Abstract: Abstract Terahertz time-domain spectroscopy has been used for the characterization of different types of olive oil, such as extra virgin, pure, extra light, and pomace olive oils. Temperature-dependent refractive indices and absorption coefficients have been measured in 0.2–1.6 THz spectral-frequency range. Temperature-dependent Sellmeier coefficients and thermo-optic coefficients of these oil samples have also been derived from the measured data. These results will benefit different oil-based optical devices requiring accurate knowledge of their refractive indices in THz range. PubDate: 2020-10-01
Abstract: Abstract Composite materials made of micron-sized oxide particle fillers in an epoxy resin matrix that can be molded into desired shapes are widely used for packaging radiofrequency and microwave-integrated circuits (ICs). To potentially employ these materials with millimeter-wave ICs (MMICs), quantitative knowledge of the composites’ dielectric properties across a broad millimeter-wave band is necessary. Here, we present non-destructive measurements of the complex relative permittivity, εr = ε′ + jε″, on some possible MMIC packaging composites consisting of silica and/or alumina microsphere fillers dispersed in an epoxy matrix. Measurements using phase-sensitive transmission over the WR3 and WR5 frequency bands (140 to 325 GHz) show that ε′ ranged from 3.6 for pure silica filler to 7.2 for pure alumina filler, with very little frequency dispersion. In all materials, the loss tangent tanδ = ε″/ε′ was between 0.01 and 0.02 in this frequency range. An analysis using the theory of two-component composites is used to extract the real permittivity of the epoxy resin. The results could be used to model performance of packaged MMICs and to design composites having a tailored value of ε′. PubDate: 2020-10-01
Abstract: Abstract We report on the observation of terahertz (THz) radiation induced band-to-band impact ionization in HgTe quantum well (QW) structures of critical thickness, which are characterized by a nearly linear energy dispersion. The THz electric field drives the carriers initializing electron-hole pair generation. The carrier multiplication is observed for photon energies less than the energy gap under the condition that the product of the radiation angular frequency ω and momentum relaxation time τl larger than unity. In this case, the charge carriers acquire high energies solely because of collisions in the presence of a high-frequency electric field. The developed microscopic theory shows that the probability of the light-induced impact ionization is proportional to \(\exp (-{E_{0}^{2}}/E^{2})\) , with the radiation electric field amplitude E and the characteristic field parameter E0. As observed in experiment, it exhibits a strong frequency dependence for ωτ ≫ 1 characterized by the characteristic field E0 linearly increasing with the radiation frequency ω. PubDate: 2020-10-01
Abstract: Abstract The effect of frequency multiplication was investigated for a fundamental-harmonic 0.263-THz kW-level gyrotron, in which a certain fraction of its radiation was observed experimentally at the doubled operating frequency. Accurate measurements of the power of such high-frequency generation were carried out based on cyclotron absorption of radiation in a HgTe/CdHgTe quantum-well heterostructure. The maximum power ratio of the second harmonic to the fundamental one is about 10−4 (an absolute power of 10–15 mW at a frequency of 0.526 THz), which agrees well with the results of simulations within the framework of the self-consistent averaged approach performed to describe the multimode generation in gyrotrons. PubDate: 2020-10-01
Abstract: Abstract A multiple-joint transmission line based on the principle of the periscope was developed to realize the flexible guidance of terahertz (THz) waves. The transmission line has a slightly larger diameter than the standard plastic hollow-pipe waveguide, sufficient stability, low transmission loss, ultrabroadband spectrum, and low dispersion. The transmission line is evaluated using a THz time-domain spectrometer consisting of reflective two-dimensional electro-optic sampling with a high-speed complementary metal-oxide semiconductor camera. PubDate: 2020-10-01
Abstract: Abstract MEMS-based phase shifters show the best performance in terms of figure of merit, but their footprints are usually large and it is difficult to achieve several bits. This paper demonstrates a miniaturized phase shifter based on slow-wave CPW and MEMS that occupy 0.47 mm2. A total phase shift of 152° was obtained with a maximum insertion loss of 3 dB, resulting in a figure of merit of 50°/dB at 60 GHz. The 3-bit device showed an insertion loss variation of 1.3 dB and return loss better than 13 dB. The pull-in and pull-out voltages were measured to be 17 V and 10 V, respectively. The presented device is well suited for mm-wave phased array applications. Thanks to the proposed concept, more bits could be easily achieved and much higher frequencies could be addressed. PubDate: 2020-10-01
