Terahertz Electromagnetic Waves Research Articles

Dielectric properties of Pouteria sapota compounds in terahertz frequency range

The analysis and a mathematical approximation of the dielectric properties of Pouteria sapota (Mamey) pulp compounds in the Terahertz (THz) range are presented. Since THz electromagnetic waves show a high sensitivity to detect water content when interacting with organic materials, this technique was used to determine the effective dielectric function (EDF) of a wet and a dry sample from the THz spectrum data. By fitting the EDF to the Landau-Lifshitz-Looyenga (LLL) dielectric mixture equation, the theoretical dielectric functions of each pulp compounds were approximated. The approach proposed here not only considered dry matter and water content in the Pouteria sapota pulp, as has been reported in the literature for organic samples, but also included the contribution of the volatile components (VC) that allowed a better adjustment to the experimental results. The water and VC content increase the value of the real and imaginary part of the calculated EDF of the wet sample. The use of the non-destructive THz spectroscopy technique allows determining the dielectric properties of fruits that have been dehydrated. Therefore, the methodology proposed here can be used as a method of in situ and ex situ quality control in the agri-food industry.

Comparative analysis of machine learning techniques for metamaterial absorber performance in terahertz applications

Metamaterial absorbers (MMAs) have tremendous potential for controlling and modulating Terahertz electromagnetic (EM) waves. It is challenging to design MMAs for optimal performance using conventional methods due to their time-consuming and computationally demanding nature. Machine learning (ML) regressor approaches have emerged as potential tools for optimizing the predictive modeling of MMAs in recent years. This article examined different regression techniques, such as Decision Tree, k-Nearest Neighbors, Random Forest, Extra Trees (ET), Extreme Gradient Boosting, Bagging, and Categorical Boosting. The primary goal is to assess the effectiveness of each regressor technique in forecasting the performance of MMAs using various performance metrics. The study focuses on a representative spectrum of MMAs, chosen for its key features that are common across a wide range of MMAs. The extra tree Regressor outperforms other models in the comparative analysis due to its capacity to integrate multiple decision trees and include randomness into the feature selection process, resulting in improved predictive skills. The ET regressor forecasts MMA performance remarkably well for a 70 % test size, with a Root Mean Squared Error of 0.0251, a Mean Absolute Error of 0.0074, a Mean Squared Error of 0.006, and an Adjusted R-squared of 0.9873. These findings can assist researchers in optimizing metamaterial absorber designs for Terahertz applications using ML regressor techniques and provide insights into how these methods can be generalized to other spectra with similar characteristics.

Возбуждение терагерцовых плазмонных мод в графеновом квадратном микрорезонаторе

We consider a graphene square located in the interface between two half-spaces with different dielectric constants. Using a self-consistent electromagnetic approach based on the integral equation method, we solve the problem of a linearly polarized terahertz electromagnetic wave scattering by the graphene square. The spectra of extinction, absorption, and scattering cross sections reveal the various plasmon modes excitation in graphene square. The properties of the plasmon modes different types are discussed.

A millimeter-wave atomic receiver

Rydberg quantum sensors are sensitive to radio-frequency fields across an ultra-wide frequency range spanning megahertz to terahertz electromagnetic waves resonant with Rydberg atom dipole transitions. Here, we demonstrate an atomic millimeter-wave heterodyne receiver employing continuous-wave lasers stabilized to an optical frequency comb. We characterize the atomic receiver in the W-band at a signal frequency of f = 95.992 512 GHz and demonstrate a sensitivity of 7.9 μV/m/Hz with a linear dynamic range in power greater than 70 dB. We develop frequency selectivity metrics for atomic receivers and demonstrate their use in our millimeter-wave receiver, including signal rejection levels at signal frequency offsets Δf/f = 10−4, 10−5, and 10−6; 3, 6, 9, and 12-dB bandwidths; filter roll-off; and shape factor analysis. Our work represents an important advance toward future studies and applications of atomic receiver science and technology in weak millimeter-wave and high-frequency signal detection.

