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Design development of continuous-wave 240 GHz gyrotron and 300 GHz gyrotrons with output power about 200–1000 kW for fusion research at advanced plasmas with intense magnetic field is presented. Main goal of such gyrotrons is application for EC complexes of IGNITOR and DEMO tokamaks. This paper includes task motivation and existing technical basis, results of calculation, design, technical requirements...
We demonstrate the generation of 11ps terahertz pulses from metal-metal (MM) quantum cascade lasers (QCLs) at 77K via active mode-locking. Contrary to popular belief that a long gain recovery time is required, we demonstrate that the dominant factor necessary for active pulse generation is in fact the synchronization between the propagating electronic microwave modulation and the generated THz pulses...
Shallow donor impurities in silicon, once frozen out at low temperature, share many properties in common with free hydrogen atoms [1]. They have long been the subject of spectroscopic investigation, but it is only very recently [2,3] that it has been possible to investigate the time-domain dynamics of orbital excitations such as the 1s to 2p, due to the difficulty of obtaining short, intense pulses...
We have demonstrated phase sensitive detectors of coherent terahertz radiation that use single nanowires as active elements. The single GaAs/AlGaAs nanowires acted as sensitive photoconductive elements within a gold antenna structure on quartz. The detectors were also implemented in a terahertz time domain spectroscopy (THz-TDS) system. Our devices show great promise as near-field terahertz sensors...
THz mixtenna chips and quasi-optical mixers for focal plane imaging applications are presented. Firstly, the THz Schottky diodes with the cutoff frequency of 3.2 THz are designed and fabricated. Then, the mixtenna chips are designed, by extending the two pads of the diode to a differentially-fed antenna. Finally, the 325 GHz quasi-optical mixer and mixer array, which consist of the mixtenna chip integrated...
The design and the performance of a 4.7-THz local oscillator (LO) for the GREAT (German REceiver for Astronomy at Terahertz frequencies) heterodyne spectrometer on SOFIA, the Stratospheric Observatory for Infrared Astronomy, are presented. The LO is based on a quantum-cascade laser, which is mounted in a compact mechanical cryocooler. It delivers up to 150 μW output power into a nearly Gaussian shaped...
Carrier dynamics in single-layered graphene has been studied by reflection-type of THz time domain spectroscopic ellipsometry (THz-TDSE) and optical-pump/THz-probe spectroscopy (OPTP). We successfully determined the frequency-dependent complex sheet conductivity of photo-excited monolayer Graphene from 0.3 to 16 THz. By analyzing with Drude model, the time dependence of Drude weight and carrier scattering...
In this paper, we review our recent progress on low dimensional room-temperature, high-gain, and broad-spectrum photodetectors based on InAs nanowires. Several novel infrared photodetectors based on InAs nanowire are fabricated showing a high photo-gain at room temperature.
In this paper, a low-voltage high-efficiency 330 GHz tunable gyrotron is investigated, which operates on the open-cavity TE6,2 mode and employs an electron beam with voltage of 20 kV, current of 0.5A, and pitch factor of 1.5. Simulation achieves a competitive efficiency, more than 40%, under the assumption of using copper circuit wall and electron beam of 5% velocity spread. Besides, the investigation...
With the application requirement and technology development, the necessity and tendency of Synthetic Aperture Radar (SAR) imaging within the framework of multidimensional space joint-observation, which are polarimetry, frequency, angle, time series and etc., evoke catholic interests in SAR imaging research nowadays. Recent research progress on the Multidimensional Space Joint-observation SAR (MSJosSAR)...
We study temperature dependence of terahertz dielectric properties of ferroelectric lead germanate crystal. Observed soft mode behavior directly indicates transition from displacement to order-disorder type of ferroelectric in the vicinity of phase transition point.
The nonlinear optical response of silicene in terahertz (THz) regime is theoretically studied. Since the intrinsic spin-orbit coupling gap of silicene is in the order of 1 THz, the optical response in few THz frequency regime is strongly enhanced by linear and nonlinear conductivity peaks. Furthermore, these conductivity peaks are gate-tunable. The frequency-tripling nonlinear optical current can...
A beam characterization of a THz beam generated from the organic crystal DSTMS is presented. The simple, collinear phase-matching geometry for this crystal results in an M2 factor below 1.5, resulting in a focused field strength of more than 4 MV/cm.
An ultra-high extinction THz polarizer with a tri-layer wire-grid structure was proposed here. The performance of the THz polarizer was theoretically evaluated through FEM software (HFSS), and showed a greater than 90 dB extinction ratio at 1 THz, which is the highest as far as the authors' knowledge.
Terahertz (THz) radiation has attracted a lot of attentions due to its fascinating potential applications. Wavefront modulation of the THz radiation will benefit the applications of THz imaging and communication. We propose a novel approach to actively modulate the wave-front of the THz radiation based on the THz hologram formed with photon-generated carriers. The diffracted THz beam will come into...
Ultrafast electron field emission from gold resonant antennas induced by strong terahertz (THz) transient is investigated using two THz pulse experiments. It is shown that UV emission from nitrogen plasma generated by liberated electrons is a good indication of the local electric field at the antenna tip. Using this method resonant properties of antennas fabricated on high resistivity silicon are...
The effect of a strong electric field (≥ 4kV/cm) on the properties of surface plasmon polaritons (SPP's) is investigated. It is found that when a strong enough field is applied, the TM SPP dispersion can move up into the light cone. It is further found that when considering the problem of exciting SPP's using a prisim (ie. excitation from evanescent waves), optical bistability is possible as a sufficiently...
We demonstrate the ability to control the transmission of freely propagating terahertz radiation using a magnetic field. This is accomplished using a periodic array of subwavelength apertures fabricated in a free-standing metal foil coated in cobalt. Using a transverse magnetic field that corresponds to the Voigt geometry, we observe a quadratic change in the terahertz amplitude as a function of the...
We demonstrate bright low-frequency terahertz (<5 THz) radiation confined to a diffraction-limited spot size by wavefront manipulation. Focusing to a lambda-cubic volume provides bright THz radiation at the PW/m2 level.
The application of an intense THz field results a nonlinear transmission in high resistivity silicon. Upon increasing field strength, the transmission falls from 70% to 62% due to carrier generation through THz-induced impact ionization and subsequent absorption of the THz field by free electrons.
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