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A Method and System for Enhancing the Resolution of Terahertz Imaging
Abstract
In this paper, a comprehensive method and process for developing Terahertz (THz) images with enhanced resolution is introduced. This method is implemented as a system comprised of various resolution and quality enhancement techniques. In this system, filters in time and frequency domains are used to filter out noise, low-frequency spectrum, and diffraction distortions. Point Spread Function (PSF) is modeled and the quality of the output image is enhanced further through deconvolution. This process can be fully implemented on THz Time-Domain Spectroscopy (TDS) systems, and partially on continuous-wave THz imaging systems. A user-friendly software program comprising a graphical user interface (GUI) has been developed by the author for automation of implementation of the proposed system. By taking advantage of the proposed process, researchers and industrial sectors can achieve a substantial enhancement on the quality and resolution of their THz images.
Supplementary resource (1)
... In the literature, different techniques were proposed for improving resolution and the signal to noise ratio of THz images [20][21][22]. An approach to beam shape compensation in THz CT reconstruction was described in [23]. ...
... The transformation in Eq. (26) applied to q ϕ (t) defines the desired matrix H p . We will further refer to the solution of the system in Eq. (22) as reconstruction with sinogram space correction (RSSC). ...
... (22), the inverted matrices W R −1 and C −1 can be applied as preconditioners to the systems in Eq. (20) or Eq. (22). Applying C −1 as the preconditioner to Eq. (20) gives the following system of linear equations: ...
Terahertz (THz) computed tomography is an emerging nondestructive and non-ionizing imaging method. Most THz reconstruction methods rely on the Radon transform, originating from x-ray imaging, in which x rays propagate in straight lines. However, a THz beam has a finite width, and ignoring its shape results in blurred reconstructed images. Moreover, accounting for the THz beam model in a straightforward way in an iterative reconstruction method results in extreme demands in memory and in slow convergence. In this paper, we propose an efficient iterative reconstruction that incorporates the THz beam shape, while avoiding the above disadvantages. Both simulation and real experiments show that our approach results in improved resolution recovery in the reconstructed image. Furthermore, we propose a suitable preconditioner to improve the convergence speed of our reconstruction.
... A variety of approaches for remotely sensed images can be categorized as optics-based methods, interpolation methods, and machine-learning methods [6][7][8][9]. Optics-based methods such as dielectric cube terajet generation, wide-aperture lens, and solid-immersion technique have been proposed for enhancing the resolution of imaging systems [10][11][12][13][14]. Compared with optics-based methods, the idea behind machine-learning methods is to learn the potential relationships between low-resolution and high-resolution domains from an external training set, then to generate the final super-resolved image using this prior knowledge and machine-learning methods can improve the reconstruction quality in parallel with optics-based methods. ...
... A variety of approaches for remotely sensed images can be categorized as optics-based methods, interpolation methods, and machine-learning methods [6][7][8][9]. Optics-based methods such as dielectric cube terajet generation, wide-aperture lens, and solidimmersion technique have been proposed for enhancing the resolution of imaging systems [10][11][12][13][14]. Compared with optics-based methods, the idea behind machine-learning methods is to learn the potential relationships between low-resolution and high-resolution domains from an external training set, then to generate the final super-resolved image using this prior knowledge and machinelearning methods can improve the reconstruction quality in parallel with optics-based methods. ...
... Remote Sens. 2019,11, 2333 ...
Single-image super-resolution (SR) is an effective approach to enhance spatial resolution for numerous applications such as object detection and classification when the resolution of sensors is limited. Although deep convolutional neural networks (CNNs) proposed for this purpose in recent years have outperformed relatively shallow models, enormous parameters bring the risk of overfitting. In addition, due to the different scale of objects in images, the hierarchical features of deep CNN contain additional information for SR tasks, while most CNN models have not fully utilized these features. In this paper, we proposed a deep yet concise network to address these problems. Our network consists of two main structures: (1) recursive inference block based on dense connection reuse of local low-level features, and recursive learning is applied to control the model parameters while increasing the receptive fields; (2) a bidirectional convolutional LSTM (BiConvLSTM) layer is introduced to learn the correlations of features from each recursion and adaptively select the complementary information for the reconstruction layer. Experiments on multispectral satellite images, panchromatic satellite images, and nature high-resolution remote-sensing images showed that our proposed model outperformed state-of-the-art methods while utilizing fewer parameters, and ablation studies demonstrated the effectiveness of a BiConvLSTM layer for an image SR task.
