Terahertz Imaging Research Articles

Multifunctional Reconfigurable Vanadium Dioxide Integrated Metasurface for Reflection, Asymmetric Transmission and Cross‐Polarization Conversion in Terahertz Region

AbstractIntegrating reconfigurable and diverse functionalities into a single metasurface at terahertz (THz) frequencies is an emerging research topic that faces significant challenges. Here, a reconfigurable THz metasurface is proposed, offering diversified functionalities based on the phase transition of vanadium dioxide (). The proposed metasurface can switch from wideband reflection to wideband cross‐polarization conversion (CPC) and asymmetric transmission (AT) for linearly polarized waves. When is in its fully metallic state, the metasurface efficiently reflects normally incident waves ranging from 0.2 to 2.8 THz, with total reflection exceeding 0.8. When is in its insulating state, the metasurface achieves nearly perfect wideband cross‐polarization conversion with CPC efficiency above 0.99 from 0.46 to 2.67 THz, and excellent AT effect with efficiency over 0.9 from 0.65 to 2.29 THz for normal incidence of linearly polarized waves. Moreover, the high efficiency of CPC and AT effects is maintained over a wide range of incident angles. The proposed switchable metasurface with diverse functionalities is expected to enable cutting‐edge research and innovative applications in THz communication, sensing, and imaging.

Hybrid network with difficult–easy learning for concealed object detection in imbalanced terahertz image dataset

Hybrid network with difficult–easy learning for concealed object detection in imbalanced terahertz image dataset

Vanadium dioxide and a graphene-based ultra-wideband absorption and polarization conversion switchable terahertz device

A multifunctional terahertz functional device based on vanadium dioxide (VO2) and graphene is proposed, which can realize ultra-wideband absorption and polarization conversion. When the VO2 is in the insulating state, the device can achieve the polarization conversion function to convert the incident wave into the corresponding cross-polarized wave. Polarization conversion ratios (PCRs) can exceed 90% in the 2.1–8 THz frequency range; in the 3–7.5 THz range, the PCR can be more than 95%. When VO2 is in the metallic state and the graphene Fermi energy is 0.9 eV, the device can realize a broadband absorption function. An absorption rate of more than 90% can be achieved over a wide frequency range of 3.3–7.7 THz. In addition, the polarization conversion device can maintain high performance in broadband polarization conversion at incident angles no greater than 40°. The absorber device also exhibits insensitivity to both incident and polarization angles. These advantages which make the proposed multifunctional terahertz functional device have a wide range of applications in the fields of terahertz imaging, sensing, communication, and so on.

Broadband polarization-insensitive terahertz absorber based on Ta2O5 snowflake-like structure

Abstract In this paper, an ultra-broadband tantalum pentoxide (Ta2O5) all-dielectric metamaterial absorber is proposed from the far-infrared to the terahertz range, which exhibits polarization-insensitive and wide-angle characteristics. The absorber consistently demonstrates an impressive absorption rate exceeding 90% within the frequency range from 1.1 THz to 20 THz. Such an absorber can have a bandwidth and relative bandwidth ratio of 18.9 THz and 179.15%. We employ an equivalent circuit model to simulate the performance of the absorber using transmission line theory, which facilitates near-perfect absorption by finely tuning the geometrical parameters of the structure to match the input impedance with that of free space. Detailed calculations and analyses of the electric and magnetic field distributions, as well as power loss density, are presented to clarifying the underlying mechanisms of absorption. The design of the proposed absorber inherently provides insensitivity to polarization angles and sustains superior absorption efficiency at substantial incidence angles. An exhaustive exploration of the influence of structural variations on the performance of the absorber has been conducted. Looking ahead, the proposed absorber can be potential application in enhanced terahertz imaging, terahertz communication systems, novel energy harvesting solutions and broad scientific research endeavours.

