Abstract: Terahertz (THz) absorption spectra of cross-linked polyurethane films were observed at 30–180 °C. The obtained spectra displayed characteristic absorption bands. A band observed at 2.6 THz was assigned to the C=O···N-H hydrogen bond vibration using previously reported data. DFT calculation results confirmed that a band at 7.5 THz arises owing to the torsional vibration of cross-linked molecules containing urethane bonds. The slight changes in the temperature-dependent spectra were analyzed by the first derivative of absorption intensity with temperature. The thermal behavior of absorption intensities indicates the dynamics of hydrogen bonds and cross-linked structures in polyurethane films. PubDate: 2020-03-01
Abstract: Abstract Metasurface is an artificial material composed of a series of subwavelength structure units and has unique electromagnetic characteristics. Based on the ability of manipulating the phase, amplitude, and polarization of electromagnetic wave, various kinds of metasurfaces are designed to realize wavefront manipulations, such as beam focusing, beam steering, vector beams generating, and holographic imaging. This paper reviews the design methods of metasurfaces for wavefront modulation and evolution of the metasurfaces designed for wavefront manipulation in the terahertz (THz) region. The metasurfaces can be divided into two categories: passive and active metasurfaces. For the passive metasurfaces, the single-functional metasurfaces, multifunctional metasurfaces, and high diffraction efficient metasurfaces designed for various THz wavefront shaping, such as focusing, imaging, and special beams generating, are reviewed. For the active metasurfaces, the metasurfaces with fixed structure and all-optical metasurfaces without fixed structure for THz wavefront modulation are summarized. Furthermore, a comparison on the performance of different kinds of metasurfaces for THz wavefront modulation is presented and the development direction and challenges of the THz wavefront modulation metasurfaces in the future are discussed. PubDate: 2020-02-21
Abstract: Abstract The effects of small amounts of enantiomeric impurities on the vibrational modes of polycrystalline L-histidine (His) were examined using high-frequency-accurate terahertz (THz) spectroscopy. Samples of polycrystalline L-His in its monoclinic form (form B) were grown from an ethanol-rich solvent. D-histidine, ranging from 0.05 to 10.0% in content, was added to the L-His/ethanol-rich solution prior to recrystallization. The crystal forms of all recrystallized samples were first confirmed by high-frequency-accurate THz spectra recorded at 70 K and room-temperature powder X-ray diffraction spectra. A detailed analysis of two spectral features (1.54 and 1.80 THz) measured at 10 K showed that the absorption lines had a narrow linewidth, as well as broad spectral components that could be fitted by Gaussian curves corresponding to defects within the crystals. The shifts in peak frequency with varying amounts of D-His additive in the polycrystalline L-His samples were examined in detail. It was observed that the peak frequency shifted to lower frequencies when small amounts of D-histidine were present and then to higher frequencies when relatively large amounts were present. At low concentrations, D-His molecules are thought to act as impurities that disturb the normal mode vibrations of L-His crystals; however, above certain concentrations, D-His contributes to the formation of a DL racemic crystal. Although the effects of small amounts of D-His molecules in L-His crystals were detectable, quantitative determination of trace amounts of impurity was limited by the complex crystallography of L-His, which includes defects and racemic compounds. PubDate: 2020-02-21
Abstract: Abstract The automatic classification of paraffin-embedded prostate cancer tissue biopsy samples in terms of the Gleason scale is proposed using terahertz (THz) spectroscopy and machine learning. The samples with normal tissues (N=80) and prostate cancer tissues corresponded to the Gleason 4 (N=10), and 8 (N=13) scores, were analyzed. Absorption spectra of paraffin-embedded prostate cancer and healthy tissues were measured in the 0.2–1.5 THz range. The principal component analysis, support vector machine (SVM), and “majority vote” classification were applied to analyze experimental data. The original algorithm of spatial regions of interest selecting was developed to reduce the influence of the plastic base of a paraffin block on the results of a sample classification. The predictive model of the experimental spectral data of the paraffin blocks in the THz range was created using a set of the “One-Vs-One” binary SVM classifiers. We used multiple random splitting of the spectral data on the training and testing sets in 60%:40% proportion to teach the SVM classifiers. Validation of the predictive model showed 100% accuracy of the classification of the samples from the testing set. PubDate: 2020-02-21