Abstract: Abstract Strontium molybdate (SrMoO3) thin films are grown epitaxially by pulsed laser deposition onto gadolinium scandate (GdScO3) substrates and characterized in the terahertz (THz) and visible part of the electromagnetic spectrum. X-ray diffraction measurements prove a high crystallinity and phase-pure growth of the thin films. The high-quality SrMoO3 thin films feature a room temperature DC conductivity of around \(3{\frac {1}{\mu {\Omega } m}}\) . SrMoO3 is characterized in the THz frequency range by time domain spectroscopy. The resulting AC conductivity is in excellent agreement with the DC value. A Lorentz-Drude oscillator approach models the THz and visible conductivity of SrMoO3 very well. We compare the results of the SrMoO3 thin films to a standard, sputtered gold film, with a resulting THz conductivity of \(8{\frac {1}{\mu {\Omega } m}}\) . The comparison demonstrates that oxide thin film–based devices can play an important role in future THz technology. PubDate: 2020-10-01
Abstract: Abstract Material parameter extraction algorithms are studied and simplified both for transmission-reflection and transmission-only methods. The simplified relations, which are closed-form in some cases, are analyzed to establish the uncertainty sensitivity coefficients and therefore, to clarify the main uncertainty contributions and reduce the systematic and random errors. Simple closed-form expressions presented in this paper show the sensitivity of the extracted permittivity to each input parameter such as S21 (phase and amplitude), frequency, and the material thickness. Results are presented for several material slabs for three waveguide frequency ranges 75–110 GHz, 140–220 GHz, and 500–750 GHz using VNA-based free-space technique in the THz domain. Comparison of results (and the associated uncertainties) between different algorithms can help to choose the optimal one suitable for lossy or low-loss materials, and thin or thicker slabs. This can explain why the same set of S-parameters data usually gives different final results (permittivity and permeability) with different algorithms and verify the reliability of the calibration and extraction process. PubDate: 2020-10-01
Abstract: Abstract New method of measurement of dielectric constant and loss tangent for small samples of lossy dielectrics, including plain semiconductors, using non-oversized resonators being parts of the recently developed cavity switches for sub-terahertz bands is proposed. Strong influence of the tangent loss value to the Q-factor of the resonator is calculated and observed in experiments. A simple monosemantic algorithm to retrieve explicit mathematical expressions for the complex permittivity from the experimental data is demonstrated. The advantage of the new method over the known ones is much lower (up to million times) volume of the sample needed to analyze. PubDate: 2020-09-17
Abstract: Abstract Analysis of dielectric properties of several materials, BN, AlN with BN additive, crystalline quartz, MgAl2O4 spinel, 6H-SiC in the millimeter and submillimeter (terahertz) ranges, is presented. Basing on the experimentally measured dielectric losses of these materials and on their thermal and mechanical properties, we made a calculation to estimate the maximal achievable output power that can be transmitted through disks made of such materials. Prospects for using these materials for gyrotron barrier window production are discussed. PubDate: 2020-09-16
Abstract: Abstract The analysis of terahertz transmission through semiconducting thin films has proven to be an excellent tool for investigating optoelectronic properties of novel materials. Terahertz time-domain spectroscopy (THz-TDS) can provide information about phonon modes of the crystal, as well as the electrical conductivity of the sample. When paired with photoexcitation, optical-pump-THz-probe (OPTP) technique can be used to gain an insight into the transient photoconductivity of the semiconductor, revealing the dynamics and the mobility of photoexcited charge carriers. As the relation between the conductivity of the material and the THz transmission function is generally complicated, simple analytical expressions have been developed to enable straightforward calculations of frequency-dependent conductivity from THz-TDS data in the regime of optically thin samples. Here, we assess the accuracy of these approximated analytical