Synergistic terahertz shielding effects of electrically conductive MXene and shape-controlled magnetic nickel in polyvinylidene fluoride (PVDF) composites

Terahertz (THz) shielding materials have garnered significant attention due to various potential applications in wireless communication, sensing, and defense. In this study, we present efficient THz shielding performance of polyvinylidene fluoride (PVDF) polymer composites incorporated with a mixture of MXene and shape-controlled nickel (Ni) nanoparticles. The PVDF/MXene/Ni composites demonstrated exceptional absorption coefficient and shielding effectiveness against unwanted THz electromagnetic waves, owing to the remarkable synergistic effect between electrically conductive Ti3C2Tx MXene and magnetic nickel nanoparticles. Notably, magnetic properties of Ni nanoparticles were controlled by external magnetic field applied during the synthesis process. Flower-like Ni nanoparticles synthesized under an external magnetic field exhibited enhanced magnetic properties compared to spherical Ni nanoparticles synthesized without an external field. As a result, a 20-µm thick PVDF/MXene/Ni composite film, with a 10 wt.% loading of flower-like Ni nanoparticles, demonstrated the THz shielding effectiveness of 34 dB at 1 THz. This shielding performance surpasses those of composite containing spherical Ni nanoparticles (26.7 dB) as well as previously reported shielding materials. These findings highlight an effective approach to tailor the THz shielding properties of polymer composites by modifying the composition and morphology of conductive and magnetic fillers.

Exploring the impedance-matching effect in terahertz reflection imaging of skin tissue.

Terahertz (THz) electromagnetic waves, known for their unique response to water, offer promising opportunities for next-generation biomedical diagnostics and novel cancer therapy technologies. This study investigated the impedance-matching effect, which enhances the efficiency of THz wave delivery into tissues and compensates for the signal distortion induced by the refractive index mismatch between the target and the sample substrate. Three candidate biocompatible materials, water, glycerol, and petroleum jelly were applied to a skin phantom and compared using THz two-dimensional imaging and time-of-flight imaging methods. Finally, we successfully demonstrated impedance-matching effect on mouse skin tissues.

The absorption properties of ZrO2 nanoparticles in the THz and sub-THz frequency ranges.

As terahertz (THz) and sub-THz region electromagnetic waves are becoming vital for industrial applications such as 5G wireless communication, so too are THz and sub-THz wave absorbing materials. Herein, we report the optical properties of monoclinic zirconia (m-ZrO2) nanoparticles in these frequency regions, with different crystalline sizes. The crystalline sizes of the three samples, measured by transmission electron microscopy, are 93 ± 23 nm (denoted 1), 28 ± 14 nm (denoted 2) and 2.6 ± 0.7 nm (denoted 3). X-ray diffraction and Raman spectra show that 1 and 2 have high crystallinity whereas 3 shows peak broadening due to its small crystalline size. Terahertz time-domain spectroscopy (THz-TDS) measurements of pelletised samples show that the small crystalline size sample exhibits larger absorption, e.g., the absorbance value at 300 GHz is 0.18 mm-1 (1), 0.04 mm-1 (2) and 1.11 mm-1 (3), and the related dielectric loss value (ε'') is 0.04 (1), 0.01 (2) and 0.82 (3), respectively. This is considered to be due to the proportional increase in surface water molecules for the small particle size sample due to the relative increase in surface area and under-coordinated atoms, shown by IR spectra. These results show that small crystalline size m-ZrO2 nanoparticles have potential as THz and sub-THz wave absorbing materials, which are crucial for noise reduction in THz and sub-THz wave technologies.

Influence of terahertz waves on the binding of choline to choline acetyltransferase: insights from molecular dynamics simulations.

Acetylcholine (Ach) is a common neurotransmitter in the central nervous system (CNS) and peripheral nervous system (PNS). It is one of the neurotransmitters in the autonomic nervous system and the main neurotransmitter in all autonomic ganglia. Experiments have confirmed that electromagnetic waves can affect the synthesis of animal neurotransmitters, but the microscopic effects of electromagnetic waves in the terahertz (THz) frequency band are still unclear. Based on density functional theory (DFT) and molecular dynamics (MD) simulation methods, this paper studies the effect of THz electromagnetic waves on the binding of choline to choline acetyltransferase (ChAT). By emitting THz waves that resonate with the characteristic vibration mode of choline near the active site, it was found that THz waves with a frequency of 45.3 THz affected the binding of choline to ChAT, especially the binding of the active site histidine His324 to choline. The main evidence is that under the action of THz waves, the binding free energy of choline to histidine His324 and ChAT at the active site was significantly reduced compared to noE, which may have a potential impact on the enzymatic synthesis of Ach. It is expected to achieve the purpose of regulating the synthesis of the neurotransmitter Ach under the action of THz waves and treat certain nervous system diseases.

