Preliminary research on insect damage detection in pecans using terahertz spectroscopy

Grape pomace (GP) and pecan shell (PS) are agro-industrial by-products rich in healthy bioactives, mainly phenolic compounds (PC) and dietary fiber (DF). Raw GP and PS were chemically characterized, and the adsorption isotherms of their PC-insoluble DF (IDF) complexes were evaluated. GP and PS statistically differed (p < 0.05) in chemical and physicochemical properties. Total PC, total flavonoids, and IDF were higher in GP. Thirteen and eight PC were identified in GP and PS by HPLC-MS/MS chromatography, finding isoquercetin and type B dimer and trimer procyanidins as the most abundant compounds. PC-IDF (phenolic compounds-insoluble dietary fiber) adsorption isotherms were determined by both spectroscopic (Freundlich) and HPLC-MS/MS (Freundlich/Langmuir) techniques, observing that PS IDF presented higher PC adsorption at all tested concentrations. Epicatechin, isoquercetin, and quercetin were the main identified PC in both by-products, and were able to fit to both Langmuir and Freundlich isotherm by HPLC-MS/MS.

The terahertz (THz) time-domain spectroscopy technique was used to characterize the cooling process of solid n-alkanes. The THz waveforms of n-octadecane, n-nonadecane, n-eicosane, n-heneicosane, n-docosane, and n-pentacosane were obtained with the cooling time using the aforementioned noncontact optical method. The peak values of the THz signal were found to be related to the cooling temperature of n-alkanes. The THz wave was sensitive to the size and structure of particles in the liquid; therefore, the crystallization process of n-alkanes was monitored. An empirical equation based on signal attenuation was proposed to quantitatively distinguish the content change of structural order in the samples. Results present a new noncontact optical approach for characterizing wax crystallization via THz time-domain spectroscopy.

In this study, the vibrational modes of nitrofurantoin, urea, and cocrystal were characterized by terahertz time domain spectroscopy and Raman spectroscopy. The results indicate that both terahertz and Raman spectroscopy can effectively identify cocrystal and parent materials, with terahertz spectroscopy being more prominent. Meanwhile, the vibrational modes and structural optimization of four theoretical cocrystal configurations of the two drugs were simulated by density functional theory (DFT). Comparing the theoretical results with the experimental results shows that theoretical configuration D is most consistent with the experimental results. The results provide rich information on the pharmaceutical cocrystal to improve the physicochemical properties and pharmacological activities of specific pharmaceuticals at the molecular level. It shows that terahertz and Raman vibrational spectroscopy combined with DFT calculated is an effective method to analyze weak intermolecular interactions and characterize the molecules with cocrystal.

The rotational mode and vibrational mode of many biomolecules are all located in terahertz band, and the photon energy of terahertz wave is low, which is only millivolt level, and will not damage the internal structure of molecules, so terahertz time-domain spectroscopy technology has a good application prospect in biological detection. In addition, most biomolecules can only exert their biological activity under liquid conditions, so it is necessary to study the interaction between biomolecules in liquid environment. In the study of the interaction between biomolecules and terahertz wave in solution, minimizing the absorption of terahertz waves by water molecules has become a research hotspot in recent years. The microfluidic chip reduces the absorption of terahertz waves by reducing the distance between the measured sample and the terahertz waves, and has important application value in the detection of biological samples. By changing the concentration of hydrogen ions in the aqueous solution, different pH values can be obtained. In this paper, Zeonor 1420rs, which has high transmission rate to terahertz wave, is selected as the material of microfluidic chip, and the microchannel depth is 50 μm. Then we combine terahertz technology and microfluidic technology, and prepare three kinds of strong acid solutions with different pH values. By analyzing the terahertz time-domain and frequency-domain spectra of strong acid solutions, we can explore the influence of different pH solutions on terahertz wave absorption, and provide necessary reference for the application of terahertz technology in the field of biochemistry.

Terahertz (THz) and sub-terahertz imaging and spectroscopy are becoming increasingly popular nondestructive evaluation techniques for damage detection and characterization of materials. THz radiation is being used for inspecting ceramic foam tiles used in TPS (Thermal Protection System), thick polymer composites and polymer tiles that are not good conductors of ultrasonic waves. Capability of THz electromagnetic waves in detecting heat induced damage in porous materials is investigated in this paper. Porous pumice stone blocks are subjected to long time heat exposures to produce heat induced damage in the block. The dielectric properties extracted from THz TDS (Time Domain Spectroscopy) measurements are compared for different levels of heat exposure. Experimental results show noticeable and consistent change in dielectric properties with increasing levels of heat exposure, well before its melting point.

