the parametric amplification of signals. The instability regions of the parametric generation in this tunable THz device are calculated depending on the magnitude and frequency of the pumping TEM-wave by computing the determinant of an obtained system of linearized equations. [ABSTRACT FROM AUTHOR]

Frequencies of pure rotational transitions of molecular anions or cations which are important in the studies of interstellar chemistry are measured precisely by using an Evenson-type tunable far-infrared spectrometer (TuFIR) based on the difference frequency generation of two CO2 lasers. The specs (accuracy, sensitivity) of the spectrometer is introduced together with the examples of the studies of cations (HeH+, H2D+, etc), and anions (OH-, OD-). Also, recent improvement of the spectrometer using extended negative glow discharge cell is shown with preliminary studies of N2H+ [ABSTRACT FROM AUTHOR]

Terahertz signal is generated from Low temperature gallium arsenide photoconductive dipole antennas (gap =5μm, length = 20μm) by focusing 15 fs laser pulses and applying 12V DC across it. Terahertz intensity is detected by Pyroelectric detector (THZ1.5MB-USB). The collinear autocorrelation arrangement provides dynamic fringes which are allowed to be incident on photoconductive antennas to study the variation in terahertz intensity with respect to delay between laser pulses. Interestingly, the profile of THz intensity variation was similar to interferometric autocorrelation signal of laser pulses. The THz power attenuation with its propagation distance in atmosphere was measured. [ABSTRACT FROM AUTHOR]

Microstructured optical fibers (MOFs) have wavelength scale periodic microstructure running along their length. Their core and two-dimensional microstructured cladding might be based on varied geometries and materials, enabling light guidance due to different propagation mechanisms over an extremely large wavelength range, extending to the terahertz (THz) frequency region. As a result, these fibers have revolutionized the optical fiber technology by means of creating new degrees of freedom in the fiber design, fabrication and applicability. We analytically study the modal properties of terahertz microstructured optical fiber (THz MOF), by using our analytical field model, developed for optical waveguides. [ABSTRACT FROM AUTHOR]

Conventional and commercially available inline wall thickness monitoring systems for pipe extrusion are usually based on ultrasonic or x-ray technology. Disadvantages of ultrasonic systems are the usual need of water as a coupling media and the high damping in thick walled or foamed pipes. For x-ray systems special safety requirements have to be taken into account because of the ionizing radiation. The terahertz (THz) technology offers a novel approach to solve these problems. THz waves have many properties which are suitable for the non-destructive testing of plastics. The absorption of electrical isolators is typically very low and the radiation is non-ionizing in comparison to x-rays. Through the electromagnetic origin of the THz waves they can be used for contact free measurements. Foams show a much lower absorption in contrast to acoustic waves. The developed system uses THz pulses which are generated by stimulating photoconductive switches with femtosecond laser pulses. The time of flight of THz pulses can be determined with a resolution in the magnitude of several ten femtoseconds. Hence the thickness of an object like plastic pipes can be determined with a high accuracy by measuring the time delay between two reflections on materials interfaces e.g. at the pipe's inner and outer surface, similar to the ultrasonic technique. Knowing the refractive index of the sample the absolute layer thickness from the transit time difference can be calculated easily. This method in principle also allows the measurement of multilayer systems and the characterization of foamed pipes. [ABSTRACT FROM AUTHOR]

Considerable effort has been devoted in recent years to developing an accurate and computationally-viable theoretical treatment of the THz response of semiconductor nanostructures that are excited by ultrashort optical pulses. Although most approaches, such as the semiconductor Bloch equations, employ an electron-hole basis, we have developed an excitonic approach that has significant advantages in many situations. Our approach includes the exchange interaction between excitons, the effects of the Pauli exclusion principle for the excitons (which are composite Bosons), and the dipole-dipole interactions between excitons. In this paper we review our excitonic formalism and apply it to examine the THz absorption of optically-excited CdSe nanorods and 2D GaAs quantum wells. [ABSTRACT FROM AUTHOR]

Many biological and chemical compounds have unique absorption features in the THz (0.1 -- 10 THz) region, making the use of THz waves attractive for imaging in defense, security, biomedical imaging, and monitoring of industrial processes. Unlike optical radiation, THz frequencies can pass through many substances such as paper, clothing, ceramic, etc. with little attenuation. The use of currently available THz systems is limited by lack of high-power, sources as well as sensitive detectors and detector arrays operating at room temperature. Here we present a novel, high power THz source based on intracavity downconverison of optical pulses. The source delivers 6 ps pulses at 1.5 THz, with an average power of >300 μW and peak powers >450 mW. We propose an imaging method based on frequency upconverison that is ideally suited to use the narrow bandwidth and high peak powers produced by the source. By upconverting the THz image to the infrared, commercially available detectors can be used for real time imaging. [ABSTRACT FROM AUTHOR]

