Terahertz Imaging Research Articles

THz-Based Monitoring of the Deformation of the Inner Woven Structure of Glass-Fiber Reinforced Polymers

This work exploits terahertz (THz) imaging in the real-space and the spatial-frequency domain to determine nondestructively the evolution of the main characteristics of a woven glass-fiber reinforced polymer (GFRP) laminate in three-dimensions in-situ during a tensile test. These characteristics include tow orientation and the weave period. The surface and the inner plies can be investigated with the proposed method. The results show how the tows tend to realign with the load direction. In addition, we detect a decrease in the weave period, manifesting a distortion of the surface and inner layers as the stress is increased. Contrary to conventional optical inspection, THz inspection can provide quantitative measurements in all material layers and not only on the surface. This technique could be helpful to monitor distortion and damage due to mechanical stress or defects incorporated during the manufacturing process and to validate parameters and results of draping process modeling, especially for the inner plies.

Simultaneous Terahertz Imaging With Information and Power Transfer (STIIPT)

Terahertz (THz) band transmission has the potential to revolutionize future-generation wireless networks by jointly meeting communication and non-communication demands of their connected devices. Recent advances in THz semiconductor technologies and antenna design are closing the THz band gap in millimeter-scale devices which are often battery-limited. The localization of these mobile devices in future wireless networks is of paramount significance for beamforming to overcome THz propagation loss. Consequently, prospective signal processing techniques with co-design architectures are emerging either for joint communication and sensing or for simultaneous information and power transfer. In this paper, we present a novel approach toward Simultaneous Terahertz Imaging with Information and Power Transfer (STIIPT) from a base station transceiver to an Integrated Receiver (IntRx). Leveraging the proposed non-linear rectenna model for energy harvesting, a customized On-Off Keying (cOOK) modulation scheme is proposed for simultaneous communication through a non-linear THz channel while generating a radar-like image to localize the IntRx acting as a target. The proposed theoretical models are corroborated with simulations using a THz band GaAs Schottky diode to demonstrate STIIPT performances which also emphasize the significance of ranging information to optimize rate–energy transfer tradeoff under the cOOK modulation scheme.

Antireflection Coatings Based on PEDOT:PSS Conductive Polymer Using d-Sorbitol Additives for Terahertz Spectroscopy and Imaging.

Optical antireflection has been employed for a variety of applications in terahertz spectroscopy and detectors. However, current methods encounter challenges in terms of cost, bandwidth, structural complexity, and performance. In this study, a low-cost, broadband, and easily processed THz antireflection coating scheme based on the model of impedance-matching effect is proposed, using a 6 wt % d-sorbitol-doped poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (s-PEDOT:PSS) film. By adjusting the thickness of the s-PEDOT:PSS film, these biocompatible conductive polymers enable a significant reduction of Fresnel reflection and operate over a broad bandwidth between 0.2 and 2.2 THz. Applying the antireflective coating to the surface of the sample substrate and electro-optic probe crystal in THz spectroscopy and near-field imaging shows that the spectral resolution is significantly improved, and the devices exhibit more excellent intended performance. The findings of this study could aid in improving the measurement capability of various THz time-domain spectroscopy and imaging system.

Research on nondestructive detection of pine nut quality based on terahertz imaging

ObjectivePine nuts are of great nutritional and medicinal value, but they cannot avoid such defects as mildew and insect-eating during storage. Because of their hard shells, the internal quality detection of pine nuts is a major problem for industrial sorting. For this reason, it is of great significance to carry out rapid nondestructive detection of the internal quality of pine nuts. MethodsIn this paper, a rapid and nondestructive detection of pine nuts for mildew and plumpness based on terahertz transmission imaging technology was carried out. Firstly, the terahertz transmission images of pine nut samples were acquired and the terahertz spectral signals of four different regions of interest were extracted for analysis. Secondly, in order to reduce the interference of external environment on the acquisition of terahertz spectra, the terahertz spectra were pre-processedby several methods, such as ALS, AirPLS, BEADS and SNV + Detrending, and then three qualitative discriminant models, namely, Linear Discriminant Analysis (LDA), Support Vector Machine (SVM) and XGBoost integration learning, were established respectively to explore the optimal qualitative discriminant model for the detection of pine nut quality. Finally, the terahertz transmission image of pine nuts was subjected to image processing. The main feature gamut extraction combined with channel separation strategy were adopted. and then the automatic threshold segmentation algorithm was applied to perform binary threshold segmentation on the separated image, thus the plumpness of the pine nuts was calculated by calculating the ratio of the pixel points of the shell and the kernel. ResultsThe prediction set of BEADS + XGBoost model was established after data preprocessing with the optimal effect and the accuracy of 98.61%. The acquired terahertz images of pine nuts were extracted by the main feature gamut and the images of channel B were extracted by using channel separation. Finally, the automatic threshold segmentation of channel B was performed by using the maximum one-dimensional entropy, which can well realize the visual detection of the inner shell kernel of pine nuts. ConclusionTerahertz imaging technology can achieve rapid and nondestructive detection of pine mildew as well as pine nut plumpness. This study provides a new rapid and nondestructive effective method for pine nut quality detection, which can provide technical reference for other shelled nut quality detection and has significant practical value.