... The blind methods only utilize the prior information of the blurred image itself, and then use regularization term (such as Wong et al. applied the total variation [35] and Krishnan et al. utilized the normalized sparsity measurement [36]) to estimate the PSF and improve its spatial resolution subsequently. Apart from blurred images, the non-blind methods also apply measured [37] or simulated [38], [39] PSF to restore the THz images. ...
... Previous studies have already revealed that the key aspect of THz image restoration is the construction of the PSF. In this study, we applied the mathematical modeling of the PSF proposed by Ahi [39], which has been proved technically feasible in many studies [13], [38], [50], and could be illustrated in Fig. 1 and expressed as: ...
Terahertz time-domain spectroscopy imaging system (THz-TDS) is becoming a promising tool for packaged integrated circuit (IC) failure detection due to its nonmetal penetrability and low radiation. However, two major obstacles are hindering the industrial application of the THz-TDS based IC detection method: 1) the low resolution of THz images may affect the detection accuracy; 2) the failure detection tasks are always carried out manually, which is inefficient and inaccurate. Thus, in this paper, we firstly enhanced the quality of IC THz images with a deconvolution algorithm and a mathematically simulated point spread function (PSF), and then we proposed a deep convolutional neural network (CNN) based failure detection framework to achieve end-to-end IC inspection automatically. Besides, we introduced transfer learning to overcome the limitation of the IC dataset size. The result demonstrated that our proposed method achieved excellent performance concerning both accuracy and efficiency.
... MISR refers to improving the resolution of THz images from overlapping oversampled THz images; whereas 2) single image superresolution (SISR) aims at reconstructing a high-resolution (HR) THz image from a single low-resolution (LR) THz image. Lucy-Richardson deconvolution algorithm with estimated [18,19], measured [9], or simulated [20,21] PSF was one of the most widely used SISR methods for THz image restoration. Since lots of THz images are acquired by the raster-scanning method, and this imaging process can be modeled as a convolution operation of the object and the point spread function (PSF). ...
... In addition to the blur kernel, noise is recognized as an important factor that influences the quality of the THz images. As there is always much noise in the THz signal, directly resolving an image without denoising may amplify the noise, leading to visually unpleasant results [20]. Considering the characteristics of the THz image, white Gaussian noise with different standard deviations was set up as the noise template. ...
This study proposes a super-resolution (SR) method for terahertz time-domain spectroscopy (THz-TDS) images, combining a convolutional neural network (CNN) and a mathematical degradation model. The mathematical degradation model considers three possible factors affecting the quality of THz images: the blur kernel, noise, and down-sampler. Specifically, the blur kernel characterizes the continual change of image blur extent with the imaging distance. The designed CNN learns from the degradation model and then copes with the distance dependent image restoration problem based on the learned mapping between the low and high-resolution image pairs. The designed two-stage comparative experiment shows that the proposed method significantly improved the quality of the THz images. To be specific, our proposed method enhanced the resolution by a factor of 1.95 to 0.61 mm with respect to the diffraction limit. In addition, our method achieved the greatest improvement in terms of image quality, with an increase of 4.35 in PSNR and 0.10 in SSIM. We believe that our method could offer a satisfactory solution for THz-TDs image SR applications.
... In 2020, Zhang et al. proposed a method for THz image restoration using wavelet denoising with PSF model that enhanced the resolution and quality on the imaging detection of the IC chips with complex internal structures. 34 Ahi et al. used THz-TDS methods combined with PSF-based image restoration techniques to characterize and identify quality control issues that can be used for authentication of packaged ICs 30,193 shown in Fig. 18. This method was able to detect the presence of unexpected materials in counterfeit devices, blacktopping layers (used by counterfeiters to hide the original label and overprint a false one), shape and dimensions of hidden structures, sanded and contaminated devices, differences between internal structures of counterfeit and authentic devices, such as misshapen die-frames and bond-wires. ...