Summary form only given. The terahertz (THz) band of the electromagnetic spectrum has long held interest in the science and engineering community for its unique applications in imaging and spectroscopy. However, due to the difficulty in generating and receiving THz radiation, only very recently practical and efficient THz systems become available. Due to recent advances in the development and manufacturing of commercially available THz spectroscopy and imaging system many of these applications are coming to fruition. As such, the practical implementation of THz in applications needs to be investigated in order to determine the advantages of THz technology. In this work, THz spectroscopy of photovoltaic semiconductor devices will be performed throughout various critical stages in the fabrication process. Due to the few millimeter penetrating and none-destructive properties of THz radiation it will be possible to monitor semiconductor wafers using THz spectroscopy. This technology has a potential to provide insight into the development of wafer defects earlier in the fabrication process. Using a pulsed Terahertz imaging and spectroscopy system at the University of Arkansas, our group will investigate the electrical properties of semiconductor photovoltaics and will conduct THz imaging of defected ones. Testing the wafers throughout various key steps in fabrication is a process known to produce a functional device. This will then be repeated for the same process, only now with a critical fatal defect such as a buried crack, surface crack or a defect in the doping level. These defects will be intentionally inserted at an early stage to determine if THz spectroscopy can be used to detect them early on. THz characterization of the semiconductors for the impurity doping levels and crystal orientation to be studied will be performed to determine the electrical properties in this frequency range. This will allow for accurate numerical simulation to compare with the experimental results.

Terahertz Spectroscopy Tracks Proteolysis by a Joint Analysis of Absorptance and Debye Model

The use of plant-derived secondary metabolites to induce disease resistance in plants has been well documented. In this study, total phenolics and total proanthocyanidins (PACs) were extracted from the shell and husk of pecan ( Carya illinoinensis) and quantified. Under greenhouse conditions, the extracts were foliarly applied to chile pepper ( Capsicum annuum, cultivar NM 6-4) to compare their ability to induce resistance in plants inoculated with a virulent isolate of Phytophthora capsici. Extractions yielded total phenolic contents of 673 ± 28 and 293 ± 24 mg/g dry weight (DW) for shell and husk and total PAC contents of 1,770.10 ± 158.62 and 901.30 ± 22.84 mg/g DW for shell and husk, respectively. Chile plants treated with total phenolic and PAC extracts from pecan husk or shell or 0.1% salicylic acid, a plant-defense hormone, remained asymptomatic throughout the study for 3 weeks, whereas plants treated with water, which served as a control, died. Both total phenolic and total PAC extracts derived from pecan byproducts can be used within a greenhouse setting as elicitors to induce resistance response in chile pepper against plant pathogens such as P. capsici.

As a novel, label-free and non-invasive detection modality, terahertz (THz, 1THz=1012Hz) spectroscopy has been widely used in various areas. For instance, in the biomedical field, it has great potentials to provide real-time scanning of living cells and tissues due to its unique advantages. Significant achievements have been reached in cell detection, related to bacterial identification, cancer cell characterization and blood cell detection. In tissue detection, the THz spectroscopy can be used to provide real-time scanning of living tissues and fast diagnosis. Furthermore, a single system which integrated THz spectroscopy and THz imaging would be able to collect information more sensitive and comprehensive. However, the clinical adaption of THz spectroscopy is still a controversial issue attributed to some intrinsic limitations and technical bottlenecks. In this article, both the application of THz spectroscopy in cell and tissue detection and the existing challenges and strategies to accelerate clinical applications were reviewed comprehensively.(Chin J Lab Med, 2018, 41: 348-351) Key words: Spectrum analysis; Biosensing techniques

The topcoat thickness of thermal barrier coating (TBC) samples was measured using terahertz (THz) waves. The index of refraction of yttria stabilized zirconia (YSZ), which is the topcoat material, is necessary to obtain the topcoat thickness from time domain reflectometry. Time domain THz spectroscopy was applied to YSZ samples, and the index of refraction was measured to be 4.8 in the frequency region 0.1-1.2 THz. The topcoat thickness of 6 different TBC samples, which varied from 300 to 600 μm, was measured using THz waves. The results were in agreement with microscope observation results to within measurement error. In addition, the topcoat thickness profile of a turbine blade sample was measured with 2.4 μm resolution. The profile showed a standard deviation of 4 μm, which reflects the actual variation in the topcoat thickness. The results showed that THz waves are effective for high resolution measurement of the topcoat thickness.