Theoretical and numerical investigation of an ultra-broadband near-perfect absorption in auxetic metamaterials

In this paper, we present and numerically investigate an auxetic metamaterial absorber based on vanadium dioxide (VO2), achieving more than 90% absorption of the incident terahertz (THz) waves between 4.15 THz to 8.43 THz with an average absorption of 98.4%. To our knowledge, the absorption bandwidth is higher than previously reported VO2-based absorbers. The proposed absorber contains two VO2 based resonator rings placed diagonally at the top of the dielectric substrate in such a way as to create an auxetic shape and provide more design space than the existing absorbers. Considering the vector nature of electromagnetic fields in three-dimensional space, numerical analysis is performed while keeping the phase-changing material VO2 in the metallic state. According to the full wave simulation, it is shown that under normal incidence, the proposed absorber provides almost flat and near-unity absorption, which covers from 4.82 THz to 7.53 THz. We describe the physical mechanism of the absorber through impedance matching theory, and the absorber performance is evaluated by observing the electric field distributions at various frequencies. The proposed structure also exhibits a satisfactory tunable range from 2% to 100%, which may satisfy the requirements of re-configurable metamaterial absorbers. We also show that the absorber provides better wide-angle absorption performance than the existing VO2-based models for both transverse polarization i.e., transverse electric (TE) and transverse magnetic (TM) modes. Owing to the higher absorption bandwidth and better tunable range, the proposed auxetic metamaterial has great potential applications in THz imaging, detectors, and sensing.

Probing ozone effects on European hornbeam (Carpinus betulus L. and Ostrya carpinifolia Scop.) leaf water content through THz imaging and dynamic stomatal response

We investigated the impact of ozone exposure on Hornbeam using a novel dual approach based on Terahertz (THz) imaging in a free-air ozone exposure experiment (three ozone levels: ambient; 1.5 times ambient; twice ambient). The research aims at unraveling the physiological responses induced by elevated ozone levels on water dynamics. THz imaging unveiled dynamic changes in leaf water content, providing a non-invasive approach to leaf water monitoring. Leaf gas exchange measurements assessed stomatal responses to light variation. Our findings showcase a compelling correlation between elevated ozone levels and reduction in photosynthetic rate and impairment of stomatal function, i.e. “stomatal sluggishness”, indicative of nuanced regulatory mechanism. Stomatal sluggishness was particularly evident in Carpinus betulus (CB) compared to Ostrya carpinifolia (OC) and was linked to reduction in photosynthetic capacity. THz-based imaging techniques confirmed this result indicating a negative effect of O3 on leaf-level total water content. In addition, spatial analysis of leaf water status using these techniques also highlighted that the negative effect of O3 on water status was progressing even in less sensitive OC plants though visible foliar injury was not detected. In fact, OC showed a relative dry area of 1.6 ± 1.6 % in the control group and 3.8 ± 1.3 % under high ozone levels. THz-based imaging techniques provided a deep understanding of O3 behavior in plants and may be recommended for precision biosensing in the early detection of O3-induced damage. The integration of THz imaging and physiological analysis resulted in comprehensive understanding of Hornbeam acclimation response to ozone exposure.

High efficient bi-functional metasurface with linear polarization conversion and asymmetric transmission for terahertz region

Abstract Polarization control is crucial in contemporary photonics, but achieving broad bandwidths and high efficiencies remains a significant challenge. Here, we propose a wideband and high-efficiency metasurface designed for linear polarization conversion and asymmetric transmission (AT) in the terahertz region based on a three-layer chiral structure. By integrating a double-split ring and strip resonator, the design can improve both wide bandwidth and high-efficiency cross-polarization conversion and AT capabilities for a linearly polarized wave. The operating band for AT is 0.73-3.01 THz with an efficiency above 0.8, while the polarization conversion ratio exceeds 0.99 across the 0.56-3.94 THz range. The underlying physical mechanism behind wideband transmission polarization conversion was fully elucidated through theoretical analysis and the characterization of surface current distribution. This designed structure can be used in terahertz imaging, sensing, and communication.