formulas in thin films of highly doped semiconductors, finding significant deviations of the calculated photoconductivity from its actual value in materials with background conductivity comparable to 102Ω− 1cm− 1. We propose an alternative analytical expression, which greatly improves the accuracy of the estimated value of the real photoconductivity, while remaining simple to implement experimentally. Our approximation remains valid in thin films with high dark conductivity of up to 104Ω− 1cm− 1 and provides a very high precision for calculating photoconductivity up to 104Ω− 1cm− 1, and therefore is highly relevant for studies of photoexcited charge-carrier dynamics in electrically doped semiconductors. Using the example of heavily doped thin films of tin-iodide perovskites, we show a simple experimental method of implementing our correction and find that the commonly used expression for photoconductivity could result in an underestimate of charge-carrier mobility by over 50%. PubDate: 2020-09-15
Abstract: Abstract In order to better understand the variation mechanism of ozone abundance in the middle atmosphere, the simultaneous monitoring of ozone and other minor molecular species, which are related to ozone depletion, is the most fundamental and critical method. A waveguide-type multiplexer was developed for the expansion of the observation frequency range of a millimeter-wave spectroradiometer, for the simultaneous observation of multiple molecular spectral lines. The proposed multiplexer contains a cascaded four-stage sideband-separating filter circuit. The waveguide circuit was designed based on electromagnetic analysis, and the pass frequency bands of stages 1–4 were 243–251 GHz, 227–235 GHz, 197–205 GHz, and 181–189 GHz. The insertion and return losses of the multiplexer were measured using vector network analyzers, each observation band was well-defined, and the bandwidths were appropriately specified. Moreover, the receiver noise temperature and the image rejection ratio (IRR) using the superconducting mixer at 4 K were measured. As a result, the increase in receiver noise due to the multiplexer compared with that of only the mixer can be attributed to the transmission loss of the waveguide circuit in the multiplexer. The IRRs were higher than 25 dB at the center of each observation band. This indicates that a high and stable IRR performance can be achieved by the waveguide-type multiplexer for the separation of sideband signals. PubDate: 2020-09-11
Abstract: Abstract Highly nonlinear optical 4-(4-dimethylaminostyryl)-1-methylpyridinium tosylate (DAST) crystals have been successfully grown. The photobleaching effect was demonstrated by irradiation with an UV light at 405 nm during 120 h. The UV-Vis and terahertz absorption spectra of DAST in crystalline states before and after photobleaching were measured and compared. Results of photobleaching modification of DAST monocrystal have been demonstrated with the help of differential interference contrast microscopy (DIC). A possible explanation of photobleaching effect has been suggested. Since absorption in the terahertz range of crystalline materials is associated with vibrational modes of the crystal lattice, we can conclude that photobleaching leads to its destruction. Taking into account the obvious change of crystalline spectra in the UV-Vis and terahertz frequency range and the absence of changes in the molecular spectra from irradiated DAST in ethanol solution, we can conclude that the photobleaching process may be a photoactivated destruction of chemical in DAST molecules. The photobleaching technique was proposed to implement quasi-phase matching (QPM) structures for efficient nonlinear conversion by optical rectification. PubDate: 2020-09-01
Abstract: Abstract A new method for diagnosis of diabetes mellitus is proposed, which uses for measurement a lyophilized blood plasma sample prepared in the form of a pellet. The paper develops a methodology for fast spectroscopic measurements of such pellets with terahertz pulse time-domain holography. For that reason, blood plasma pellets were experimentally measured by terahertz time-domain spectroscopy system in transmission mode and its characteristics were obtained to be then used in numerical simulation of pulse terahertz hologram formation and extraction of its optical properties. Thus, a demonstration of the proof-of-concept was given for the techniques of pellet inspection, which contains information about the presence of glycated proteins, reflecting a diabetic pathology. PubDate: 2020-09-01