This paper illustrates the non-destructive application of a compact and portable Terahertz (THz) system to analyze the structure of an old painting and to measure the layers of composite plastic samples. THz images from the painting reveal features that resemble the signature of the artist that is not visible in the optical or X-ray channels, which support the authenticity of the painting. On the other hand, data from a composite plastic sample is analyzed to measure the thickness of each layer and determine the presence or absence of adhesive bonding between them. The presence and position of the adhesive is clearly visible in the THz images and the measured thickness shows an excellent agreement with nominal thickness. These applications demonstrate the capabilities of THz technology for unique non-destructive inspection applications. Furthermore, available compact and portable THz systems enable to perform these inspections onsite without the need to bring the sample to the laboratory, increasing the utility and convenience of THz technology. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced polymer composites (CFRPC) are of great interest in the aerospace and automotive industries due to their exceptional mechanical properties. Carbon fibers are typically woven and inter-laced perpendicularly in warps and wefts to form a carbon fabric that can be embedded in a binding matrix. The warps and wefts can be interlaced in different patterns called weaving structures. The primary weaving structures are the plain, twill, and satin weaves, which give different mechanical composite properties. The goal of this work is to computationally investigate the dependence of CFRPC microwave and terahertz electromagnetic characteristics on weave structure. These bands are good candidates for the Nondestructive Evaluation (NDE) of CFRPC since their wavelengths are comparable to the main weave features. 3D full wave electromagnetic simulations of several different weave models have been performed using a finite element (FEM) simulator, which is able to accurately model the complex weave structure. The computational experiments demonstrate that the reflection of electromagnetic waves from CFRPC depend sensitively on weave structure. The reflection spectra calculated in this work can be used to identify the optimal frequencies for the NDE of each weave structure. [ABSTRACT FROM AUTHOR]

Detecting the early corrosion of steel that is embedded in in reinforced concrete (rebar) is a goal that would greatly facilitate the inspection and measurement of corrosion in the US physical infrastructure. Since 2010, the National Institute of Standards and Technology (NIST) has been working on a large project to develop an electromagnetic (EM) probe that detects the specific corrosion products via spectroscopic means. Several principal iron corrosion products, such as hematite and goethite, are antiferromagnetic at field temperatures. At a given applied EM frequency, which depends on temperature, these compounds undergo a unique absorption resonance that identifies the presence of these particular iron corrosion products. The frequency of the resonances tends to be on the order of 100 GHz or higher, so transmitting EM waves through the cover concrete and back out again at a detectable level has been challenging. NIST has successfully detected these two iron corrosion products, and is developing equipment and methodologies that will be capable of penetrating the typical 50 mm of cover concrete in the field. The novel part of this project is the detection of specific compounds, rather than only geometrical changes in rebar cross-section. This method has the potential of providing an early-corrosion probe for steel in reinforced concrete, and for other applications where steel is covered by various layers and coatings. [ABSTRACT FROM AUTHOR]

In order to develop an intense far-infrared radiation source, a high quality electron beam has been studied at Tohoku University, Sendai. The bunch length of the beam expected is very much shorter than terahertz (THz) wavelength, so that coherent spontaneous emission of synchrotron radiation will be a promising high brilliant far-infrared source. An undulator consisting of permanent magnets has been designed in which optional free electron laser (FEL) will be operated in free space mode. Consequently the minimum gap of the undulator is decided to be 54 mm for 0.36 mm radiation to avoid diffraction loss, and then the period length of 10 cm is employed. The undulator may cover a wavelength range from 0.18 to 0.36 mm with the beam energy of 17 MeV. Property of coherent THz radiation from the undulator and possibility of novel pre-bunched THz FEL is discussed. [ABSTRACT FROM AUTHOR]