More Than 30 000-fold Field Enhancement of Terahertz Nanoresonators Enabled by Rapid Inverse Design.

The rapid development of 6G communications using terahertz (THz) electromagnetic waves has created a demand for highly sensitive THz nanoresonators capable of detecting these waves. Among the potential candidates, THz nanogap loop arrays show promising characteristics but require significant computational resources for accurate simulation. This requirement arises because their unit cells are 10 times smaller than millimeter wavelengths, with nanogap regions that are 1 000 000 times smaller. To address this challenge, we propose a rapid inverse design method using physics-informed machine learning, employing double deep Q-learning with an analytical model of the THz nanogap loop array. In ∼39 h on a middle-level personal computer, our approach identifies the optimal structure through 200 000 iterations, achieving an experimental electric field enhancement of 32 000 at 0.2 THz, 300% stronger than prior results. Our analytical model-based approach significantly reduces the amount of computational resources required, offering a practical alternative to numerical simulation-based inverse design for THz nanodevices.

Laser-Induced Graphene/TiCN on a Polyimide/MXene Film as Interference Shielding Materials for Terahertz Electromagnetic Waves

Laser-Induced Graphene/TiCN on a Polyimide/MXene Film as Interference Shielding Materials for Terahertz Electromagnetic Waves

Исследование влияния краевых эффектов на свойства плазмонных мод в графеновой микроленте

The work devoted to theoretical studies of the excitation of plasmon modes in a graphene microribbon by an incident terahertz electromagnetic wave. The problem is solved in a three-dimensional formulation taking into account edge-field effects at the ends of graphene microribbon by using the integral equation method being developed by the authors. The influence of microribbon length (for constant width of the microribbon) on the properties of the fundamental plasmon mode is studied. Criteria for choosing either simplified two-dimensional or three-dimensional electromagnetic approaches for studying the properties of plasmon modes in graphene microribbons are formulated.

Inspection for Voids in the Grout below the Protective Duct of an External Post-Tensioning Bridge Tendon Using a THz A-Scanner

Grout voids in the tendons of a post-tensioning bridge reduce their strength. Grout voids are also severe flaws causing corrosion of the steel strands in the tendons. Detecting voids during construction and operation of the tendons is essential to prevent tendon failure, which is critical to bridge safety. This study presents a method for inspecting external tendons for voids in the grout below the protective duct pipe using terahertz electromagnetic waves. Due to low attenuation in the high-density polyethylene duct and the large reflectivity difference between the duct/grout and the duct/void interfaces, terahertz waves are suitable for detecting voids in the grout inside tendons. For this study, we developed a mobile frequency-domain terahertz A-scanner that can be used to measure terahertz A-scan data in real time. It is shown that the mobile terahertz A-scanner can be used to assess the area of the grout void in external bridge tendons.

A dynamically switchable metamaterial device with multiple functions based on vanadium dioxide

Based on the phase transition properties of vanadium dioxide (VO2), a metamaterial device is designed in this paper, which can achieve multiple switchable functions. When VO2 is in the metallic state, the proposed device can realize broadband absorption with peak absorbance more than 90 % in the frequency range of 2.40–6.73 THz and broadband linear polarization conversion (LPC) with polarization conversion ratio (PCR) greater than 90% in the range of 1.80–4.21 THz, respectively, depending on whether the incidence of terahertz (THz) electromagnetic (EM) wave is from top side or bottom side. When VO2 is in the insulated state, the proposed device can achieve LPC of half-reflection and half-transmission (HRHT) and transmissive linear-to-circular (LTC) polarization conversion. The study in this paper has guiding significance in the design of THz metamaterial devices with multiple functions.