Abstract: Terahertz spectroscopy has proven to be a powerful tool for the study of condensed phase materials, opening research directions in a number of fields ranging from the pharmaceutical to semiconducting industries. Recent developments in terahertz technology have made this technique more accessible than ever before, and an increasing number of researchers are turning to terahertz spectroscopy for analysis and characterization of advanced materials. However, unlike mid-IR techniques, there do not exist any functional group specific transitions at terahertz frequencies, making the interpretation and assignment of terahertz spectral data more complex than complementary techniques. Through the aid of computational tools, incredible insights into the atomic-level dynamics occurring at terahertz frequencies have been uncovered, yet such highly accurate simulations require more care than traditional simulation methods in order to obtain such results. This review aims to highlight the recent advances in the computational assignment of terahertz spectral data, as well as showcasing common pitfalls to avoid, in order to demonstrate the utility of simulation methods for terahertz spectral assignment. Finally, cutting edge techniques and applications will be discussed, opening the door for future work in this exciting area of terahertz science. PubDate: 2020-02-18
Abstract: There was no appropriate antireflection coating material for organic nonlinear optical (NLO) crystal used for terahertz wave generation; therefore, it was difficult to reduce Fresnel’s reflection loss of laser power used to excite the emitter crystal until we found that an amorphous polymer “Cytop” was applicable. Here, we demonstrate enhanced terahertz (THz) wave generation from an organic NLO crystal using a coating made from Cytop. With the Cytop coating, surface reflection losses of the excitation light were minimized, resulting in a significant improvement in THz wave generation over a wide frequency range. PubDate: 2020-02-15
Abstract: We analyse the use of a tunable dual wavelength Y-branch DBR laser diode for THz applications. The laser generates electrically tunable THz difference frequencies in the range between 100 and 300 GHz. The optical beats are tuned via current injection into a micro-resistor heater integrated on top of one of the distributed Bragg reflector (DBR) section of the diode. The laser is integrated in a homodyne THz system employing fiber coupled ion-implanted LT-GaAs log spiral antennas. The applicability of the developed system in THz spectroscopy is demonstrated by evaluating the spectral resonances of a THz filter as well as in THz metrology in thickness determination of a polyethylene sample. PubDate: 2020-02-15
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: 2020-02-03
Abstract: The main trends in gyrotron development are escalation of the radiated power and increasing the frequency of coherent radiation. For both trends, it is beneficial to develop gyrotrons with wide emitters because this allows one to use cryomagnets with smaller inner bore sizes. For analyzing and optimizing the operation of gyrotrons with wide emitters, it is proposed to represent such emitters as a superposition of thin rings and analyze the properties of electron beams emitted by each of these rings. The present paper consists of two parts. In the first part, the peak values of the orbital velocities and their spread are determined in all fractions of an electron beam in a gyrotron with the standard and widened emitters; also, the effect of profiling the anode on characteristics of these electron beam fractions is considered. In the second part, the interaction efficiency of electron beams produced by thin emitter rings is described and the relationship between these efficiencies and orbital-to-axial velocity ratios in these beams is discussed. PubDate: 2020-02-01
Abstract: The article demonstrates the possibility of creating an over-the-horizon locator with an information carrier of surface plasmon polaritons (SPPs), a kind of surface electromagnetic waves, of the terahertz (THz) spectral range. This possibility is based on the macroscopic propagation length (decimeters) of THz SPPs, as well as on their ability to propagate along convex surfaces of metals. Two schemes of location measurements on gold convex samples using THz SPPs were developed and tested for SPPs generated by free-electron laser radiation with a wavelength of 130 μm. It was found that the intensity of SPPs reflected by an object placed on a conducting surface about 4 cm beyond the horizon line depended on the height of the object above the surface, in accordance with the distribution of the SPP field in the air (a few millimeters or fractions of a millimeter). The distance, orientation, coordinates, and height of an object placed beyond the horizon line can be determined using the SPP locator, which can be applied for localization of hard-to-reach objects on a convex surface. PubDate: 2020-02-01
