Study on Terahertz Time‐Domain Spectroscopy of HNS by Sample Measure and Quantum Chemistry Calculation

The absorption spectra of 2,2′,4,4′,6,6′‐Hexanitrostilbene (HNS), in the frequency range between 0.2 and 4.0 THz, are calculated using quantum chemistry calculations and resulted in 1.9 THz and 3.3 THz. The experimental result offered by THz time‐domain spectroscopy and FTIR shows that the characteristic peaks are located at 1.7 THz and 3.1 THz. Compared with experimental results, all the evidences indicate that HNS has distinct characteristic peaks, which agree with the simulation results. It is believed that this method can detect this material quickly and accurately, which means a lot to the explosives detection.     

太赫兹辐射的研究及应用

太赫兹波(Terahertz。THz)是指频率在0.1~10 THz或波长为30μm~3 mm范围内的电磁波。THz频段是一个非常具有科学价值但尚未完全开发的电磁辐射区域,它在成像、医学诊断、环境科学、信息通信、生物化学及基础物理研究领域有着广阔的应用前景。本文综述了THz波的特点、发展状况,重点介绍了THz技术的应用领域,简单阐述了THz技术的发展方向及存在问题,并讨论了其发展前景。     

Terahertz absorption spectra of Fatty acids and their analogues

Absorption spectra in the terahertz (THz) region between 10 and 400 cm(-1) were measured for fatty acids and their analogues at room temperature. Saturated fatty acids such as palmitic and stearic acids had some sharp peaks, while unsaturated fatty acids such as oleic, linoleic and linolenic acids had two distinct peaks at 247 and 328 cm(-1). These peaks apparently derived from the carboxylic group because oleyl alcohol had no distinct peak. The THz absorption spectra of fatty acids may be affected by the crystalline as well as the chemical structure. The THz absorption spectra of oleic acid esters depended on ester types, although all oleic acid esters had some peaks due to the ester group. THz absorbance of fatty acids positively correlated with concentration. Based on these results, THz spectrometry may be a good analytical method for the non-destructive qualitative and quantitative evaluation of fatty acids and their analogues.     

Towards high-power single-cycle THz laser for initiating high-field-sensitive phenomena

Powerful THz radiation confined in one field period or less is an adequate tool for triggering nonlinear actions. We show results towards the realization of a tunable high-power THz source based on a laser-driven frequency conversion scheme in plasma and nonlinear crystals. A powerful THz source in combination with the future X-ray Free Electron Laser facility in Switzerland (SwissFEL) holds promise for exciting experiments in a variety of different research areas.     

Trace Identification and Visualization of Multiple Benzimidazole Pesticide Residues on Toona sinensis Leaves Using Terahertz Imaging Combined with Deep Learning

Molecular spectroscopy has been widely used to identify pesticides. The main limitation of this approach is the difficulty of identifying pesticides with similar molecular structures. When these pesticide residues are in trace and mixed states in plants, it poses great challenges for practical identification. This study proposed a state-of-the-art method for the rapid identification of trace (10 mg·L⁻¹) and multiple similar benzimidazole pesticide residues on the surface of Toona sinensis leaves, mainly including benzoyl (BNL), carbendazim (BCM), thiabendazole (TBZ), and their mixtures. The new method combines high-throughput terahertz (THz) imaging technology with a deep learning framework. To further improve the model reliability beyond the THz fingerprint peaks (BNL: 0.70, 1.07, 2.20 THz; BCM: 1.16, 1.35, 2.32 THz; TBZ: 0.92, 1.24, 1.66, 1.95, 2.58 THz), we extracted the absorption spectra in frequencies of 0.2–2.2 THz from images as the input to the deep convolution neural network (DCNN). Compared with fuzzy Sammon clustering and four back-propagation neural network (BPNN) models (TrainCGB, TrainCGF, TrainCGP, and TrainRP), DCNN achieved the highest prediction accuracies of 100%, 94.51%, 96.26%, 94.64%, 98.81%, 94.90%, 96.17%, and 96.99% for the control check group, BNL, BCM, TBZ, BNL + BCM, BNL + TBZ, BCM + TBZ, and BNL + BCM + TBZ, respectively. Taking advantage of THz imaging and DCNN, the image visualization of pesticide distribution and residue types on leaves was realized simultaneously. The results demonstrated that THz imaging and deep learning can be potentially adopted for rapid-sensing detection of trace multi-residues on leaf surfaces, which is of great significance for agriculture and food safety.     