We successfully realized phase-locking of a 2.7-THz metal-metal waveguide quantum cascade laser (QCL) to a reference, which is generated from an external microwave signal by applying two stages of frequency multiplication. The reference is the 15th harmonic of a signal at 182 GHz, which is produced by a semiconductor superlattice nonlinear device. The signal at 182 GHz is generated by a multiplier-chain (×12) from a microwave synthesizer at ∼15 GHz. Both QCL laser and reference radiations are coupled into a superconducting NbN hot electron bolometer mixer, resulting in a beat signal, which is fed into a phase-lock loop. Spectral analysis of the beat signal confirms that the QCL is phase locked. This result is a crucial demonstration for a QCL used for local oscillator and opens the possibility to extend heterodyne interferometers into the far-infrared range. [ABSTRACT FROM AUTHOR]

We discuss in this paper the design and development of a 4 K Stirling-type pulse tube cryocooler for a mobile terahertz detection system. This system integrates new heterodyne detector technology at terahertz frequencies with advancements of Stirling-type pulse tube technology that brings the advent of cooled detector sensitivities in a mobile, compact, and long duration operation system without degradation of sensitivity. To achieve this goal we reduced overall system size, input power, and temperature fluctuations and mechanical vibrations in order to maintain the detector sensitivity. The Stirling-type pulse tube cryocooler developed for this system is a hybrid design employing a He-4 pulse-tube cryocooler operating at 60 Hz and 2.5 MPa average pressure that precools a He-3 pulse tube cryocooler operating at 30 Hz and 1.0 MPa average pressure to achieve 4 K cooling for the terahertz receiver. The He-4 cryocooler employs stainless steel mesh regenerators for the first stage and ErPr spheres for the second stage, while the He-3 cryocooler employs stainless mesh for the first stage and ErPr spheres for the second stage with a layered rare-earth third stage regenerator. Design details and cooler performance goals are discussed. [ABSTRACT FROM AUTHOR]

Plasma waves are oscillations of electron density in time and space, and in deep submicron field effect transistors, typical plasma frequencies, ωp, lie in the terahertz range and do not involve any quantum transitions. Hence, using plasma wave excitation for detection and/or generation of THz oscillations is a very promising approach. In this paper, the investigation of plasma wave interaction between the plasma waves propagating in a short-channel High-Electron-Mobility Transistor (HEMT) and the radiated electromagnetic waves was carried out. Experimentally, we have demonstrated the detection of the terahertz (THz) radiation by an AlGaAs/GaAs HEMT up to third harmonic at room temperature and their resonant responses show very good agreement with the calculated results. [ABSTRACT FROM AUTHOR]

Advances in Nondestructive Evaluation demand an arsenal of techniques. This was as true twenty years ago as it is today. As materials and their associated technologies progress, so must the strategy for evaluating the quality of the product. Nondestructive Evaluation is a constant struggle to extract, from classical and quantum physics, the quintessential elements that will optimally address an inspection requirement, apart from purely academic interest. It must work to be useful and thereby advance the field. The electromagnetic spectrum, from Roentgen to Maxwell/Hertz, lies at the foundation of both classical and quantum physics. Not only has NDE plucked pearls form every nook of the spectrum ranging from X-rays to T-rays but also has managed to utilize virtually every known coupling of the spectrum to the classical mechanics of heat and sound. I will “surf” the spectrum to provide a sampling of what NDE has extracted past and present as well as try to couple to its future. [ABSTRACT FROM AUTHOR]

The ability to control and concentrate electromagnetic (EM) energy in length scales much smaller than the excitation wavelength opens up new opportunities for sensing and NDE. Ordinarily EM wave transmission through such structures becomes infinitesimally small as the slit opening becomes much smaller than the wavelength; however, extraordinary transmission through these structures has been observed by researchers, which is believed to be due to surface plasmonic effect. We have investigated THz subwavelength slit structures for sensing and NDE. In a deep slit structure with subwavelength width, not only are the waves transmitted through, but also are found to be resonating because of the reflections caused by the impedance mismatch at the open ends. As a result, this structure can sense the dielectric properties of materials, which may be in gas, liquid, or solid forms, with high sensitivity and selectivity depending on the quality factor of the cavity resonance. Subwavelength aperture allows for near field imaging of materials with spatial resolution below the Abbe diffraction limit. To understand the electromagnetic (EM) wave propagation through subwavelength structures, we have performed a computational analysis of the EM fields with finite difference time domain (FDTD) technique. Simulated responses were compared with swept-frequency cw THz waves (230–300 GHz) in straight and stepped cavities with 50 μm slits on both ends and increased width in the middle. FDTD modeling of straight slits indicated Fabry-Perot (F-P) resonance peaks with low quality factor. We modified the slit to form a stepped cavity which increased the quality factor because of increased reflection coefficient at the slit ends. We fabricated straight and stepped cavities and tested with THz radiation in the 250–300 GHz range. The stepped cavity appears to have desirable features for sensing: good transmissivity, high-Q F-P resonance, compactness, and ruggedness. [ABSTRACT FROM AUTHOR]