Evaluation of static bending caused damage of glass-fiber composite structure using terahertz inspection

Abstract Composite materials find increasing applications in modern industry and transport. However, they may lose desirable mechanical properties due to external mechanical excitations, ultraviolet radiation, moisture penetration, or other factors. Therefore, an effective way to assess the condition of materials is necessary. In this article, a nondestructive evaluation of glass fiber-reinforced composite subjected to five-stage static bending is presented. For this reason, pulsed excitation terahertz imaging was utilized, and a data processing/exploration scheme was proposed. The proposed, novel approach consists of an efficient data registration algorithm based on surface approximation (for surface roughness and unevenness elimination) and a parametrization scheme applied for the signals gained from the time response of the glass fiber-reinforced polymer layer. Obtained parameters enable the global description of the evaluated material state and prediction of failure effectively, even in the early stages of destruction.

Colossal room-temperature non-reciprocal Hall effect.

Non-reciprocal charge transport has gained significant attention due to its potential in exploring quantum symmetry and its promising applications. Traditionally, non-reciprocal transport has been observed in the longitudinal direction, with non-reciprocal resistance being a small fraction of the ohmic resistance. Here we report a transverse non-reciprocal transport phenomenon featuring a quadratic current-voltage characteristic and divergent non-reciprocity, termed the non-reciprocal Hall effect. This effect is observed in microscale Hall devices fabricated from platinum (Pt) deposited by a focused ion beam on silicon substrates. The transverse non-reciprocal Hall effect arises from the geometrically asymmetric scattering of textured Pt nanoparticles within the focused-ion-beam-deposited Pt structures. Notably, the non-reciprocal Hall effect generated in focused-ion-beam-deposited Pt electrodes can propagate to adjacent conductors such as Au and NbP through Hall current injection. Additionally, this pronounced non-reciprocal Hall effect facilitates broadband frequency mixing. These findings not only validate the non-reciprocal Hall effect concept but also open avenues for its application in terahertz communication, imaging and energy harvesting.

A novel enhancing method for terahertz imaging of integrated circuits flaw detection

This research paper addresses the current gap in understanding the intricate damage types within packaged integrated circuits (ICs). We introduce a novel and multiscale latent low-rank representation (M-LatLRR) methodology, meticulously tailored to enhance the quality of terahertz (THz) IC images and reveal latent features. By integrating THz imaging with our proposed M-LatLRR method, we aim to facilitate the precise identification of damage types within inside the packaged ICs. Firstly, the multiscale Gaussian functions are used to remove the blur and the components are obtained via averaging. Secondly, the image matrices are utilized to extract base matrices and detail matrices. With LatLRR, the multiscale detail matrices are extracted at several representation levels. The final enhanced image is reconstructed by average strategy for dealing with the detail and pre-enhancement parts. The M-LatLRR framework is universal and can be effectively applied to extract multi-level features of packaged IC images. In a comparative analysis, our method demonstrates superior capabilities in determining the failure types of fractured wire bonds, cracks, and delamination of the dielectric layer, outperforming alternative methodologies.

Scanless Spectral Imaging of Terahertz Vortex Beams Generated by High‐Resolution 3D‐Printed Spiral Phase Plates

Terahertz technology has experienced significant advances in the past years, leading to new applications in the fields of spectroscopy, imaging, and communications. This progress requires the development of dedicated optics to effectively direct, control and manipulate terahertz radiation. In this regard, 3D printing technologies have shown great potential, offering fast prototyping, high design flexibility, and good reproducibility. While traditional 3D printing techniques allow for the preparation of terahertz optical components operating at relatively low frequencies (<0.4 THz) due to their limited resolution, two‐photon polymerization lithography (TPL) exhibits high detail resolution and low surface roughness and can thus potentially enable the fabrication of high‐frequency terahertz devices. Here, as a proof of principle, spiral phase plates operating at 1 THz are designed and fabricated by means of TPL. Moreover, these samples are characterized via a rapid and scanless terahertz imaging technique customized to obtain a coherent hyperspectral analysis of the generated vortex beams at varying distances along propagation. Numerical simulations are also conducted for comparison with experiments, revealing a good agreement. Current limitations of the technique are found to be mainly related with terahertz loss in TPL polymers, and possible solutions are discussed.