... This method is implemented as a system comprised of various resolution and quality enhancement techniques.In this system, filters in time and frequency domains are used to filter out the noise, low-frequency spectrum, and diffraction distortions. Reprinted with permission from Ref.193, Copyright © 2019 Elsevier. ...
Terahertz radiation for inspection and fault detection has been of interest for the semiconductor industry since the first generation and detection of THz signals. Until recent hardware advances, THz systems lacked the signal quality and reliability for use as an effective nondestructive testing (NDT) method. Incremental advances in THz sources, detectors, and signal processing resulted in the successful applied-industrial use of THz NDT techniques on carbon fiber laminates, automotive coatings, and for detection of counterfeit pharmaceutical tablets. Semiconductor inspection and verification methods ensure the functionality and thereby safety of vital electronics for several critical industries. For this reason, the reliability and verification of a THz NDT method must exceed currently used inspection systems. With recent laboratory access to THz radiation, THz inspection methods are often compared with existing optical, electrical, and volumetric semiconductor verification techniques for their production monitoring and failure analysis viability. This review will cover THz techniques and their applications at the printed circuit board (PCB), integrated circuit (IC), and transistor/gate scales. The THz radiation gap spans between optical and electronic ranges with a millimeter-sized wavelength allowing for adequate penetration of plastic and ceramic and semiconductor materials. THz radiation can be used to determine structural features, electrical signatures in the THz range, and chemical information simultaneously. Cost and environmental limitations restricted the ability for THz NDT semiconductor inspection methods to escape the lab and succeed in the dynamic environment of a semiconductor fabrication environment. Hybridized metrology methods incorporating information from multiple inspection tools are a regime where THz spectral and structural data can be combined with existing methods such as optical, X-ray, or E-beam. THz can be used initially to offer support to the complex failure analysis and verification requirements of the semiconductor industry from nanoscale to macroscale features and components. For THz systems to become independent inspection tools used for semiconductor production monitoring, in the lab or fab, this will require a confident level of statistical process control for THz signal generation, detection, or processing. Applied industrial semiconductor device inspection will likely be a result of a combination of research into THz hardware, reconstruction techniques, and the widespread application of machine learning techniques. Many breakthroughs occurred over the years to enable successful nondestructive characterization and inspection of semiconductor devices from the nanoscale transistors to fully packaged integrated circuits and assembled PCBs. © 2021 Society of Photo-Optical Instrumentation Engineers (SPIE).
... Different forms of graphene have been presented so far, the most prominent of which are carbon nanotubes, 3D graphene, graphene nano-ribbons, and graphene-oxygen blends. Graphene is also used to control nanoscale light emission [6]. Researchers are working on a device that can control the scattering of light on a surface. ...
... For instance, THz imaging is an appropriate alternative for conventional X-rays imaging. It is less hazardous but needs image enhancements due to lower resolution [6]. ...
A dual narrow band perfect THz absorber is presented in this work. The structure includes three layers of graphene ribbons, disks, and sheet on the TOPAS dielectric layer while a golden plate is placed at the bottom to act as a fully reflecting mirror against THz waves. According to the simulations, the device is robust enough to show independent operation versus layers thicknesses variations, chemical potentials mismatches, and changing of electron relaxation time. The designed THz absorber in this work is an appropriate basic block for several applications in THz optical systems such as sensors, detectors, and modulators. The layers in the proposed device are modeled via passive circuit elements and consequently, the equivalent circuit of the device is calculated. Leveraging developed circuit model and impedance matching concept, the proposed device is designed to perfect absorption at 1 THz and 7 THz. Ample simulations are performed using MATLAB and CST to verify the superior performance of the device. The presented manuscript considers the circuit model representation for three different layers of the device. For a unique structure, highly tunable response versus chemical potential is obtained. Circuit model approach and impedance matching theory are exploited to reduce computational time regarding conventional approaches.
... This study strengthens analytically the previous results and associated assertions [28,42] made with numerical simulations to gain physical insight into the ACI's working principles in imaging. We conjecture that the theory of ACI can be potentially generalized to a wide variety of noisy and lossy linear systems including, for example, those in atmospheric imaging [46][47][48][49], time-domain spectroscopy [50,51], optical communications [52][53][54][55], P T symmetric non-Hermitian photonics [21,56,57], and even quantum computing [49,58,59]. Some discussions of how the ACI can be applied to atmospheric imaging, timedomain spectroscopy, and quantum computing can be found in Ref. [49]. ...
