Abstract: Terahertz (THz) spectroscopy is regarded as a professional technology to probe collective molecular vibrational modes and intensity of bio–molecules, which is potential to analyze amino acids and short peptides. In this study, THz spectroscopy is utilized to investigate the correlations of glycine (Gly), Serine (Ser), amino acid mixtures of Gly/Ser, and dipeptides (GS and SG) in the frequency range of 0.3–2.5 THz. The results manifest that the collective molecular vibrational intensity of Ser is larger than Gly, implying that the hydroxyl methyl group (–CH2–OH) on the side chain of Ser exhibits larger motions than that of –H in Gly. Additionally, the THz spectra of amino acid mixtures of Gly/Ser fit the linear addition of the individual amino acid absorption spectrum. Herein, the THz spectra of dipeptide GS and its isomer SG are distinguished according to the different collective molecular vibrations produced by the terminal groups on the carbon skeleton. Compared to physical amino acid mixtures of Gly/Ser, the dipeptides exhibit diverse THz spectra, indicating that the different collective vibrational modes are caused by peptide bonds and terminal groups. These solid evidences confirm that THz spectroscopy can be availably used to analyze the correlations of amino acids, amino acid mixtures, and dipeptides, as well as to identify dipeptide and its isomer from molecular level. PubDate: 2020-11-23
Abstract: At present, the terahertz time-domain spectroscopic information of each component in multi-composition compounds detection is comprehensively combined. The lower resolution level of mixed spectra has posed many difficulties in signal analysis due to the overlapping characteristic information in the mixed ones. In this paper, to compare the performances of Nonnegative Matrix Factorization (NMF), Self-modeling Mixture Analysis (SMMA) and Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) on complex systems, a binary mixture and a ternary mixture are employed during THz-TDS testing. The position of the absorption peak (PK) and the correlation coefficient are used to evaluate the decomposition effects. The experimental results show that the component spectra resolved by MCR-ALS demonstrate good consistency with the sample components. Further, MCR-ALS presents excellent results in comparison with NMF and SMMA in terms of decomposing precision and computing speed. MCR-ALS thus appears a promising algorithm to resolve the THz multi-way data, and may be useful for many applications, such as medicine quality assurance and unknown component identification in the general area of terahertz science and technology. PubDate: 2020-11-23
Abstract: In this paper, a 4 × 4 60-GHz circularly polarized (CP) magnetoelectric (ME) dipole array based on TE340-mode substrate integrated cavity (SIC) and sequential rotation feed-network (SRFN) is proposed. The ME dipole array is constructed by 3 printed circuit board (PCB) layers with its assembled size of 21 × 23 × 2.4 mm3. The top layer is the radiation layer with 2 × 2 subarrays. The second layer includes 2 × 2 TE340-mode SICs. The bottom layer is the substrate integrated waveguide (SIW)-SRFN. By combining ME dipole, SRFN and high-order-mode SIC techniques, a compact 4 × 4 array with wide axial-ratio (AR) and impedance bandwidths is obtained. The measured results show that the − 10-dB impedance bandwidth is 24% (54–68.7 GHz), the 3-dB AR bandwidth is 25.9% (52–67.5 GHz), the 3-dB gain bandwidth is 22.5% (54–67.5 GHz) and the peak gain is 19.4 dBic at 62 GHz. The proposed array is a good candidate for 60 GHz wireless communications with its advantages of compact size, wide impedance bandwidth, wide AR bandwidth and high gain. PubDate: 2020-11-10
Abstract: β-dicalcium silicate (β-C2S), the second most prevalent constituent of Ordinary Portland Cement (OPC), reacts slowly during the early stage of hydration but is responsible for long-term strength development of concrete. To accelerate the hydration during this stage, nanomaterials are incorporated in the β-C2S matrix which results in pozzolanic reactions. In this work, nanosilica has been added in different percentages, and the acceleration in hydration has been tracked by employing Terahertz spectroscopy for a period of 100 days. This study clearly demonstrates that nanosilica incorporation accelerates the early-stage hydration of β-C2S as seen by the reduction in intensity of the resonances between 500 and 520 cm−1. Density functional theory (DFT) simulations of β-C2S confirm these resonances to be due to SiO4 and CaO bending modes. The prominence of the resonance around 455 cm−1 for the nanosilica-incorporated β-C2S indicates early-stage formation of the key hydration product, calcium silicate hydrate (C–S–H). Furthermore, sharpening of the resonance around 283 cm−1 during hydration could possibly indicate the formation of more ordered structures of the hydration products. By tracking the shift of these resonances, it can be inferred that the degree of polymerization of silicates during C–S–H formation is different compared to cement and C3S. To track the structural changes during hydration, scanning electron microscopy (SEM) has been carried out which shows the formation of fiber-like morphologies, indicative of early-stage formation of C–S–H for the nanosilica-incorporated samples. This study could potentially help to engineer β-C2S-rich cement matrix with nanomaterials which could aid in reducing CO2 emission during cement manufacturing. PubDate: 2020-11-01