Ultrabroadband vanadium-dioxide-based metamaterial absorber based on two resonance modes at a terahertz frequency

A terahertz (THz) ultrabroadband metamaterial absorber consisting of a periodically patterned vanadium dioxide (VO2) array, loss-free dielectric layer, and a continuous gold film is designed. Its resonance features can be dynamically tuned by applying different temperatures to the VO2 to promote phase transformation. When the VO2 is in the metallic state, the designed metamaterial has an absorption bandwidth of 6.08 THz with an absorptivity more than 90%, from 3.84 THz to 9.92 THz. The broadband absorption is attributed to the combination of two absorption peaks localized at 4.73 THz and 9.05 THz that are based on the localized resonance mode and surface lattice resonance mode. Taking advantage of the temperature phase transition of VO2, the designed absorber can be switched between ultrabroadband absorption and near-total reflection. Its maximum modulation depth can reach 99%, and it achieves an excellent modulation effect with a bandwidth of about 6 THz. The physical mechanism of the ultrabroadband absorption is discussed through an analysis of the near-field distribution and the current density distribution of the absorption peaks. The effect of structural parameters on the absorption are also investigated. The designed metamaterial absorber could have application potential in THz imaging, THz communications and smart devices.

Performance Analysis of an Aperture-Coupled THz Antenna for Diagnosing Breast Cancer.

The most important technique for exposing early-stage breast cancer is terahertz imaging. It aids in lowering the number of breast cancer-related fatalities and enhancing the quality of life. An essential component of developing the THz imaging system for high-quality photos is choosing the right sensor. In this article, a wideband antenna for microwave imaging of breast tissue with an operating frequency of 30 GHz (107 GHz to 137 GHz) is constructed and analyzed. An aperture-coupled antenna with an optimized ground aperture is proposed and analyzed, which made it possible to obtain better and consistent impedance matching in the wideband spectrum. The variation of backscattered signal energy in body tissue is assessed with healthy breast tissue and in the presence of malignant cells. A significant difference in energy scattering is observed for both situations. The suggested antenna's linear and stable time domain characteristics make it an appropriate component for THz imaging technology.

Tunable bandstop filter using spoof surface plasmon polaritons for terahertz applications

This paper presents the design and analysis of a spoof surface plasmon polaritons (SSPP)-based bandwidth tunable bandstop filter (BSF) operating at 0.26 THz frequency. The proposed filter is comprised of SSPP T-shape resonators which are coupled to the spoof transmission line. Odd and even mode analysis is carried out for the proposed design, and using Co-EM simulation, it is shown that the designed band stop filter is capable of offering tunable bandwidth at THz frequency using a variable capacitor. In order to experimentally validate the proposed approach, design studies are carried out at microwave frequencies and the BSF is developed on a Rogers R4003C substrate of thickness 1.524 mm. The proposed filter designs have a 3 dB bandwidth tunability of 3 GHz (0.038–0.041 THz) and 50 MHz (0.55–0.60 GHz) in the THz and microwave frequency regimes, respectively. The proposed terahertz filter has the potential to be used as an ultra-compact BSF in future applications of photonic integrated circuits in terahertz imaging.