... Revealed from the simple mathematical result in Eq. (14), we conjecture that the theoretical concepts of ACI can be potentially generalized to numerous scenarios in noisy and lossy linear systems (e.g., atmospheric imaging [46][47][48][49], bioimaging [89], deep-learning-based imaging [90], structured illumination [68], tomography [91], time-domain spectroscopy [50,51], free-space optical communications [52][53][54][55], symmetric non-Hermitian photonics [21,56,57], quantum computing [49,58,59], etc.) at different frequencies. Since ACI operates down at the physical layer, all of these scenarios should benefit from ACI for improved performance. ...
Recently an optical amplification process called the plasmon injection scheme was introduced as an effective solution to overcoming losses in metamaterials. Implementations with near-field imaging applications have indicated substantial performance enhancements even in the presence of noise. This powerful and versatile compensation technique, which has since been renamed to a more generalized active convolved illumination, offers new possibilities of improving the performance of many previously conceived metamaterial-based devices and conventional imaging systems. In this work, we present, to the best of our knowledge, the first comprehensive mathematical breakdown of active convolved illumination for coherent imaging. Our analysis highlights the distinctive features of active convolved illumination, such as selective spectral amplification and correlations, and provides a rigorous understanding of the loss compensation process. These features are achieved by an auxiliary source coherently superimposed with the object field. The auxiliary source is designed to have three important properties. First, it is correlated with the object field. Second, it is defined over a finite spectral bandwidth. Third, it is amplified over that selected bandwidth. We derive the variance for the image spectrum and show that utilizing the auxiliary source with the above properties can significantly improve the spectral signal-to-noise ratio and resolution limit. Besides enhanced superresolution imaging, the theory can be potentially generalized to the compensation of information or photon loss in a wide variety of coherent and incoherent linear systems including those, for example, in atmospheric imaging, time-domain spectroscopy, ${\cal P}{\cal T}$ P T symmetric non-Hermitian photonics, and even quantum computing.
... 8,266,267 Signal processing and suppression of diffraction through filtering out the delayed beams have also proved significant enhancements in THz images. 268 A software system for automation of the implementation of the high-resolution THz imaging theory of Ref. 268 was also proposed. 269 The user interface of this system is shown in Fig. 17. ...
... 8,266,267 Signal processing and suppression of diffraction through filtering out the delayed beams have also proved significant enhancements in THz images. 268 A software system for automation of the implementation of the high-resolution THz imaging theory of Ref. 268 was also proposed. 269 The user interface of this system is shown in Fig. 17. ...
... Addressing these issues without costly upgrades to the imaging system, algorithm-based methods show promise in directly enhancing low-resolution (LR) images into super-resolution (SR) images, tackling concerns (i) and (ii). For instance, by using prior knowledge of the degradation based on the specific point spread function (PSF) measured [39][40][41] or simulated [42][43][44] from the THz imaging system, a variety of SR algorithms have been explored, including Richardson-Lucy algorithm [45] and PSF deconvolution [46]. These methods have demonstrated success in improving spatial resolution and reducing the diffractive limit, although challenges in rendering image edge and texture details remain. ...
Data acquisition, image processing, and image quality are the long-lasting issues for terahertz (THz) 3D reconstructed imaging. Existing methods are primarily designed for 2D scenarios, given the challenges associated with obtaining super-resolution (SR) data and the absence of an efficient SR 3D reconstruction framework in conventional computed tomography (CT). Here, we demonstrate BLIss, a new approach for THz SR 3D reconstruction with sparse 2D data input. BLIss seamlessly integrates conventional CT techniques and variational framework with the core of the adapted Euler-Elastica-based model. The quantitative 3D image evaluation metrics, including the standard deviation of Gaussian, mean curvatures, and the multi-scale structural similarity index measure (MS-SSIM), validate the superior smoothness and fidelity achieved with our variational framework approach compared with conventional THz CT modal. Beyond its contributions to advancing THz SR 3D reconstruction, BLIss demonstrates potential applicability in other imaging modalities, such as X-ray and MRI. This suggests extensive impacts on the broader field of imaging applications.