Abstract: Terahertz spectroscopy and density functional theory (DFT) calculations have been used to study the dipeptide l-carnosine. In this paper, we expand the range of materials described with THz measurements and DFT calculations from amino acids to dipeptides. Three clear resonances were detected in the experimental spectrum at 1.5 THz, 1.9 THz, and 2.3 THz. Additionally, we performed periodic boundary condition DFT calculations. The computational spectrum accurately reproduces the experimental one. PubDate: 2020-11-01
Abstract: We present a joint experimental and computational terahertz (THz) spectroscopy study of the most stable polymorph (form I) of an antihypertensive pharmaceutical solid, felodipine (FLD). The vibrational response has been analyzed at room temperature by combining optical (THz-TDS, FT-IR, THz-Raman) and neutron (INS) terahertz spectroscopy. With the challenging example of a large and flexible molecular solid, we illustrate the complementarity of the experimental techniques. We show how the results can be understood by employing ab initio modeling and discuss current progress in the field. To this end, we employ plane wave formulation of density functional theory (plane wave DFT) along with harmonic lattice dynamics calculations (HLD) and ab initio molecular dynamics (AIMD) simulations. Based on a comprehensive theoretical analysis, we discover an inconsistency in the commonly accepted structural model, which can be linked to a distinct librational dynamics of the side ester chains. As a result, only a moderate agreement with the experimental spectra can be achieved. We, therefore, propose an alternative structural model, effectively accounting for the influence of the large-amplitude librations and allowing for a comprehensive analysis of the vibrational resonances up to 4.5 THz. In that way, we illustrate the applicability of the computationally supported THz spectroscopy to detect subtle structural issues in molecular solids. While the provided structural model can be treated as a guess, the problem calls for further revision by means of high-resolution crystallography. The problem also draws a need of extending the THz experiments toward low-temperature conditions and single-crystal samples. On the other hand, the studied system emerges as a challenge for the DFT modeling, being extremely sensitive to the level of the theory used and the resulting description of the intermolecular forces. FLD form I can be, hence, considered as a testbed for the use of more sophisticated theoretical approaches, particularly relying on an advanced treatment of the van der Walls forces and going beyond zero-temperature conditions and harmonic approximation. PubDate: 2020-11-01
Abstract: Accurate interpretations of THz vibrational spectra using density functional theory (DFT) must account for the often significant thermal expansion exhibited by molecular crystal systems. In this study, the THz spectra of two p-nitrophenol polymorphs were obtained in the spectral bandwidth of 20–95 cm−1, and solid-state DFT was used to assign the observed absorptions to specific vibrational mode characters. Computational treatments of the crystal systems involved comparison of phonon frequencies produced from both fixed- and full-unit cell geometry optimizations using a variety of functional and basis set combinations. Arbitrary frequency scaling was applied to overcome the temperature dependence of unit cell volumes and associated phonon vibrational frequencies. Fully relaxed cell geometries producing contracted cell volumes resulted in a large overestimation of THz vibrational frequencies. Employing appropriate frequency scaling factors to the computed mode frequencies allowed for proper vibrational mode assignments and enhanced the quality of spectral simulations versus fixed-cell calculations. Optimal frequency scalars in the range of 0.675 to 0.827 were determined to best reproduce the THz spectra of the p-nitrophenol polymorphs. This treatment of calculated phonon modes offers the key advantage of eliminating the temperature correlation between the crystal structure data and the THz data acquisition. PubDate: 2020-11-01