The principles and experimental methods of terahertz (THz) wave ellipsometry are described. The procedure to determine diagonal and off-diagonal components of the complex dielectric tensor for materials using THz time domain spectroscopy in transmission and oblique reflection geometries are explained. The measured optical activity in artificial chiral grating structures is also described as applications of this technique.KeywordsOptical ActivityEllipticity AnglePolarization RotationDrude ModelDielectric TensorThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Considering the growing demand for modern facility agriculture, it is essential to develop non-destructive technologies for assessing lettuce nutritional status. To overcome the limitations of traditional methods, which are destructive and time-consuming, this study proposes a multimodal non-destructive nitrogen detection method for lettuce based on multi-source imaging. The approach integrates terahertz time-domain spectroscopy (THz-TDS) and near-infrared hyperspectral imaging (NIR-HSI) to achieve rapid and non-invasive nitrogen detection. Spectral imaging data of lettuce samples under different nitrogen gradients (20–150%) were simultaneously acquired using a THz-TDS system (0.2–1.2 THz) and a NIR-HSI system (1000–1600 nm), with image segmentation applied to remove background interference. During data processing, Savitzky–Golay smoothing, MSC (for THz data), and SNV (for NIR data) were employed for combined preprocessing, and sample partitioning was performed using the SPXY algorithm. Subsequently, SCARS/iPLS/IRIV algorithms were applied for THz feature selection, while RF/SPA/ICO methods were used for NIR feature screening, followed by nitrogen content prediction modeling with LS-SVM and KELM. Furthermore, small-sample learning was utilized to fuse crop feature information from the two modalities, providing a more comprehensive and effective detection strategy. The results demonstrated that the THz-based model with SCARS-selected power spectrum features and an RBF-kernel LS-SVM achieved the best predictive performance (R2 = 0.96, RMSE = 0.20), while the NIR-based model with ICO features and an RBF-kernel LS-SVM achieved the highest accuracy (R2 = 0.967, RMSE = 0.193). The fusion model, combining SCARS and ICO features, exhibited the best overall performance, with training accuracy of 96.25% and prediction accuracy of 95.94%. This dual-spectral technique leverages the complementary responses of nitrogen in molecular vibrations (THz) and organic chemical bonds (NIR), significantly enhancing model performance. To the best of our knowledge, this is the first study to realize the synergistic application of THz and NIR spectroscopy in nitrogen detection of facility-grown lettuce, providing a high-precision, non-destructive solution for rapid crop nutrition diagnosis.