InAs Terahertz Metalens Emitter for Focused Terahertz Beam Generation

Metasurfaces have opened doors to combining multiple photonic functionalities in a single compact device. In particular, the ability to generate short terahertz (THz) pulses with precise wavefront engineering in a single THz metasurface redefined the role metasurfaces can play in THz systems. Here, an InAs metalens emitter which generates and focuses a THz pulse beam is demonstrated using a 130 nm thick InAs metasurface designed as a binary‐phase Fresnel zone plate. The THz beam is focused to a spot of ≈430 μm at 1 THz with a short focal length of 5 mm and large numerical aperture of 0.5. Nanoscale InAs Mie resonators comprising the metasurface enable THz generation with an amplitude as high as 20 times compared to plasmonic THz emitters and several times compared to a 1 mm thick ZnTe crystal. This InAs metasurface emitter provides a new paradigm for designing THz imaging, spectroscopy, and communication systems, where THz beam generation and shaping are performed with a single device without compromising the generation efficiency, while eliminating losses and avoiding limitations of phase matching of conventional nonlinear optics approaches.

Self-supervised learning-based re-parameterization instance segmentation for hazardous object in terahertz image

Recently, terahertz imaging technology has shown widespread potential applications in the public security screening field. Previous terahertz hazardous object detection techniques primarily relied on manual recognition and image processing methods. Some solutions incorporating deep learning technologies depend on large quantities of high-quality data, making it challenging to achieve low-cost and high-performance detection. To solve the issue of dependency on large amounts of high-quality data, we propose a diversified dynamic structure You Only Look Once (DDS-YOLO) model based on masked image modeling and structural reparameterization for the instance segmentation of hazardous objects in terahertz security inspection images. To address the scarcity and annotating difficulty problems of terahertz security inspection image samples, we combine automatic data strategies with masked image modeling for self-supervised learning. We propose a multilevel feature refinement fusion mechanism to enhance the quality of learned feature representations. The backbone parameter transfer and fine-tuning training strategies are employed to achieve hazardous object instance segmentation on the terahertz dataset. To address the low detection accuracy issue caused by the poor terahertz image quality, we develop a reparameterizable hierarchical structure for the backbone, improve a multi-scale feature-integrating neck, and design a dynamically decoupled head with lower computational requirements to enhance the performance of the instance segmentation model. Experimental results demonstrate that the proposed model accurately outputs detection boxes, categories, and segmentation masks for hazardous objects with minimal training samples. The comparative experimental results indicate that the proposed model outperforms existing state-of-the-art methods in terms of detection performance. The proposed DDS-YOLO model achieves 59.3 % in mask mean Average Precision (mAP) and 61.3 % in box mAP, and the model parameters and computational requirements also meet practical application scenarios.

Feasibility and potential of terahertz spectral and imaging technology for Apple Valsa canker detection: A preliminary investigation

Apple Valsa canker (AVC) caused by the Ascomycete Valsa mali, seriously constrains the production and quality of apple fruits. The symptomless incubation characteristics of Valsa mali make it highly challenging to detect AVC at an early infection stage. After infecting the wound of apple bark, the pathogenic hyphae of AVC will expand and colonize the phloem tissue. Meanwhile, various enzymes and toxic substances released by hyphae cause the decomposition of cellulose and lignin, and the generation of poisonous secondary metabolites in bark tissue. However, these early symptoms of AVC are invisible from the bark’s appearance. Fortunately, Terahertz Spectral Imaging (ThzSI) technology with the advantage of penetrating, and fingerprinting is promising for detecting hidden or slight symptoms of the fungal infection. This study is a preliminary investigation of terahertz frequency-domain spectra for AVC in the early stage of infection. Healthy and two-week-infected apple tree branches were prepared for capturing ThzS images, and the spectral data were preprocessed by Multivariate scattering correction (MSC), Savitzky-Golay convolution smoothing (SG), and standard normal variate (SNV) respectively to remove data noise and improve data quality. Principal component analysis (PCA), competitive adaptive reweighted sampling (CARS), and random frog (RFROG) were employed to extract the spectral feature bands to eliminate redundant data and improve computational efficiency. Machine learning models were established based on the spectral features to detect AVC at an early infection stage, where 11 of them exhibited the best performance with F1-score of 99.72%. To further explore disease information in spatial spectra, imaging data were acquired using terahertz imaging technology. Based on imaging data, pseudo-color imaging, histogram equalization, and Otsu segmentation were employed to visualize early infection areas in apple barks. Furthermore, histogram feature (HF), shape feature (SF), and local binary pattern (LBP) extracted from terahertz spectral images were utilized to establish the SVM, RF, and KNN models. HF-SF-KNN and HF-SF-LBP-KNN with the best performance achieved F1-score of 98.82%. This study presents a preliminary application of terahertz spectral and imaging technology for early-stage AVC detection and demonstrates its feasibility. Additionally, it provides a new way to detect AVC, which expands the application of ThzSI technology in tree disease detection in orchards and lays the foundation for further research.