The results of both a laboratory and factory trial of a time-domain terahertz (TD-THz) multi-layer gauge for on-line process monitoring are presented. The TD-THz gauge is demonstrated on a two layer laminated plastic insulation material. The TD-THz gauge simultaneously measured the total and the individual layer thicknesses. Measurements were made while transversely scanning across a 12 foot wide sheet extruded at high speed in a factory environment. The results were analyzed for precision, accuracy, and repeatability; and demonstrated that the TD-THz gauge performed in an equivalent or superior manner to existing ionizing radiation gauges (which measure only one layer). Many dielectric materials (e.g., plastic, rubber, paper, paint) are transparent to THz pulses, and the measurement of a wide range of samples is possible. [ABSTRACT FROM AUTHOR]

We report on the end station for terahertz spectroscopy at high pressures and low temperatures at the UVSOR Facility of the Institute for Molecular Science, and present one set of experimental results for the pressure-dependent valence transition of samarium sulfide (SmS) at the temperatures of 6 and 300 K. The end station covers the photon energy range of 5 meV to 1.2 eV with pressures of up to 6 GPa and temperatures down to 3.7 K. Using the end station, the valence transition, namely, the black—golden phase transition, of SmS was clearly observed at 0.65 GPa. From Drude and Lorentz fitting of the pressure-dependent reflectivity spectra, the carrier density at the critical pressure was found to be about 10-2/SmS at both 6 and 300 K. The carrier density appear to play an important role in the valence transition. [ABSTRACT FROM AUTHOR]

We describe a technique for characterizing bolometric detectors that have sufficient sensitivity to count single terahertz photons. The device is isolated from infrared blackbody radiation and a single terahertz photon is simulated by a fast microwave pulse, where the absorbed energy of the pulse is equal to the photon energy. We have employed this technique to characterize bolometric detectors consisting of a superconducting titanium nanobridge with niobium contacts. Present devices have Tc = 0.3 K and a measured intrinsic energy resolution of approximately 6 terahertz full-width at half-maximum, near the predicted value due to intrinsic thermal fluctuation noise, with a time constant of 2 μs. An intrinsic energy resolution of 1 terahertz should be achievable by reducing the volume of the titanium nanobridge. Such a detector has important applications in future space-based terahertz astronomy missions. [ABSTRACT FROM AUTHOR]

A theoretical three-dimensional transverse magnetic (TM) mode analysis to describe the presence of interactions between surface plasma waves of carriers in a two-dimensional electron gas (2DEG) at AlGaAs/GaAs heterostructure and electromagnetic space harmonics slow waves using the so-called interdigital-gated high-electron-mobility-transistor (HEMT) plasma wave devices is presented. First, the device structure and the outline of theoretical formulation to determine the admittance of the interdigital structure are described. Then, the analysis of the space harmonics propagating through the interdigital slow-wave structures is performed. Next, the numerical procedures to solve the integral equations which are used in determining the admittance are explained. Finally, we point out and discuss the main results of the theoretical analysis where an appearance of negative conductance is obtained. [ABSTRACT FROM AUTHOR]

A rapid time domain terahertz computed axial tomography (CT-TD-THz) system is described. The system is used to reconstruct three dimensional structure of aerospace components including: Kevlar soft shell fan containment systems, silica thermal protection tiles, and sprayed on foam thermal insulation. [ABSTRACT FROM AUTHOR]

Time domain terahertz (TD-THz) non destructive evaluation (NDE) imaging is used to two-dimensionally map the thickness of yttria stabilized zirconia (YSZ) thermal barrier coatings (TBC) on aircraft engine turbine blades. Indications of thermal degradation can be seen. The method is non-contact, rapid, and requires no special preparation of the blade. [ABSTRACT FROM AUTHOR]