BICs-enhanced active terahertz wavefront modulator enabled by laser-cut graphene ribbons

Graphene-based terahertz (THz) metasurfaces combined with metallic antennas have the advantages of ultra-small thickness, electrical tunability, and fast tuning speed. However, their tuning ability is limited by non-independently tunable pixels and low modulation depth due to the ultra-small thickness of graphene. Here, we demonstrate a reconfigurable THz phase modulator with 5×5 independently tunable units enabled by switching the voltages applied on 10 graphene ribbons prepared by laser cutting. In addition, by introducing quasi-bound states in the continuum resonance through a designed double C-shaped antenna, the efficiency of the device is enhanced by 2.7–3.6 times under different graphene chemical potentials. Experimental results demonstrate that a focus can be formed, and the focal length is changed from 14.3 mm to 22.6 mm. This work provides potential for compact THz spatial light modulators that may be applied in THz communication, detection, and imaging.

Label-free detection and identification of single bacteria via terahertz near-field imaging.

In recent years, terahertz (THz) imaging has attracted much attention because of its ability to obtain physical and chemical information in a label-free, noninvasive and nonionizing manner. However, the low spatial resolution of traditional THz imaging systems and the weak dielectric response of biological samples hinder the application of this technology in the biomedical field. In this paper, we report a new THz near-field imaging method for a single bacteria, through the coupling effect of nanoscale radius of probe and platinum gold substrate, which greatly enhances THz near-field signal of biological samples. A THz super-resolution image of bacteria has been successfully obtained by strictly controlling the relevant test parameters such as tip parameters and driving amplitude. By analyzing and processing the THz spectral image, the morphology and inner structure of bacteria have been observed. The method has been used to detect and identify Escherichia coli represented by Gram-negative bacteria and Staphylococcus aureus represented by Gram-positive bacteria. This application provides a new label-free, noninvasive and nonionizing testing protocol for the detection of single bacteria.

Tutorial: Real-time coherent terahertz imaging of objects moving in one direction with constant speed

Pulsed terahertz (THz) electric fields enable various coherent THz imaging modes, such as reflection tomography, spectral imaging, and computed tomography (CT) for nondestructive inspection, quality control, and material characterization. The extension of coherent THz imaging modes to moving objects has been regarded as key to their social implementation. This Tutorial focuses on two-dimensional spatiotemporal (2D-ST) THz imaging of objects moving in one direction with constant speed as a promising means of enabling real-time coherent THz imaging. In 2D-ST THz imaging, the temporal waveform and line image of the THz pulse are simultaneously acquired without the need for mechanical scanning of the time delay and sample position using a combination of non-collinear 2D free-space electro-optic sampling with THz line-imaging optics. This 2D-ST THz imaging boosts the imaging rates of THz reflection tomography, THz spectral imaging, and THz CT to levels that are applicable to moving objects. The advanced THz reflection tomography and THz spectral imaging that result from the assistance of 2D-ST THz imaging achieve real-time line imaging of cross sections and spectral signatures, respectively. Subsequently, this enables in-line total inspection of objects moving on a translation stage or a conveyor belt. A THz CT system using real-time line projection of a THz beam is effectively applied to a 2D spectral cross section of a continuously rotating object. 2D-ST THz imaging enables the functional THz imaging of moving objects in various practical applications.

Enhance Terahertz Image Resolution via Complex-valued convolutional neural networks

Terahertz imaging technology is widely used in nondestructive testing (NDT), human security inspection, far-field imaging systems, and so on. Improving image resolution is always a topic of discussion. For visible image and image super-resolution, the convolutional neural network (CNN) has achieved a better improvement effect than the traditional algorithm. Compared to optics, it is quite feasible to gain both the amplitude and phase information in THz imaging, the Complex-valued convolutional neural network (CV-CNN) is better suited for the field of terahertz imaging than the real-valued neural network. In this paper, A lightweight complex-valued neural network is constructed which enhances terahertz imaging in a vector network analyzer (VNA) imaging system. Compared to CNN, CV-CNN has a better peak signal-to-noise ratio (PSNR) and structural similarity (SSIM), and it is significantly less vulnerable to overfitting. Phase information can be used well at the same time, which is impossible for CNN. The network is trained using the MNIST dataset and verified by using simulated and measured data obtained from a 200Ghz imager.