Graphene–metamaterial hybridization for enhanced terahertz response

Improving the interaction of graphene with terahertz (THz) waves in experiment – through experimental measurement is a challenge for THz detectors, modulators, and other THz photonic components based on graphene. Hybridization of graphene with metamaterials leads to a strong THz response enhancement. Here, we observed maximum enhancement of 33.0% in non-resonant region and 23.8% in resonant region with the hybridization of graphene and metamaterials in experiment. A coupling model as well as numerical calculation has been carried out to fully investigate the influence of this coupling. The results suggest that there exists an exponential relationship between coupling strength and THz response in both resonant and non-resonant region, while the resonant frequency shift shows a linear growth with coupling strength. The bandwidth of the resonance shows exponential increasing with the damping constant. Correspondingly, the numerical calculation shows the similar dependency with the electrical conductivity of the graphene overlayer. This suggests a higher conductivity for stronger coupling. Substrates could also bring the remote phonon scattering, charge transfer, and dielectric effect, which show the influence such as low dielectric constant for high coupling.     

Near field enhancement for THz switching and THz nonlinear spectroscopy applications

Slits in thin metal sheets and split-ring resonators (SRR) featuring gaps on the micro- or nano-scale are shown to be a promising tool for THz switching or THz nonlinear spectroscopy applications. Both structures show strong field enhancement in the gap region due to light-induced current flows and capacitive charging across the gap. Whereas nano-slits allow for broadband enhancement the resonant behavior of the SRRs leads to narrowband amplification and results in field enhancement of tens of thousands. This property is particularly beneficial for the realization of nonlinear THz experiments.     

New polaritonic materials in the THz range made of directionally solidified halide eutectics

Directionally solidified alkali halide binary eutectics have been recently proposed as THz polaritonic metamaterials based on their ordered microstructure and the suitable phonon–polariton resonances in the THz range of the spectrum. In the present work we focus on the search of new available eutectic systems both binary and ternary eutectics with well-ordered fibrous or lamellar microstructures and interparticle distances from 1 to several tens of microns. Simple effective homogenization models have been used to calculate effective permittivity and transmittance in the THz range of eutectic slices. This lets us identify the electromagnetic spectral ranges where hyperbolic dispersion is expected together with a significant transmittance value. The hyperbolic dispersion range shifts with microstructure size, that is, with growth parameters, showing that the materials response can be finely tuned by the manufacture conditions. Applications with these materials cover the electromagnetic range from 5 to 20 THz.     

Development of Nd (III)-Based Terahertz Absorbers Revealing Temperature Dependent Near-Infrared Luminescence

Molecular vibrations in the solid-state, detectable in the terahertz (THz) region, are the subject of research to further develop THz technologies. To observe such vibrations in terahertz time-domain spectroscopy (THz-TDS) and low-frequency (LF) Raman spectroscopy, two supramolecular assemblies with the formula [Nd^III (phen)₃ (NCX)₃] 0.3EtOH (X = S, 1-S; Se, 1-Se) were designed and prepared. Both compounds show several THz-TDS and LF-Raman peaks in the sub-THz range, with the lowest frequencies of 0.65 and 0.59 THz for 1-S and 1-Se, and 0.75 and 0.61 THz for 1-S and 1-Se, respectively. The peak redshift was observed due to the substitution of SCN⁻ by SeCN⁻. Additionally, temperature-dependent TDS-THz studies showed a thermal blueshift phenomenon, as the peak position shifted to 0.68 THz for 1-S and 0.62 THz for 1-Se at 10 K. Based on ab initio calculations, sub-THz vibrations were ascribed to the swaying of the three thiocyanate/selenocyanate. Moreover, both samples exhibited near-infrared (NIR) emission from Nd (III), and very good thermometric properties in the 300–150 K range, comparable to neodymium (III) oxide-based thermometers and higher than previously reported complexes. Moreover, the temperature dependence of fluorescence and THz spectroscopy analysis showed that the reduction in anharmonic thermal vibrations leads to a significant increase in the intensity and a reduction in the width of the emission and LF absorption peaks. These studies provide the basis for developing new routes to adjust the LF vibrational absorption.     