Characteristic identification of steel corrosion products based on terahertz transmission spectroscopy

Corrosion of steel materials (such as rebars and steel plates) usually occurs under coating materials. It is, with the existing non-destructive testing (NDT) technologies, difficult to accurately determine the corrosion of steel materials. The terahertz (THz) electromagnetic waves can penetrate the non-polar materials, and theoretically experience an absorption resonance with some corrosion products crystals (i.e., iron oxides). The THz spectroscopy has, therefore, a great application potential to characteristically identify the corrosion of steel materials. In the present study, the optical parameters and, especially, the positions of the characteristic absorption peaks of the corrosion products were systematically investigated and identified using the THz system. The experimental results showed that the refractive index of the different corrosion product samples was in the range of 2.7–3.4 in the effective frequency range of 0 THz - 1.2 THz, and the content of Fe3O4 had a large influence on the optical parameters in the corrosion product mixtures. To measure the characteristic absorption peaks of the corrosion products, the effective frequency range was extended to 0 THz - 10 THz by an assembled broadband THz system. Within this range, the positions of the characteristic absorption peaks were identified as 3.4 THz, 4.2 THz, 4.85 THz, and 5.8 THz for Fe2O3, and as 3.6 THz, 4.05 THz, 5 THz, and 5.45 THz for α-FeOOH. For Fe2O3 and α-FeOOH, the intensities of the absorption peaks at 4.85 THz and 3.6 THz, respectively, showed a linear relationship with the mass content. Furthermore, the THz spectroscopy could accurately identify Fe2O3 and α-FeOOH in the corrosion product mixtures by comparing with the results from IR spectroscopy and Raman spectroscopy. This means that the occurrence of steel corrosion could be detected according to the characteristic absorption peaks of Fe2O3 and α-FeOOH. The experimental results provide a basis for the application of THz spectroscopy in the NDT of the corrosion of steel materials.

High-Field Nonresonant Response of Zundel Cations to Intense Terahertz Radiation

The fundamental unit for comprehending the physicochemical properties of water, the Zundel cation configuration H5O2+, has yet to be exhaustively evaluated in terms of its interaction with terahertz (THz) electromagnetic waves, characterized by sub-picosecond oscillation periods or pulse widths. In this study, we embark on an investigation of the broad resonance and high-field nonresonant effects of intense THz radiation (ITR) on Zundel cations, utilizing a multifaceted methodological approach that includes density functional theory (DFT) calculations, finite difference time domain (FDTD) algorithm of the Schrödinger equation, and ab initio molecular dynamics (AIMD) simulations. Our analysis reveals that the proton potential energy surface (PES) varies in response to the external electric (E) field, suggesting that the interaction frequency of the central proton with the electromagnetic wave encompasses the THz band. This resonance effect is associated with proton behavior that may oscillate or demonstrate periodic tunneling. Moreover, our work uncovers the high-field nonresonant effects of ITR on Zundel cations, manifesting in proton transfer and vibrational excitation of the system. Our findings contribute to the understanding of the interaction between Zundel species and electromagnetic waves by presenting a microscopic view of proton transfer as informed by wavefunction evolution.

All-dielectric terahertz metalens using 3D-printing

Terahertz (THz) electromagnetic waves offer a new way of understanding various phenomena, owing to their non-ionizing properties resulting from their low energy, and non-destructive inspection features in dielectrics. Although THz emitters and detectors have been developed with much interest, the performance of optics in the THz band is still limited by the properties of the available materials. Metamaterials, which are the artificial subwavelength structures for implementing optical properties that do not exist in intrinsic materials, proposed as a new approach. However, their fabrication requires a complex semiconductor manufacturing process for its elaborate structure. In this study, we demonstrated the fabrication of all-dielectric THz metamaterial optics using an accessible 3D printing technology. A meta-element consisting of a square pillar exhibited full-wave phase controllability. The metalens was designed for 0.2 THz. The metalens fabricated using a low-cost LCD-type 3D printer and photopolymer resin showed good focusing performance and polarization insensitivity.

Изменения иммунологических показателей у пострадавших с политравмой при применении в комплексном лечении электромагнитных волн терагерцевого диапазона