Terahertz (THz) wave science and technology have been found countless applications in biomedical imaging, security screening, and non-destructive testing as they approach maturity. However, due to the challenge of high ambient moisture absorption, the development of remote open-air broadband THz spectroscopy technology is lagging behind the compelling need that exists in homeland security, astronomy and environmental monitoring. Furthermore, the underlying physical mechanisms behind the interaction between the THz wave and laserinduced plasma which responds strongly to electromagnetic waves have not been fully understood. This review aims to explain the light-plasma interaction at THz frequencies within a semiclassical framework along with experimental study of the femtosecond-laserinduced nitrogen plasma fluorescence under the illumination of single-cycle THz pulses. The results indicate that THz-radiation-enhanced-emission-of-fluorescence (THz-REEF) is dominated by electron kinetics in the THz field and the electron-impact excitation of gas molecules/ions. The information of the time-dependent THz field can be recovered from the measured time-resolved THz-REEF from single-color laser induced plasma with the help of the bias as local oscillator. The calculations and experimental verification lead to complete understanding of the science behind these effects and push forward to extend their capabilities in related applications such as remote THz sensing, plasma diagnostics and ultrafast photoluminescence modulation. Systematic studies in selected gases including neon, argon, krypton, xenon, methane (CH4), ethane (C2H6), propane (C3H8), and n-butane (C4H10) gases were performed to obtain an improved understanding of the THz-REEF. The dependences of the enhanced fluorescence on the THz field, laser excitation intensity, gas pressure, and intrinsic atomic properties were experimentally characterized. Both narrow line emission and broad continuum emission of the gas plasma were enhanced by the THz field. Their fluorescence enhancement ratios and time-resolved enhanced fluorescence were largely dependent on the scattering cross section and ionization potential of atoms. For the first time, we demonstrated a novel ‘all-optical’ technique of broadband THz wave remote sensing by coherently manipulating the fluorescence emission from asymmetrically ionized gas plasma that interacted with THz waves. By studying the ultrafast electron dynamics under the single cycle THz radiation, we found that the fluorescence emission from laser-induced air plasma was highly dependent on the THz electric field and the symmetry of the electron drift velocity distribution created by two-color laser fields. The time-resolved THz-REEF can be tailored by switching the relative two-color phase and laser polarizations. Owing to the high atmospheric transparency and omni-directional emission pattern of fluorescence, this technique can be used to measure THz pulses at standoff distances with minimal water vapor absorption and unlimited directionality for optical signal collection. The coherent THz wave detection at a distance of 10 m had been demonstrated. The combination of this method and previously demonstrated remote THz generation would eventually make remote THz spectroscopy available. We also introduced a unique plasma diagnostic method utilizing the THz-wave-enhanced fluorescence emission from the excited atoms or molecules. The electron relaxation time and plasma density were deduced through applying the electron impact excitation/ionization and electron-ion recombination processes to the measured time-delay-dependent enhanced fluorescence. The electron collision dynamics of nitrogen plasma excited at different gas pressures and laser pulse energies were systematically investigated. This plasma diagnostic method offers picosecond temporal resolution and is capable of omnidirectional optical signal collection. The ultrafast quenching dynamics of laser-pulse-induced photoluminescence in semiconductors under the radiation of single-cycle THz pulses was studied. It was found that the quenching in both cadmium telluride (CdTe) and gallium arsenide (GaAs) was linearly proportional to the intensity of incident THz waves and reaches up to 17% and 4% respectively at the peak intensity of 13 MW/cm2. The THz-wave-induced heating of the carriers and lattice and the subsequent decreased efficiency of photocarrier generation and recombination were most likely to be responsible for the quenching. This is potentially useful for the applications of a non-invasive ultrafast light modulator for photoluminescence devices with picoseconds switching time in the fields of the light-emitting devices and optical communication.

Terahertz time-domain spectroscopy (THz-TDs) has been used to record the resonant frequencies of plasmon oscillations excited in samples of heterostructures with three InxGa1-xN/GaN quantum wells (QWs) by laser pulses with a duration of 130 fs in the temperature range from 90 to 170 K. Fast Fourier transform (FFT) of the time dependence of the electric field of THz-pulses made it possible to obtain the frequency spectra of the power and phase shift of THz-radiation, the interpretation of which made it possible to estimate the pulse relaxation time, mobility and effective mass of two-dimensional electron gas (2DEG) in the heterostructures. Using a series of frequency spectra of the power and phase shift of THz-radiation, the temperature dependences of the effective mass and relaxation time of the 2DEG pulse were obtained. Mobility value 2DEG obtained by the THz-TDs is in good agreement with the data of Hall measurements. Keywords: heterostructures, pulse relaxation time, 2DEG, terahertz radiation, terahertz spectroscopy.

Graphene, made of carbon atoms arranged in a honeycomb lattice, has already attracted intense research and commercial interest in recent years. Early research focused on its remarkable electronic properties, such as the observation of massless Dirac fermions and the half-integer quantum Hall effect. Now graphene is finding application in touch-screen displays, as channels in high-frequency transistors and in graphene-based integrated circuits. The unique properties of graphene have also attracted various researches on carrier dynamics using THz spectroscopy. Here we present an experimental demonstration of monolayer CVD grown graphene via THz time-domain spectroscopy, as well as optical pump terahertz probe system. We observe that the maximum transmittance of the graphene is nearly 96% compared to the ambient signal. However, under the excitation of different optical pump fluences, it is found that unlike the semiconductor material, its transmitted amplitude is enhanced accordingly. We observed a wide-band modulation of the terahertz transmission at the range of 0.3-1.6 THz and a large modulation depth of 16.4% with a certain optical excitation. We attributed it to suppression of the air-adsorbed graphene photoconductivity due to an increase in the carrier scattering rate induced by the increase in the free-carrier concentration by photoexcitation. The obtained results not only highlight the influence of air conditions on how THz characterizations would guide the design and fabrication of graphene-based terahertz modulators and optoelectronic devices, but also show that graphene exhibits the potential for terahertz broadband transmission enhancement with photoexcitation.