This paper investigates the use of surface plasmonic effect in slit resonators for sensing and imaging in the terahertz regime. The transmittance of electromagnetic (EM) waves through a narrow aperture becomes negligible when the aperture size becomes much smaller than the wavelength. However, with the resonant excitation of charge density waves in the metal/air interface, called the surface plasmons, an extraordinary transmittance has been observed through such apertures. Using slit resonators of 50 to 100 μm width, we have demonstrated enhanced transmission of THz radiation through the slits. The ability to concentrate the EM radiation through a sub-wavelength aperture bodes well for detection of chemical or biological materials with high sensitivity or for super-resolution imaging of materials for NDE. We present results of using a slit resonator for chemical detection and for super-resolution imaging of materials for NDE. [ABSTRACT FROM AUTHOR]

The development of electromagnetic metamaterials by micro/nanomanufacturing at SSLS has led to matrix-embedded or substrate-supported rod-split-ring-based samples reaching left-handed pass-bands at 216 THz or 1.39 μm and to free-space S-string bi-layer chips at 2.2 THz. Potential applications of metamaterials range from sub-wavelength resolution imaging over invisibility cloaking to advanced antennae and are relevant to fields including microscopy, lithography, electromagnetic shielding, and telecommunication. [ABSTRACT FROM AUTHOR]

Coherent Cherenkov radiation (CCR) wakefields are produced when a compressed electron beam travels along the axis of a hollow cylindrical dielectric tube. In a dielectric wakefield accelerator (DWA) these wakefields accelerate either a trailing electron bunch or the tail of the driving bunch, depending on the modal structure of the radiation. For an appropriate choice of dielectric structure geometry and beam parameters the device operates in a single-mode regime, producing sinusoidal wakefields with wavelengths in the THz range. We report on preliminary results of an experiment at UCLA studying the potential of a DWA structure to produce high-power, narrow-band THz radiation. First measurements include observation of 1 MW peak-power pulses of coherent broadband radiation from a compact dipole beam dump magnet. [ABSTRACT FROM AUTHOR]

We present observation and characterization of microjoule-MV/cm-level THz pulses from a laser wakefield accelerator. THz emitted as coherent transition radiation from the plasma-vacuum boundary is collected and refocused by off-axis parabolas to a test stand where a suite of diagnostics is performed, including energy measurement by a Golay cell and electro-optic sampling of the spatio-temporal electric field using a probe pulse split from the main laser. Frequency Domain Holography is also implemented for the first time to capture spatio-temporal field distributions in a single shot. The four techniques strongly corroborate detection of THz pulses of ∼0.4 ps duration, with peak fields of several hundred kV/cm and energies of 5–10 μJ. The advantages and disadvantages of each technique are discussed. [ABSTRACT FROM AUTHOR]

The Terahertz (THz) window of the electromagnetic spectrum has been partially explored but almost unexploited commercially. In recent years there has been an increased interest and a technological boost in THz research for detection systems, material characterization and imaging. Among many hot topics the researchers are interested in medical applications, and protein characterization. We present a general overview of the field showing some of the handicaps and promises of this region of the electromagnetic spectrum. [ABSTRACT FROM AUTHOR]

The simulation of a tunable dual wavelength birefringent filter is presented with the ultimate goal of developing a novel optical in the terahertz (THz) region of the electromagnetic spectrum for medical applications. THz radiation is located in the electromagnetic around 300 μm. For a long time this frequency domain has been named the “THz-gap” due to the lack of sources and detector systems. Here we present a novel alternative for a THz laser source based on the novel birefringent filter presented. The filter is constructed using four stages quartz plates placed at Brewster angle. The design of the filter was done using a zero diving angle to facilitate its construction, and the tunability in the overall response of the filter is achieved by rotating only one of the plates. The filter was design to have a center response around 980 nm with the peak separation of 2.50 nm to 3.75, corresponding to frequency separation between 0.8 THz to 1.2 THz, compatible with broadband laser sources in the 980 nm vicinity. [ABSTRACT FROM AUTHOR]

This article describes a non-contact single-sided terahertz electromagnetic measurement and imaging method that simultaneously characterizes microstructural (e.g. spatially-lateral density) and thickness variation in dielectric (insulating) materials. The method was demonstrated for Space Shuttle External Tank sprayed-on foam insulation. It is believed that this method can be used for applications where microstructural and/or thickness variation in dielectric materials require precision mapping. Scale-up to more complex shapes such as cylindrical structures and structures with beveled regions would appear to be feasible. [ABSTRACT FROM AUTHOR]