Large depth-of-focus achievement based on an aspheric lens with a ring

Terahertz (THz) imaging technology has been widely studied because of its easy penetration of non-polar materials and low photon energy. To acquire a beam featuring both excellent transverse spatial resolution and a considerable depth-of-focus (DOF) to fulfill the demands of two-dimensional and three-dimensional THz imaging, this paper presents an aspheric lens with ring (ALR). The ALR has a controlled diameter of 50 mm, can be machined by 3D-printed technology, and does not need to use complex imaging optical paths to achieve the large DOF function. In a transmitted point-scan imaging system with a 140 GHz light source, the lens can achieve both a resolution of 6 mm and an effective DOF of 66.4 mm for objects greater than 27 mm from the lens surface.

Cryogenically cooled periodically poled lithium niobate wafer stacks for multi-cycle terahertz pulses

We report on the generation of high-power narrow-bandwidth terahertz (THz) pulses by cryogenic cooling of hand-made periodically poled lithium niobate (PPLN) wafer stacks. As a proof-of-concept, we cool stacks with up to 48 wafers down to 97 K and achieve few-percent bandwidths at a center frequency of 0.39 THz, with pulse energy up to 0.42 mJ and average power of 21 mW. Supported by modeling, we observe effective cooling of PPLN wafer stacks that not only reduces terahertz absorption but critically maintains the micrometer-scale inter-wafer gaps for optimal terahertz transmission. Our results unlock the potential for scaling these large-area sources to greater numbers of wafers to push both the energy and bandwidth beyond current capability, opening up possibilities in areas such as terahertz-driven particle acceleration, terahertz imaging, and control over material properties.

Integrated identification and detection of hydration state and its evolution using terahertz technology

The accurate detection of dehydration processes in hydrated drugs can reveal various intermolecular vibration modes mediated by hydrogen bonds between water molecules and other components, which underpin the further development of pharmaceutical science, food safety and biophysics. Herein, terahertz (THz) technology is utilized to investigate the dehydration state of d(+)-Raffinose pentahydrate (Rf·5H2O), in conjunction with imaging-based point by point scanning data acquisition and barcodes methods, to establish an innovative platform integrated identification, trace detection, and application capabilities. Our study demonstrates that the dehydration process of Rf·5H2O can be dynamically monitored through the evolution of its THz absorption peaks, offering more precise results compared to XRD and Raman spectroscopies. Moreover, the absorbance spectra data collected at each individual pixel is utilized to build visualized THz images, achieving an ultralow minimum content required for detection of 0.032 μg/(50 μm)2. Additionally, we introduce a THz spectra-barcode conversion system that not only ensures efficient electronic recordkeeping but also enhances user readability, thereby facilitating the practical applications of THz technology.