Abstract: The low-frequency (terahertz) dynamics of condensed phase materials provide valuable insight into numerous bulk phenomena. However, the assignment and interpretation of experimental results require computational methods due to the complex mode types that depend on weak intermolecular forces. Solid-state density functional theory has been used in this regard with great success, yet the selection of specific computational parameters, namely the chosen basis set and density functional, has a profound influence on the accuracy of predicted spectra. In this work, the role of these two parameters is investigated in a series of organic molecular crystals, in order to assess the ability of various methods to reproduce intermolecular forces, and subsequently experimental terahertz spectra. Specifically, naphthalene, oxalic acid, and thymine were chosen based on the varied intermolecular interactions present in each material. The results highlight that unconstrained geometry optimizations can be used as an initial proxy for the accuracy of interatomic forces, with errors in the calculated geometries indicative of subsequent errors in the calculated low-frequency vibrational spectra, providing a powerful metric for the validation of theoretical results. Finally, the origins of the observed shortcomings are analyzed, providing a basic framework for further studies on related materials. PubDate: 2020-11-01
Abstract: Hydroquinone (HQ) and its clathrate are materials of growing interests, due to their promising applications in energy related science and industries. Recently, many studies have been performed to understand the properties of these materials. Terahertz (THz) spectroscopy is a powerful tool for studying the properties of these materials by non-destructively probing their low-energy (meV) dynamics. Although terahertz spectra of HQ and its clathrates have been measured, a report on the correspondence between THz spectra and low frequency dynamics is still lacking. In this paper, we measure the temperature-dependent THz spectra of both α-HQ (the non-clathrate form) and β-HQ clathrate, with Ar and CO2 as guest species. We also perform density functional theory (DFT) simulations on these materials, in order to assign the spectral features. We find an excellent match between the experimental and the DFT calculated spectra. Using the simulation result, we build connections between the THz spectra and the atomic motions in these materials. In addition, we also perform DFT simulations on β-HQ-He, β-HQ-Ne, and β-HQ-Kr to study the patterns in the change of THz spectra as the guest species changes. PubDate: 2020-11-01
Abstract: High-resolution and broadband THz spectra of the crystals of nine polycyclic aromatic hydrocarbons (PAHs) are presented. Five PAHs are comprised of ortho-fused benzene rings and the other four of peri-fused benzene rings. THz mode assignment is performed by using the anthracene and pyrene crystals as examples. The performance of the PBE functional augmented by Grimme’s two dispersion correction terms, D* and D3, respectively, are rigorously evaluated against the experimental criteria of frequency and isotope shift (IS). The D* and D3 terms use empirical and semi-classical approach for correcting the London-type dispersion interactions, respectively. The nature of each THz mode simulated by PBE-D* and that by PBE-D3 is quantitatively compared in terms of the percentage contributions of the intermolecular and the intramolecular vibrations to the vibrational energy. We find that the two methods have equivalent performance in reproducing the frequencies, ISs, and nature of THz modes of both the anthracene and pyrene crystals. PubDate: 2020-11-01
Abstract: The sub-200 cm−1 (sub-6 THz) vibrations of molecular crystals provide identifying features that are characteristic of each solid sample under study. These distinctive vibrational spectra have driven the development of new techniques and instrumentation in analytical spectroscopy. As terahertz time-domain spectroscopy and low-frequency Raman spectroscopy become increasingly prevalent in non-specialist laboratories, the need for a common set of spectral standards for use across these techniques becomes imperative. To meet this need, α-lactose monohydrate, biotin, and L-cystine are proposed here as molecular standards to evaluate instrument performance with both terahertz and Raman spectroscopies, as well serve as benchmarks for quantum mechanical simulations and analyses of these spectra. These substances all reveal a series of readily discernable peaks across the low-frequency region and also over a range of temperatures (295–50 K) making them even more useful. The often overlooked aspect of detailed spectral interpretation and assignment is directly addressed with rigorous solid-state density functional theory simulations of the three compounds based on a standard computational framework. By investigating these proposed molecular crystal standards with commonly available experimental and theoretical approaches, a set of realistic performance expectations can be achieved for both commercial instrumentation and software being used in low-frequency vibrational spectroscopy. PubDate: 2020-11-01