The terahertz dynamics of glass-forming liquids has been studied by terahertz time domain spectroscopy. The real and imaginary parts of dielectric constants have been determined accurately by the transmission measurements using the dual-thickness geometry. The complex dielectric spectra of aqueous solutions of ethylene glycol clearly show the existence of relaxation processes at room temperature. As the concentration of ethylene glycol increases, the relaxation time of the main relaxation process becomes longer. It is attributed to the destruction of the hydrogen-bonded tetrahedral structure of water and the formation of clusters consists of water and ethylene glycole molecules. [ABSTRACT FROM AUTHOR]

Nanophotonics—the manipulation of light with nanomaterials—is a booming subject, its success owing to the host of nanoscale fabrication techniques now at our disposal. However, for the characterization of such nanomaterials it is expedient to turn to other types of waves with a wavelength commensurate with the nanostructure in question. One such choice is acoustic waves of nanometre wavelength. The aim of this article is to provide an introduction to laser picosecond acoustics, a means by which gigahertz-terahertz ultrasonic waves can be generated and detected by ultrashort light pulses. This method can therefore be used to characterize materials with nanometre spatial resolution. In this article we review the theoretical background for opaque single-layer thin film isotropic samples with reference to key experiments. Solids including metals and semiconductors are discussed, although liquids and, conceivably, gases, are not excluded. [ABSTRACT FROM AUTHOR]

Nanometric InGaAs-based High Electron Mobility Transistors (HEMTs) have been shown experimentally to generate and detect THz radiation. The observed phenomena can be attributed to the presence of plasma waves inside the transistor channel. To investigate these oscillations we have performed Monte Carlo simulations of InGaAs homogeneous channels of different length and thickness. To detect the presence of plasma waves we have calculated the spectral density of voltage fluctuations under thermodynamic equilibrium at 300 K. Results confirm the presence of 3D and 2D plasma modes according to the topology of the channel. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

The technology, methods, and examples of high speed time domain terahertz (T-Ray) imaging non-destructive examination (NDE) for 2 and 3 dimensional structural and material content characterization are discussed. T-Ray imaging can be utilized for non-contact transmission and/or monostatic reflection inspection of non-conductive materials such as plastics, foam, composites, ceramics, paper, wood and glass. Example subsurface homeland security images of concealed items in baggage and on personnel are shown. We tabulate attenuation and penetration characteristics through a selection of building materials, and demonstrate the ability of T-ray instrumentation to sub-surface image building structures such as wall framing and interior wiring and conduits. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Foam shedding from the shuttle external tank remains a critical problem regarding Shuttle orbiter safety. Flaws present in the foam can result in initiation sites for foam loss, and NASA is continuing to look at improving existing NDE methods for foam inspection as well as developing new methods. Terahertz NDE, greatly enhanced over the last several years with respect to its use for external tank foam inspection, has been a focus for continued improvement through signal and image processing improvements. In this study, results from various signal processing approaches to improve terahertz image flaw resolution for external tank foam are described. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

Development of intense terahertz radiation source is progressing at UVSOR-II, based on the mechanism of coherent synchrotron radiation (CSR). The terahertz CSR has successfully been produced by two methods. When the storage ring is operated in the single bunch mode with a sufficiently high beam current, intense bursts of terahertz radiation are emitted. Micro-structures in the longitudinal density distribution of the electron bunches created by a beam instability may be the origin of the radiation. The duration of the bursts is typically 100 micro-seconds. The peak intensity is 10000 times higher than that of the normal synchrotron radiation. The bursts appear chaotically or quasi-periodically depending on the beam current with a typical interval of 10 milli-seconds. It has been also demonstrated that the terahertz CSR could be produced by the laser-bunch slicing method. The density modulation produced on the electron bunch by the laser is the origin of CSR. The repetition rate of the terahertz pulses is 1 kHz, which is same as the laser repetition rate. The intensity per pulse is 105 times higher than that of the normal SR. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