Research on wheat impurity identification method based on terahertz imaging technology

The traditional detection of impurities in wheat has difficulties such as low precision, time-consuming, and cumbersome, therefore, it is important to study the method of rapid and accurate detection of impurities in wheat for correctly assessing the quality grade of wheat. Terahertz (THz) technology has many superior properties such as transient, broadband, low-energy, and penetrating, which can realize rapid and nondestructive detection of wheat quality. In this study, a classification and recognition algorithm AHA-RetinaNet-X for wheat impurity terahertz images based on RetinaNet and Artificial hummingbird algorithm (AHA) is proposed.A THz three-dimensional tomography imaging system is used to image wheat and its impurities, and two THz image datasets, respectively the wheat and impurity dataset for verifying the classification effect of wheat and impurities and the impurity dataset for verifying the classification effect of impurities. The experimental results show that the AHA-RetinaNet-X model outperforms other detection and classification models in terms of accuracy, F1-score, precision, recall, and specificity, and is able to achieve 96.1%, 94.9%, 95.2%, 95.8%, 95.5%, 95.3%, and 93.3% for the wheat and impurity dataset and the impurity dataset, respectively, 95.6%, 96.3%, and 95.2%, and the mAP value of AHA-RetinaNet-X is also higher than the other models and can reach 92.1%. The combination of THz imaging technology and AHA-RetinaNet-X can realize the classification and identification of wheat and impurities, which provides a new method for the non-contact rapid nondestructive detection and identification of wheat and impurities, and also provides a reference for the research of the identification and detection methods of other substances.

High gain MIMO antenna with multiband characterization for terahertz applications

Recently, the relevance of multiple input multiple output (MIMO) technology has increased due to the growing user base and the surge in demand for high data rates. It provides an essential answer to the pressing requirement for high-capacity communication systems compliant with new standards. In this study, a four-element MIMO antenna with specific properties—a thickness of 0.0224 mm and a relative permittivity of 4.3 —is presented on a polyamide substrate. It is intended to operate in terahertz (THz) applications at 0.395 and 0.629 THz. The configuration of the antenna is based on a traditional fork antenna. At the resonant frequencies, this MIMO antenna can achieve bandwidths of 9.53 GHz and 24.19 GHz, with reflection coefficients of -16.19 dB and -24.27 dB, respectively. Moreover, a gain of 5.17 dB and 3.19 dB are obtained at the second and first resonant frequencies, respectively. In the effort to maintain the operational band and enhance gain, a significant challenge arises from the insufficient isolation between the four antenna elements within the limited space. A Defected Ground Structure (DGS) is constructed and the inter-element distance is carefully investigated in order to effectively address this issue. Mutual coupling effects are effectively reduced with this antenna, and an amazing MIMO diversity response is shown. Additionally, it achieves a mean effective gain of less than -3, envelope correlation coefficients below 0.0125, diversity gain of around 10 dB, and total active reflection coefficients below -4 dB. The suggested design was simulated using the ansys High-Frequency Structure Simulator program. These enhancements show that the recommended MIMO antenna is a suitable choice for terahertz applications like 5 G networks of the future and terahertz imaging systems used in medical settings for tissue imaging and cancer diagnosis.

A novel framework for concealed weapons detection using passive terahertz imaging

ABSTRACT In this work, a novel framework for non-destructive, non-intrusive and completely automatic concealed weapon detection under human clothing for entry control and security check applications using passive terahertz (THz) images has been given. The technique models the problem of concealed weapon detection as that of binary segmentation, where the pixels corresponding to concealed weapons are foreground while the remaining are background. The core idea of the proposed framework is to first oversegment the THz image into superpixels and subsequently multiple handcrafted features are extracted from each superpixel. Finally, a machine learning-based classifier is used to perform binary classification thereby classifying each superpixel as foreground or background such that each pixel in a foreground superpixel is a foreground pixel and vice-versa for background. It is worth mentioning that this technique does not require a perfect segmentation to extract features from, rather an oversegmentation suffices. Furthermore, both global features like those based on image saliency and local features like those based on intensity are extracted considering the image properties. The detailed experiments and comparative analysis confirm that the proposed technique efficiently detects concealed weapons, exhibiting superior performance compared to state-of-the-art binary segmentation methods for the purpose.