Abstract: A new waveguide stepped septum-type circular polarizer (SST-CP) was developed to operate in the 500-GHz band for radio astronomical and planetary atmospheric observations. In a previous study, we developed a practical SST-CP for the 230-GHz band. However, several issues prevent this device being easily scaled down to the 500-GHz band, such as manufacturing dimensional errors and waveguide flange position errors. In this study, we developed a new waveguide flange with a high-accuracy position determination mechanism and a very small size of 10 × 10 mm. We also developed a new fabrication technique to obtain very good flatness for the device’s blank materials by high-accuracy polishing using a resin fixture. Using these new methods, the manufactured 500-GHz band SST-CP achieved a cross-polarization talk level of better than – 30 dB at 465–505 GHz, a device surface flatness of within 3 μm, and also the horizontal positioning error of ± 3 μm. These results indicate that the developed 500-GHz band SST-CP has high performance in the high-frequency band, and thus the new manufacturing methods are effective in the 500-GHz band. PubDate: 2020-10-29
Abstract: The phase stability of a 140-GHz, 1-kW pulsed gyro-amplifier system and the phase dependence on the cathode voltage were experimentally measured. To optimize the measurement precision, the amplifier was operated at 47 kV and 1 A, where the output power was ∼30 W. The phase was determined to be stable both pulse-to-pulse and during each pulse, so far as the cathode voltage and electron beam current are constant. The phase variation with voltage was measured and found to be 130 ± 30°/kV, in excellent agreement with simulations. The electron gun used in this device is non-adiabatic, resulting in a steep slope of the beam pitch factor with respect to cathode voltage. This was discovered to be the dominant factor in the phase dependence on voltage. The use of an adiabatic electron gun is predicted to yield a significantly smaller phase sensitivity to voltage, and thus a more phase-stable performance. To our knowledge, these are the first phase measurements reported for a gyro-amplifier operating at a frequency above W-band. PubDate: 2020-10-27
Abstract: This paper presents the results of a study on improving the performance parameters such as the impedance bandwidth, radiation gain and efficiency, as well as suppressing substrate loss of an innovative antenna for on-chip implementation for millimetre-wave and terahertz integrated-circuits. This was achieved by using the metamaterial and the substrate-integrated waveguide (SIW) technologies. The on-chip antenna structure comprises five alternating layers of metallization and silicon. An array of circular radiation patches with metamaterial-inspired crossed-shaped slots are etched on the top metallization layer below which is a silicon layer whose bottom surface is metalized to create a ground plane. Implemented in the silicon layer below is a cavity above which is no ground plane. Underneath this silicon layer is where an open-ended microstrip feedline is located which is used to excite the antenna. The feed mechanism is based on the coupling of the electromagnetic energy from the bottom silicon layer to the top circular patches through the cavity. To suppress surface waves and reduce substrate loss, the SIW concept is applied at the top silicon layer by implementing the metallic via holes at the periphery of the structure that connect the top layer to the ground plane. The proposed on-chip antenna has an average measured radiation gain and efficiency of 6.9 dBi and 53%, respectively, over its operational frequency range from 0.285–0.325 THz. The proposed on-chip antenna has dimensions of 1.35 × 1 × 0.06 mm3. The antenna is shown to be viable for applications in millimetre-waves and terahertz integrated-circuits. PubDate: 2020-10-26
Abstract: Solid-state sources consisting of cascaded Schottky diode-based frequency multipliers are commonly used to generate power at millimeter and submillimeter wave bands. In order to achieve increasing power at those frequencies, first-stage frequency multipliers have to handle a large amount of input power and generate enough output power to drive higher frequency multiplication stages. To that end, an appropriate electro-thermal design of multiplier circuits can result in improvement of power-handling capabilities without degradation of conversion efficiency. This work proposes an approach for the design of frequency multipliers where both electrical and thermal considerations are self-consistently taken into account along the optimization of the Schottky diode structure and the circuit layout. The proposed methodology is validated with the design and test of a split-block waveguide single-chip tripler circuit with output frequency in the 225–325-GHz band. Good agreement between predicted performance and measured results is obtained for a wide input power range and different ambient temperatures. PubDate: 2020-10-16
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: 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: 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
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