Unsupervised Terahertz Image Restoration Based on CycleGan

Terahertz (THz) is considered as one of the key technologies for sixth generation communications, military, medical imaging and industrial inspection. THz images are susceptible to degradation due to system noise and point spread functions during transmission. The existing deep learning methods use ground truth and input images for supervised training that can recover THz images very well. But it’s difficult to obtain labeled THz data in practical application. In this paper, we propose an attentional adversarial cycle generation network for THz image restoration (CycleTHz) based on CycleGan to address this problem. The CycleTHz generates clean images firstly by an attention-guided generation network and then discriminates the quality of the generators by an attention discriminator. In addition, RGB color loss is used for image channels for constraint. To the best of our knowledge, this is the first THz dataset to be trained using an unsupervised approach. Extensive experiments show that the proposed method improves the PSNR and SSIM by 43.4% and 101.7% compared with CycleGan, which is a benchmark method for the unsupervised development in THz image restoration. The code is available at https://github.com/hellogry/UnsupervisedCycleTHz

Broadband THz Edge-Enhanced Imaging Technology Based on Electro-Optic Sampling

Based on the radial Hilbert transform of spatial filtering and electro-optical sampling, we propose a broadband terahertz edge-enhanced imaging method in this work. Instead of using a narrow-band THz vortex phase plate, here we use vortex ultrashort laser pulse to realize the THz enhancement imaging which can be applied for few-cycle THz imaging. The principle of edge enhancement is analyzed theoretically, and the analytical expression of the final terahertz imaging is derived. We found that edge-enhancement imaging corresponds to the nonlinear term of the THz field, and the background and linear terms can be suppressed when the static birefringent phase is set to zero. The simulations show that our method can effectively improve the contrast and signal-to-noise ratio of terahertz imaging.

Data-Driven-Based Terahertz Image Restoration

Terahertz (THz) has been widely used in radar, remote sensing, atmospheric and environmental monitoring, real-time biological information extraction, and medical diagnosis. However, the long-wavelength property of THz leads to low imaging resolution. Due to the expensive nature of experimental equipment and the difficulty of obtaining terahertz sources, most scientific institutions have difficulty in obtaining large amounts of terahertz dataset. In addition, it is difficult for convolution neural networks to recover low-resolution complex images because of the phase information contained in the images. In this paper, we build a terahertz imaging system and propose a Progressive Multi-scale Augmented Neural Network (PMANN) to address this problem. PMANN integrates self-attention and channel-attention for image contextual feature extraction by the proposed progressive network. Compared with the Complex convolution Neural Network (CCNN) in private cartoon datasets, PMANN improves the quantitative metrics on PSNR by about 18.6%. It also obtains better visual quality than CCNN. Moreover, we publicly and synthesize the first grayscale image MinistTHz dataset. Compared to SwinTransformer, PMANN improves the PSNR by 28.2%. The experimental results on the THz dataset demonstrate that the proposed model achieves better performances than the state-of-the-art models. Our synthetic THz dataset is open at https://github.com/szpxmu/PMANN.

Research on Rice Seed Fullness Detection Method Based on Terahertz Imaging Technology and Feature Extraction Method

Research on Rice Seed Fullness Detection Method Based on Terahertz Imaging Technology and Feature Extraction Method

Inner defect detection of glass fiber reinforced polymer sandwich panel using pulsed terahertz imaging based on smoothing and derivative

We conducted an experimental investigation to localize and detect internal defects in a sandwich panel made of glass-fiber-reinforced polymer (GFRP) using terahertz (THz) time-domain spectroscopy and signal processing techniques, which included smoothing and derivative analysis to enhance the defect detection performance. A simulated defect was created using a peel ply and scanned using a pulsed THz scanner with a reflection geometry specifically designed for this purpose. The reflected THz wave was analyzed to improve defect detection performance through time-domain signal processing using smoothing and derivative analysis. To quantify the performance of the proposed signal processing method, we introduced the Defect to Noise Ratio and evaluated its effectiveness accordingly. As a result, noise was removed and defect detection performance was enhanced, enabling the visualization of foreign insert defects in the GFRP sandwich panel.