Abstract: Abstract The design of a magnetron-injection gun (MIG) suitable for using in the CW 3-rd harmonic gyrotron with an output frequency of 1.185 THz and an output power about 100 W, intended for DNP/NMR spectroscopy applications is described. To improve the mode selection, several electron optics schemes are investigated. The MIG allows using both single-beam and double-beam schemes of the gyrotron operation with two different operating modes. Requirements for the electron beam parameters were found. The design of the triode MIG forming two generating beams with a quality suitable for 3-rd harmonic operation is suggested and optimized. The specific feature of the MIG is the possibility to form either two beams or one beam using the same electrode geometry and different number of emitting rings. The suggested MIG is not sensitive to small misalignment and reasonable manufacturing errors. A key feature of the design is that to improve the mode selection an electron beam with increased velocity spread is proposed. It is shown that it is possible to form two beams with practically equal and quite high pitch factors in all range of operating currents and at the same time small enough coefficient of reflection from the magnetic mirror in spite of large enough value of velocity spread. PubDate: 2020-09-01
Abstract: Abstract The microstrip meander-line traveling wave tube (TWT) is a kind of small sized, low voltage, low cost, and easy to fabricate TWT. It is an attractive choice for many applications, such as communication systems, phased array radars, security detectors, and electronic counter measure (ECM) instruments. In this paper, a Ka band U-shape meander-line slow wave structure (SWS) supported by a diamond rod is presented. The dispersion characteristics of this meander-line SWS are analyzed using CST MW Studio. The S-parameter simulation results show that the reflection is below − 15 dB between 32 and 38 GHz, and the particle-in-cell (PIC) simulation reveals that this meander-line TWT can produce 57 W output power at 36 GHz, when it is driven by a 8.1 kV, 0.04 A sheet beam with rectangular cross-sectional area of 1.0 mm × 0.12 mm. This meander-line SWS is manufactured, assembled, and tested. The good agreement between the simulated and measured S11 has been achieved and the mismatch between the simulated and measured S21 has been discussed at the end. PubDate: 2020-09-01
Abstract: Abstract Terahertz single-photon detection and imaging have attracted full attention recently. Nonlinear optical frequency up-conversion is a promising technique that can be expected to satisfy this demand thanks to its high sensitivity and fast response. In this paper, theoretical analysis and numerical calculations based on the organic salt 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal were performed to show that the optimization of the detection of terahertz is different from that of the generation of terahertz, including the use of difference-frequency generation (DFG) technique, the larger thickness of the crystal, and especially the selection of the polarization direction of the pumping laser. For two different polarization configurations, the photons of the up-converted signal light both can be amplified compared with the number of incident terahertz photons under some optimal designs of the nonlinear frequency conversion process. Therefore, terahertz single-photon detection can be realized with single-photon detectors (SPDs) or even possibly with ordinary avalanche photodiodes (APDs). Furthermore, for terahertz single-photon imaging, the frequency up-conversion with the pumping laser polarized along b-axis of DAST crystal has a better performance, which is rarely used in terahertz generation. PubDate: 2020-08-12
Abstract: Abstract We have demonstrated and developed a W-band 30 W continuous-wave folded waveguide traveling wave tube (CW FWTWT) with 13.5-kV operation voltage and 4.8 dB gain flatness. In this paper, we quantitatively study the relationship between operating voltage and the key geometric dimensions of a folded waveguide, which is the key to the design of the low-voltage TWT. In addition, problems of improving beam transmission and suppressing oscillations are also studied, and the related design methodologies of the long-range electron gun, periodic permanent magnet (PPM) focusing system, low-voltage standing wave ratio (VSWR) coupling system, and horseshoe attenuator are discussed. On the basis of the above analysis, we have designed and fabricated two types of W-band CW TWTs. The test results show that beam transmissions are over 98.5% at DC and better than 96.5% at the highest peak RF power. Measured saturated output powers are greater than 33.6 W with saturated gain more than 42.2 dB within frequency range of 91–99 GHz, and an improved gain flatness of 4.8 dB is achieved. PubDate: 2020-07-30
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