Modulation of terahertz properties of 3D ceramic photonic crystals via post-creation non-metal anion doping treatment

Ceramic photonic crystals have wide application potentials for THz technology due to their characteristic strong chemical stability and high temperature resistance for demanding environment. However, it is still a challenge to modulate the THz properties of as-prepared ceramic TPCs via post-creation treatments to satisfy different application requirements. In this work, the THz properties of rutile TiO₂ ceramic 3D-TPCs were modulated by non-metal anion doping of either carbon or nitrogen, which was firstly created by the direct-writing technique with a TiO₂ sol-gel ink followed by the high temperature sintering process at 1200°C. Calcination of the pristine rutile TiO₂ ceramic 3D-TPCs in either CO or NH₃ atmosphere successfully doped either C or N dopants into rutile TiO₂ ceramic 3D-TPCs, respectively, which changed their dielectric properties and subsequently induced different THz properties on these ceramic 3D-TPCs with the same structure. Their THz properties were found to be largely affected by the specific nature of the dopant. Compared with the pristine rutile TiO₂ ceramic 3D-TPC, the bandgap positions of both C-doped and N-doped rutile TiO₂ ceramic 3D-TPCs largely shifted toward the higher frequency, while the N-doped sample demonstrated a much broader bandgap. Thus, non-metal anion doping could be an effective post-creation approach to modulate the THz properties of ceramic TPCs.     

THz dielectric properties of AlPO4-BPO4-SiO2 ternaries

0.05AlPO₄-0.05BPO₄-0.90SiO₂ glass-ceramic and glass-free 0.45AlPO₄-0.45BPO₄-0.10SiO₂ ceramic have been prepared by a solid-state reaction process and their terahertz (THz) dielectric responses were characterized by time-domain spectroscopy (TDS) techniques in the frequency range from 0.2 to 1.6 THz. Both samples demonstrate little dielectric dispersions in the (sub) THz frequency range, having dielectric permittivities (ε_r) of ∼3.35(7) and ∼4.06(8), respectively. The dielectric responses are dominated by the phonons and electronic polarizations in the (sub) THz frequency range. The contributions to the relative permittivities from electronic polarization ε_{r ∞} are 1.85(4) and 1.99(4), which are ∼55% and ∼49% of the total ε_r at THz frequencies, respectively_. The dielectric losses of both samples continuously increase with increasing frequency in the THz band, while the loss factor of glass-ceramic is one order of magnitude higher than that of glass-free ceramic. The glass-free sample exhibits extremely high Q×f values of 100,000–190,000 GHz in the THz band, being much higher than that of the glass-ceramic (∼7000–25,000 GHz).     

Extraordinary terahertz absorption bands observed in micro/nanostructured Au/polystyrene sphere arrays

Terahertz (THz) time-domain spectroscopy is carried out for micro/nanostructured periodic Au/dielectric sphere arrays on Si substrate. We find that the metal-insulator transition can be achieved in THz bandwidth via varying sample parameters such as the thickness of the Au shell and the diameter of the Au/dielectric sphere. The Au/polystyrene sphere arrays do not show metallic THz response when the Au shell thickness is larger than 10 nm and the sphere diameter is smaller than 500 nm. This effect is in sharp contrast to the observations in flat Au films on Si substrate. Interestingly, the Au/polystyrene sphere arrays with a 5-nm-thick Au shell show extraordinary THz absorption bands or metallic optical conductance when the diameter of the sphere is larger than 200 nm. This effect is related to the quantum confinement effect in which the electrons in the structure are trapped in the sphere potential well of the gold shell.     

Terahertz spectroscopic estimation of crystallinity of poly(phenylene sulfide)

As a crystalline polymer, various properties of poly(phenylene sulfide) (PPS) depend on its crystallinity. Evaluation of crystallinity of PPS is examined by terahertz (THz) absorption spectroscopy in this research. For sheets of PPS thermally annealed differently, sharp absorption peaks appearing at 3.2 and 4.2 THz are well proportional to the crystallinity estimated by X‐ray diffraction, differential scanning calorimetry, and Fourier transform infrared (FTIR) absorption spectroscopy. Therefore, THz absorption spectroscopy can be a good tool for estimating the crystallinity of PPS. Especially, it is useful for thick PPS sheets, for which FTIR spectroscopy in transmission mode is difficult to apply due to the saturation of absorbance. Since THz spectroscopy and FTIR spectroscopy are complementary in the vibrations that the two methods can detect, using the two methods should be effective to improve the accuracy of estimation of crystallinity. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46427.     