We discuss future experimental work proposed to study the performance of a cylindrical dielectric wakefield accelerating structure as a coherent Cerenkov radiation source at the Neptune laboratory at UCLA. The Cerenkov wakefield acceleration experiment carried out recently by UCLA/SLAC/USC, using the ultrashort and high charge beam (Q = 3 nC, σz = 20 micron) at the SLAC FFTB, demonstrated electromagnetic wakes at the few GV/m level. The motivation of our prospective experiment is to investigate the operation of a similar scenario using the comparatively long pulse, low charge beam (Q = 0.5 nC, σz = 200 micron) at UCLA Neptune. The field amplitude produced in this setup would be one to two orders of magnitude lower, at the few tens to few 100 MV/m level. Such a decelerating field would extract a significant amount of energy from a low-energy beam in a distance on the order of a few centimeters, allowing the use of short dielectric structures. We discuss details of the geometry and composition of the structures to be used in the experiment. We also examine the possibility of a future dedicated facility at UCLA Neptune based on a hybrid photoinjector currently in development. The intrinsic bunch compression capabilities and improved beam parameters (σz = 100 micron, Q = 1 nC) of the photoinjector would allow the creation of a high power radiation source in the terahertz regime. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

IFEL and FEL techniques can be used to modulate an electron beam on the scale of the radiation wavelength. However, the lack of a high power radiation source in the 100–300 μm range hinders the progress on THz IFEL microbunching. In this paper, we discuss microbunching of an electron beam using a single-pass FEL seeded with a low power THz pulse generated by frequency mixing of CO2 laser lines in a GaAs nonlinear crystal. A narrowband THz seed source is pumped by a dual beam TEA CO2 laser and can be tuned in the 1–3 THz range. The THz radiation is guided through a hollow waveguide inside the planar FEL undulator driven by a photoinjector. By using a time-dependent FEL code GENESIS 1.3, we optimized the undulator parameters and analyzed the dynamics of the modulated electron beam. By using a ∼ 8 MeV electron beam with a peak current of 40 A and a ∼1kW THz seed with wavelength 200 μm, the energy modulation up to 1.3% can be achieved in a ∼1.8-m long undulator with a constant period of 2.7 cm. At present, the THz seed source is built and fully characterized. The results of transmission measurements for THz waveguides are also discussed. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The transmission and penetration capability of electromagnetic waves with terahertz frequencies promises nondestructive subsurface inspection capabilities using low energy, non-harmful radiation. The development of compact and portable terahertz frequency radiation sources and detectors is crucial to the practical implementation of future terahertz based nondestructive evaluation tools for aerospace, medical, security, and electronic industries. Recent progress in the bandstructure engineering of multi-quantum well heterostructures offers the potential to design miniature, efficient, high power, and direct terahertz emitters using standard semiconductor materials and fabrication techniques. Using a recently developed terahertz spectrometer, combined with near and mid-infrared spectroscopy data, we report on the transmission characteristics of aerospace coatings over a broad spectrum from near-infrared to terahertz frequencies. The design of quantum cascade emitters based on the obtained transmission data is presented. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

A compact source of monoenergetic x-rays, generated via Compton backscattering, has been developed in a collaboration between UC Davis and SLAC. The source consists of a 5.5 cell X-band photoinjector, a 1.05 m long high gradient accelerator structure and an interaction chamber where a high power (TW), short pulse (sub-ps) infrared laser beam is brought into a nearly head-on collision with a high quality focused electron beam. Successful completion of this project will result in the capability of generating a monoenergetic x-ray beam, continuously tunable from 20 – 85 keV. We have completed a series of measurements leading up to the generation of monoenergetic X-rays. Measurements of essential electron beam parameters and the techniques used in establishing electron/photon collisions will be presented. We discuss the design of an improved interaction chamber, future electro-optic experiments using this chamber and plans for expanding the overall program to the generation of Terahertz radiation. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

TeraHertz emission from High Electron Mobility Transistors has been recently measured from experiments. The experiments show emission spectra with two peaks in the TeraHertz range: one around 1 THz is sensitive to drain and gate voltages, and another one around 5 THz which is fixed. In order to get physical insight into the microscopic mechanism at the basis of the radiation emission we have performed a Monte Carlo simulation of the measured transistors using the current noise spectra as sensitive probes to detect the presence of electrical instabilities. Numerical results are found to be in good agreement with experiments confirming the presence of an oscillatory dynamics in the TeraHertz range. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

At UVSOR-II, Institute for Molecular Science, a new beamline for novel pump-and-probe spectroscopy experiments combining terahertz (THz) coherent synchrotron radiation and vacuum-ultraviolet coherent harmonic generation from same electron bunches in the storage ring interacted with an amplitude-modulated pulse laser introduced from the outside of the storage ring. The purpose of the beamline is to elucidate the electronic structure relating to physical properties of solids by using selective excitations of low energy electronic and vibrational structure. The beamline design is reported. [ABSTRACT FROM AUTHOR]