Introduction Improving the results of treatment and rehabilitation is essential for polytrauma patients. The aim of the study was to prove the effectiveness of terahertz electromagnetic waves (TEW) in polytrauma patients early after traumatic event to improve immune parameters. Material and methods The study included 20 polytrauma patients who were divided into two groups. Patients of the treatment group (n = 10) received TEW early after traumatic event in addition to standard treatment using the damage control approach. Control patients (n = 10) received no TEW therapy in the complex of therapeutic measures. Immunological studies were performed during treatment. The effectiveness of rehabilitation was assessed at a long term using an evaluation scale. Results The mean rehabilitation score was significantly higher in the treatment group compared to the controls. The treatment group showed significantly higher production of TNFα and IL-2, spontaneous production of IFNγ by lymphocytes, phagocyte activity (NBT-test) and absorption activity of monocytes after the use of TEW in comparison with controls. An increased level of IgG and IgM was also noted in patients of the treatment group. Discussion The findings showed that the use of TEW in polytrauma patients early after traumatic event resulted in the increase in cytotoxic activity of lymphocytes and phagocytic cells. The immunostimulating effect of TEW was also achieved through boosting humoral immunity. Conclusion The TEW used early after traumatic event in polytrauma patients resulted in strengthening of the immune system which contributed to more effective rehabilitation.

Terahertz amplification and lasing by using transverse electric modes in a two-layer-graphene-dielectric waveguide structure with direct current

We study for the first time the interaction between the waveguide modes of graphene structure and freely propagating terahertz (THz) electromagnetic waves (this interaction takes place within the light cone). We revealed a new and rather unexpected physical phenomenon by showing that freely incident THz electromagnetic waves can resonate with the surface transverse electric (TE) modes of the graphene waveguide in virtue of these modes having their dispersions in the vicinity of the light cone. The dispersion and amplification of surface TE modes in a dielectric waveguide covered with two graphene layers biased by direct current (DC), as well the amplification and lasing of incident THz wave by excitation of TE mode resonances, are investigated. The DC flows perpendicular to the direction of the surface wave propagation and creates the capacitive complex conductivity of graphene at THz frequencies, which is necessary for the existence of surface TE modes in graphene. The real part of graphene conductivity can be negative at THz frequencies due to DC in graphene which leads to amplification and lasing of THz radiation. Such structure can be of great practical importance because an external THz wave can be amplified or generated in lasing process without using special coupling elements commonly needed for ensuring the interaction between external THz wave and surface waveguide modes. The use of a two-layer graphene structure makes it possible to reduce the charge–carrier drift velocity required for reaching the lasing threshold at those resonances, as compared to a structure with a single graphene layer.

Non-destructive detection of thin micro-defects in glass reinforced polymer composites using a terahertz electro–magnetic wave based on a convolution neural network

This study presents a non-destructive evaluation (NDE) method that utilizes a convolutional neural network (CNN) based terahertz (THz) signal processing to quantitively distinguish overlapping signals from micro-defects in glass fiber reinforced polymer (GFRP) composites. Theoretical electro–magnetic wave propagation models for describing the propagation of the THz signal into the GFRP composite were devised considering the interaction of THz with the micro-defects in the GFRP composite. THz signals were simulated with respect to the thickness of the defects, and transformed into two-dimensional (2D) spectrograms using a short-term Fourier transform (STFT). The converted 2D-STFT images were used to train the CNN deep learning model to classify the size and thickness of the defects. A probability map of the classification of the defect thickness was derived from the trained CNN deep learning model. Using the probability map based on the CNN model, the depth and size of the micro-defects thinner than 20 μm inside the GFRP composites could be successfully differentiated with 95% accuracy. This research proposes a promising approach for analyzing micro-defects in composite materials by quantitatively evaluating the extent of signal overlap, going beyond the traditional approach of training the model based solely on the signal.

Excitation of terahertz radiation in p-n-heterostructures based on a-Si:H/c-Si

Studies on the generation of terahertz (THz) radiation in p-n-heterostructures based on a-Si:H/c-Si upon their photoexcitation by a femtosecond titanium-sapphire laser with a wavelength of 800 nm are presented. The properties of observed THz radiation allow to explain its nature by excitation of fast photocurrent of nonequilibrium charge carriers created in the region of the potential barrier under femtosecond interband photoexcitation of the structure. The fast photocurrent, in turn, emits THz electromagnetic waves. The waveforms and amplitude spectra of the observed THz radiation reflect the dynamics of photoexcited charge carriers in the structures. The intensity of THz radiation observed in the studied p-n-heterostructures based on a-Si:H/c-Si is comparable to that generated in n-InAs crystals, which are widely used as emitters in systems of THz time-domain spectroscopy. Therefore, a-Si:H/c-Si p-n-heterostructures can be used as THz emitters for need of THz spectroscopy. Keywords: femtosecond laser photoexcitation, heterostructures, fast photocurrent, terahertz electromagnetic radiation.