Terahertz Time‐Domain Spectroscopy of 0.73Pb(Mg1/3Nb2/3)O3–0.27PbTiO3 Single Crystal

Lead magnesium niobate titanate is an important ferroelectric material. In this study, the terahertz (THz) transmission properties of a 0.73Pb(Mg_1/3Nb_2/3)O₃–0.27PbTiO₃ single crystal were investigated using a time‐domain spectroscopy method. Complex refractive index and dielectric dispersion functions were determined from the amplitude and phase information derived from time‐domain responses. Based on calculations, it was concluded that the room‐temperature dielectric constant of the single crystal equal to ~30 at 1 THz. This result could be a useful reference for development of ferroelectric‐material‐based THz components and devices.     

Scalable synthesis of pyrrolic N-doped graphene by atmospheric pressure chemical vapor deposition and its terahertz response

Atmospheric pressure chemical vapor deposition is employed to synthesize the N-doped graphene, which is mainly composed of pyrrolic type N bonding configuration with a controllable doping concentration from ∼1.6% to ∼6.4%. Transmission electron microscope, X-ray photoelectron spectroscopy, and Raman spectrometer are used for characterizing the pyrrolic N-doped graphene. X-ray photoelectron spectra confirm the dominant pyrrolic N bonding configuration, which is consistent with the Raman spectra compared with the pristine graphene. THz time-domain spectroscopy, four-probe DC electrical measurements, and visible spectroscopy are also utilized to analyze doping concentration qualitatively.The investigation suggests that THz wave is sensitive to the dopant concentration, which is relevant to the conductivity, while the visible light is insensitive to the dopant concentration. Our results further extend the synthesis method of N-doped graphene and the new type doped graphene might have potential applications in electrochemistry, electronics, photonics, and THz devices.     

The intensive terahertz electroluminescence induced by Bloch oscillations in SiC natural superlattices

We report on efficient terahertz (THz) emission from high-electric-field-biased SiC structures with a natural superlattice at liquid helium temperatures. The emission spectrum demonstrates a single line, the maximum of which shifts linearly with increases in bias field. We attribute this emission to steady-state Bloch oscillations of electrons in the SiC natural superlattice. The properties of the THz emission agree fairly with the parameters of the Bloch oscillator regime, which have been proven by high-field electron transport studies of SiC structures with natural superlattices.     

Tunable Terahertz Wavelength Conversion Based on Optofluidic Infiltrated Rib Silicon Waveguides

Silicon - Terahertz (THz) sources have attracted special attention for various applications. Compared to the standard silica-based optical fibers, silicon waveguides have more advantages such as...     

Characterization of dielectric properties of oxide materials in frequency range from GHz to THz

We measured complex dielectric permittivity using THz time-domain spectroscopy (THz-TDS) to clarify the dielectric properties of oxide materials in a frequency range from GHz to THz. Piezoelectric and ferromagnetic oxide single crystals, such as quartz (SiO₂), zinc oxide (ZnO), Bi substituted rear-earth iron garnet (BiRIG), and LiTaO₃ (LT), were used. We obtained the complex dielectric permittivity of these materials in a frequency range from 100 GHz to 2 THz. The ɛ′ and ɛ″ obtained for SiO₂ were in agreement with previous reports. We observed dielectric relaxation in ZnO crystal from 100 GHz to 1 THz, which originated from n-type conductivity. In the BiRIG, the values of the dielectric permittivity increased as the frequency increased, and the values of the dielectric permittivity with the magnetic field were smaller than those without the magnetic field throughout the measured frequency range. In a comparison between congruent LiTaO₃ (CLT) and stoichiometric LiTaO₃ (SLT), the ɛ₃₃ of the CLT was very similar to that of the SLT, but a lot of difference was between the ɛ₁₁ of evident CLT and SLT within the measured frequency region. We determined that the point defects had profound effect on the dielectric performance of the LT.     

Tunable Terahertz Circularly Polarized Dielectric Resonator Antenna with the Higher Order Modes

Silicon - A tunable terahertz (THz) dielectric resonator (DR) antenna (DRA) with circular polarization operating with multiple higher order modes is numerically studied and implemented. A...