Laser beams of high intensities are routinely used for IFEL experiments. Such beams can potentially destroy microbunching diagnostic tools such as coherent transition radiation foils due to their low damage thresholds. Near-field Fresnel diffraction scheme for termination of CO2 laser beam has been experimentally studied and is presented in this paper. Novel THz camera was utilized for such study. [ABSTRACT FROM AUTHOR]

Mode-locking of a terahertz quantum cascade laser is achieved using multimode injection seeding. Contrary to standard methods that rely on gain modulation, here a fixed phase relationship is directly imprinted to the laser modes. In this work, we demonstrate the generation of 9 ps phase mode-locked pulses around 2.75 THz. A direct measurement of the emitted field phase shows that it results from the phase of the initial injection. [ABSTRACT FROM AUTHOR]

The influence of a magnetic field on Landau levels (LLs) in graphene-based devices is described via the magnetooptical response induced by terahertz (THz) radiation. For single-layer graphene, the resonance energies of the transitions between the on Landau levels (LLs) such as L1, L2 and L3 fit quite well to the terahertz spectral range at low magnetic fields. Also, the calculations for the terahertz photoresponse (photoconductivity) in the presence of low magnetic fields, the reported calculations for the scattering rate of LLs, recent and our experimental results of photoresponse measurements yield that singlelayer graphene is suitable for the detection of terahertz radiation. [ABSTRACT FROM AUTHOR]

Results of Monte Carlo simulation of InP MOSFET performance under impact ionization in the conducting channel are presented. The drain current oscillations up to 0.5 THz frequency due to Gunn effect are demonstrated. [ABSTRACT FROM AUTHOR]

The polarity change of THz electromagnetic waves radiated from single-crystalline graphite and polycrystalline graphite films has been studied to identify the main generation mechanism in conventional reflective THz time-domain spectroscopy scheme. The excitation wavelength variation around the K-valley produces no significant changes in THz field strength. We further found that THz waves become fully dispersed without polarity change in lateral detection geometry. [ABSTRACT FROM AUTHOR]

We analyze the formation of nonequilibrium states in optically pumped graphene layers and in forwardbiased graphene structures with lateral p-i-n junctions and consider the conditions of population inversion and lasing. The model used accounts for intraband and interband relaxation processes as well as deviation of the optical phonon system from equilibrium. As shown, optical pumping suffers from a significant heating of both the electron-hole plasma and the optical phonon system, which can suppress the formation of population inversion. In the graphene structures with p-i-n junction, the injected electrons and holes have relatively low energies, so that the effect of cooling can be rather pronounced, providing a significant advantage of the injection pumping in realization of graphene terahertz lasers. [ABSTRACT FROM AUTHOR]

Magneto infrared absorption measurements have been performed on HgTe/CdHgTe quantum wells with different thicknesses grown on (013) GaAs substrate. Cyclotron resonance effective masses, inter-Landau-level transition energies and their dependence on magnetic field are measured. The measured intersubband energies are in good agreement with the theoretically calculated values [1]. Strong spin-orbit interaction is responsible for cyclotron resonance splitting in asymmetric quantum wells. We demonstrate that the increase of the quantum well thickness leads to a semimetallic state, allowing for simultaneous observation of holes and electron transitions. [ABSTRACT FROM AUTHOR]

We report on efficient terahertz electroluminescence in the region of 1.5-2 THz from high electric field biased 6H-SiC n+-n--n+ structures with a natural superlattice at 7 K. The properties of the terahertz emission allow it to be attributed to spontaneous radiation resulting from electron Bloch oscillations in SiC natural superlattice. The use of the unique object, namely, natural superlattice of SiC allowed us to demonstrate a whole series of remarkable effects of Wannier-Stark localization and to get the intensive terahertz emission under steady-state electrical excitation of Bloch oscillations. [ABSTRACT FROM AUTHOR]

The temperature dependent response and transport characteristics of an integrated bolometric-plasmonic terahertz detector have been measured. A sensitive bolometric detection element and a grating-gated GaAs/AlGaAs high electron mobility transistor (HEMT) with a resonant plasmonic response 40 GHz in width over a broad temperature range are monolithically integrated. Prior work has demonstrated that the low temperature response results from a hot electron detection mechanism; the hot electron effect remains prominent at 100 K. The terahertz response drops by nearly two orders of magnitude from 20 K to 100 K, consistent with a temperature dependent energy relaxation time. [ABSTRACT FROM AUTHOR]