Abstract: This paper presents a D-band (110–170 GHz) transmitter/receiver (Tx/Rx) chipset with end-fire on-chip antennas (OCAs) using 0.13-μm SiGe BiCMOS technology. The input LO signal frequency of the chipset is 31.25 GHz. A 62.5-GHz frequency doubler, a D-band 2nd-harmonic up-conversion mixer, a power amplifier (PA), and an end-fire antenna are integrated in the Tx. An end-fire antenna, a D-band low-noise amplifier (LNA), a 2nd-harmonic down-conversion mixer, and a 62.5-GHz frequency doubler are integrated in the Rx. The end-fire OCA has gain and efficiency of 4.1 dBi and 83%, respectively. Maximum measured effective isotropic radiation power (EIRP) of 9.2 dBm and conversion gain of 21 dB are achieved for the Tx and Rx, respectively. The Tx/Rx are packaged on PCBs through wire bonding and chip-to-chip wireless communication using 16QAM modulation with the data rate of 4 Gb/s is demonstrated. The DC power consumption of the whole chip is ~1150 mW, and the total chip size is 2 × 3.5 mm2. PubDate: 2020-01-23
Abstract: We have constructed a numerical model for injection-seeded terahertz-wave parametric generators in which the pump depletion, the duration of the pump pulse, as well as the spatial modes of the pump and seed beams are taken into consideration. Compared with other models which only provide the parametric gain under the assumption of low pump depletion, our model can make the predictions on the energy as well as spatial and temporal profiles of the generated terahertz-wave. Besides, the appropriate value of nonlinear coefficient of MgO:LiNbO3 in terahertz regime is difficult to find in the published works. To solve this problem, Miller’s rule is applied for the wavelength scaling of the nonlinear coefficient during calculation. We present a detailed description of the model and show that its predictions agree well with the reported experimental results. The presented model allows the performance to be estimated when designing an injection-seeded terahertz-wave parametric generator. PubDate: 2020-01-11
Abstract: A nonlinear finite-difference time-domain (FDTD) formalism is developed to model dispersive second-order nonlinear effects in photonic arrangements and optical devices. The dispersion of the second-order nonlinear susceptibility, χ(2), is based on the Faust-Henry model, which makes no implicit assumption on the relationship between the linear and nonlinear dispersion. Unlike other models for χ(2) dispersion, the Faust-Henry model accurately describes a broad range of crystal classes, including the \( \overline{4}3m \) crystal class, which is essential to generating radiation in the terahertz frequency regime. As such, the developed formalism based on the Faust-Henry dispersion model overcomes limitations imposed by previous FDTD methods for modelling second-order nonlinear effects. PubDate: 2020-01-11
Abstract: Acousto-optic modulation of terahertz free-electron laser radiation was investigated using diffraction on an acoustic wave in liquefied sulfur hexafluoride at room temperature. At the wavelength 130 μ m, the diffraction efficiency in Bragg regime of interaction 1% was obtained at 75 V driving electrical signal applied to the transducer terminals. PubDate: 2020-01-08
Abstract: This paper reports for the first time on a micromachined interposer platform for characterizing highly miniaturized multi-port sub-THz waveguide components. The reduced size of such devices does often not allow to connect them to conventional waveguide flanges. We demonstrate the micromachined interposer concept by characterizing a miniaturized, three-port, 220–330-GHz turnstile orthomode transducer. The interposer contains low-loss micromachined waveguides for routing the ports of the device under test to standard waveguide flanges and integrated micromachined matched loads for terminating the unused ports. In addition to the interposer, the measurement setup consists of a micromachined square-to-rectangular waveguide transition. These two devices enable the characterization of such a complex microwave component in four different configurations with a standard two-port measurement setup. In addition, the design of the interposer allows for independent characterization of its sub-components and, thus, for accurate de-embedding from the measured data, as demonstrated in this paper. The measurement setup can be custom-designed for each silicon micromachined device under test and co-fabricated in the same wafer due to the batch nature of this process. The solution presented here avoids the need of CNC-milled test-fixtures or waveguide pieces that deteriorate the performance of the device under test and reduce the measurement accuracy. PubDate: 2020-01-08