Curved Synthetic Aperture Radar for Near-Field Terahertz Imaging

High resolution imaging technique will play an essential role in the future smart imaging applications to provide potential solutions for accurate detection, monitoring and classification of the target. The potential of synthetic aperture radar (SAR) systems to acquire high resolution images has opened new frontiers and led to its exploration in new applications. The conventional microwave radar imaging suffers limited range resolution in the sub-millimeter wave range and optical imaging methods are constrained by the diffraction limit. In the terahertz (THz) regime of the spectrum, the SAR systems could be operated in all the weather, all the time and are capable of penetrating through clouds, smoke, dust etc. to achieve high resolution images beyond the diffraction limit. The curved SAR presents a SAR system created by circularly encircling the target scene. The Circular SAR and Spiral SAR which are based on a circular and cylindrical spiral scanning trajectory, respectively are the scanning methods used for curved SAR. In this paper, we have explored a new toolbox that enables the rapid development of near-field THz-SAR imaging systems and generation of large near-field THz imaging scenarios with the goal to support and facilitate data driven research such as deep learning in the THz community through synthetically generated SAR images as well as its potential for THz based high resolution and 2D/3D near-field imaging application. Initially, the potential advantages of THz-SAR over Microwave SAR and optical imaging methods are presented. Further, a frequency modulated continuous wave (FMCW) radar testbed is simulated in multi-input multi-output (MIMO) configuration for the target frequency band. Moreover, the circular and cylindrical SAR scanning methods are explored for near-field imaging of point targets to obtain 2D and 3D THz-SAR images, respectively. Also, the radar images are reconstructed using the Back Propagation and Polar Formatting Algorithms, focusing on short range applications like indoor environments. Finally, the SAR performance evaluation metrics are reported and a roadmap for future work is presented.

THz Imaging to Map the Lateral Microporosity Distribution in Carbonate Rocks

Terahertz (THz) spectroscopy is a nondestructive tool used in many industries to analyze materials, including measuring the water content and the distribution of water in biological samples. THz time-domain spectroscopy (THz-TDS) measures the dielectric and structural properties of a sample by probing it with an ultrafast THz pulse and measuring the change in amplitude and phase. In this study, we demonstrate the use of THz-TDS imaging to quickly map lateral (i.e., two-dimensional) variations in microporosity (ϕμ) using the THz attenuation due to water in the pores after clearing the large pores via centrifugation. Three carbonate rock plugs with differing ϕ and pore-size distributions were subsampled for this study. Three water saturation states were produced for each sample: saturated, centrifuged, and dry. At each saturation state, the sample is weighed and imaged using THz-TDS to spatially map and measure ϕμ. The results show that for each sample the ϕμ obtained using THz-TDS imaging is in excellent agreement with that obtained from both mass balance and MICP. In addition, the THz-TDS maps show significant differences in the spatial distribution of the microporosity for samples having similar composition. This method provides a means to measure ϕ and ϕμ while mapping the spatial distribution of ϕμ toward improved petrophysical characterization of carbonate reservoir rocks.

Terahertz Constant Velocity Flying Spot for 3D Tomographic Imaging.

This work reports on a terahertz tomography technique using constant velocity flying spot scanning as illumination. This technique is essentially based on the combination of a hyperspectral thermoconverter and an infrared camera used as a sensor, a source of terahertz radiation held on a translation scanner, and a vial of hydroalcoholic gel used as a sample and mounted on a rotating stage for the measurement of its absorbance at several angular positions. From the projections made in 2.5 h and expressed in terms of sinograms, the 3D volume of the absorption coefficient of the vial is reconstructed by a back-projection method based on the inverse Radon transform. This result confirms that this technique is usable on samples of complex and nonaxisymmetric shapes; moreover, it allows 3D qualitative chemical information with a possible phase separation in the terahertz spectral range to be obtained in heterogeneous and complex semitransparent media.

Terahertz super-resolution imaging based on a confocal waveguide and a slider-crank scanning mechanism.

Due to the limitation of Abbe diffraction limit, the traditional terahertz (THz) continuous wave imaging methods based on lenses or mirrors are difficult to achieve super-resolution. Here we present a confocal waveguide scanning method for THz reflective super-resolution imaging. In the method, a low loss THz hollow waveguide is used to replace the traditional terahertz lens or parabolic mirror. Through optimizing the size of the waveguide, we can achieve far field subwavelength focusing at 0.1THz and achieve super-resolution terahertz imaging. In addition, a slider-crank high-speed scanning mechanism is used in the scanning system, and the imaging speed is more than 10 times faster than the traditional step scanning system based on linear guides.