Abstract: The quality of sunflower seed usually influences the load and product quality. The plumpness, expressing as the percent of the kernel and shell (%), is a crucial indicator reflecting the vigor of the sunflower seed. Investigations were carried out to measure the plumpness of intact sunflower seed by the use of terahertz (THz) transmittance imaging. The THz data of the worms, defects, and sound sunflower seeds were scanned using a THz time-domain transmittance imaging system. Followed THz imaging, the shell and kernel of the sample were separated carefully and recorded RGB images as the reference. Compared with the shell, the absorption coefficients and time domain signals of the kernel showed a significant difference in 0.5–2.0 THz. The characteristic images in 0.5–2.0 THz and control groups of 0.5–1.0, 1.0–1.5, and 1.5–2.0 THz were extracted, and the defects, kernel, and shell could be discriminated clearly in 0.5–2.0 THz. The models of the plumpness were developed between the THz and RGB images processed by the threshold segmentation. The determination coefficient and root mean square error of prediction (RMSEP) were 0.91 and 4% for an independent prediction set, respectively. The results suggested that use of THz transmittance imaging in measurement of plumpness of the intact sunflower seed was feasible. In addition, THz imaging provided a novel quality assessment solution for the coated samples. PubDate: 2020-01-08
Abstract: In this paper, two different millimeter-wave printed dipole array (PDA) antenna prototypes for high-gain applications are proposed. The antenna prototypes are designed, simulated, fabricated, and experimentally tested. A good agreement between the simulated and measured results is achieved. Calculated results show that those antenna prototypes can operate from 51.5 to 74 GHz with a fractional impedance bandwidth of 35.9%. Due to limitations in the measurement facilities, the antenna return loss is measured up to 65 GHz only. At 60 GHz, the two antenna prototypes have maximum realized peak gain of 14.0 dBi and 23.29 dBi, respectively, with a gain variation of 3 dBi and 1.4 dBi, respectively, across the whole frequency band of interest with stable radiation patterns over the operating band. The proposed PDA antenna prototypes achieve good side lobe suppression, excellent front-to-back ratio in both E- and H-planes and low cross-polarization levels over the entire frequency range. PubDate: 2020-01-08
Abstract: Using terahertz time-domain spectroscopy, the frequency-dependent conductivities of polyaniline emeraldine salt-polyethylene (PAni-PE) pellets were measured at different mass concentrations. THz conductivities were compared to the behavior of DC conductivities measured using impedance spectroscopy. The DC conductivity behavior with mass concentration showed a low percolation threshold. The frequency-dependent behavior in the THz region follows the Mott-Davis behavior which shows stronger correlation at higher PAni concentration. At the same time, the conductivity increases exponentially with increasing PAni concentration over the frequency range studied without an apparent percolation threshold. The mechanisms in the two regions studied suggest that there is more dominant localization in the THz regime in contrast with a more dominant percolative transport in the Hz-MHz region. Temperature-dependent measurements showed a decreasing value of parameter S with increasing temperature consistent with a correlated barrier hopping model. Lastly, the parameter S increases in magnitude with a decreasing amount of PAni in the composites reflective of varying conducting and nonconducting compositions. PubDate: 2020-01-06
Abstract: In this work, a 300-GHz 7 × 7 detector array based on a 65-nm Si CMOS technology has been developed and transmission imaging was performed using the detector array without any optical elements. The detector array consists of a 7-by-7 arrangement of 49 pixels in a full-chip size of 4 mm × 4 mm. The unit pixel is composed of a single direct detector with a differential common-gate configuration and a differential patch antenna. The fabricated CMOS detector array chip was mounted on a planar package for electrical characterization and imaging. The measured responsivity and the noise equivalent power (NEP) showed best values of 3599 V/W and 12.46 pW/Hz1/2, respectively, at 303 GHz. Various THz imaging experiments were carried out based on the packaged CMOS detector array with a setup that does not require optical elements or the raster scan of the target object. PubDate: 2020-01-04
Abstract: The paper presents a series of model experiments devoted to the influence of a signal reflected from a non-resonant load on the gyrotron operation regime. The non-resonant reflector used in the experiments was in the form of an iris with remote control of its position in the waveguide. The possibility of frequency tuning in the band determined by the Q-factor of the gyrotron resonator and power modulation in a range of up to 20% was demonstrated in the experiment. PubDate: 2019-12-20
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