... More IC packaging features are need to be examined include the silicon die, lead frame within epoxy polymer, and the interposer, which are integral to modern advanced IC packaging. Therefore, non-destructive physical inspection using X-ray, 17 SAM (Scanning Acoustic Microscopy), THz-TDS, [18][19][20][21][22] have been used to analyze ICs. These methods have also been used to validate the authenticity of semiconductors devices for counterfeit detection purpose. ...
... Terahertz (THz) band, spanning a frequency range of 0.1-10 THz, has a wide range of applications in safety detection [1][2][3], sensors [4,5], communication [6,7], and various other fields. Metamaterials, artificially designed and manufactured materials, possess exceptional capabilities for manipulating electromagnetic waves [8][9][10]. ...
An absorber composed of stacked graphene rings, vanadium dioxide (VO 2 ) patches and metal reflectors separated by dielectric layers is proposed and numerically simulated using the finite element method (FEM). This absorber exhibits a multitude of absorption modes, including ultra-wideband, double-narrow band, and switchable multi-band perfect absorption. The physical mechanism behind perfect absorption is thoroughly investigated through the utilization of impedance matching theory, multiple interference theory (MIT), and coupled mode theory (CMT). Detailed analysis of the electric field further reveals the underlying physical
phenomena. Notably, the absorber showcases the ability to dynamically adjust absorption performance by modifying the conductivity of (VO 2 ) and the Fermi level of graphene, while maintaining consistently high absorption levels. The proposed multimode absorber maintains good performance within wide incidence angle. Based on the above advantages, this absorber holds promising potential for advancements in energy harvesting and sensor technologies.
... The experimentally obtained PSF increases the uncertainty of this parameter owing to the influence of the pinhole diameter and noise and cannot attenuate the terahertz beam in the object. Ahi et al. [19] mathematically modeled the PSF of a THz timedomain spectroscopy (THz-TDS) system and used the results to simulate and improve the resolution of a transmission THz imaging system. Using this concept, Zhang et al. [20] used a wavelet-based approach to denoise the data to obtain a THz image of a packaged integrated circuit. ...
Terahertz (THz) imaging has broad application prospects in nondestructive testing (NDT). However, compared to visible light, it has a low resolution and limited depth of field owing to its long wavelength. To address these limitations, a three-dimensional point-spread function based on a simulated THz time-domain spectroscopy system combined with the Lucy-Richardson method is proposed for image defocus restoration. The experimental results show that the contrast of THz images at different defocus positions is 2-5 times higher than that of the original image after restoration using the proposed method, and the resolution is improved by approximately 60-80%. Moreover, the maximum contrast enhancement at the boundaries of tiny features can reach 1.81 times. For a hole-defect sample, the relative error of the defect area at different depths is <5%. The proposed method solves the problem of defocus in THz images, enhances the resolution of these images, and facilitates NDT.
... Nevertheless, THz imaging suffers from low spatial resolution as compared to imaging in the UV and visible spectral ranges due to its comparatively large wavelength (0.3-3 mm (0.1 − 10THz)) and the fundamental diffraction limit. Different techniques were proposed to improve the resolution like near-field scanning [8], aspherical lenses [9], solid immersion [3,10], THz microjets with dielectric spherical particles [11], neural networks [12], structured illumination [13] and, also, the use of different methods of numerical analysis [14][15][16][17]. Very recently, a wide field of view aspheric lens with a diameter of 264 mm has been used in a THz imaging system at 0.1 THz allowing rapid imaging with extended depth-of-field (DOF) to 85 mm [18]. ...
We report on resolution enhancement of sub-terahertz (THz) images by using the terajet effect. A mesoscale cuboid dielectric particle, used to establish the terajet, was placed in front of an object located at the focus of the THz beam. The object under study was based on a printed circuit board (PCB) perforated with different holes with diameters ranging from 1.8 to 3.0 mm and separated from each other by a distance that varies from 0.25 to 4 mm. The sample was illuminated by a continuous wave source at a frequency of 0.3 THz and the image was obtained using a sensor based on a strained-Si Field-Effect Transistor. The image was formed pixel-by-pixel in a transmission mode configuration. A clearer image with enhanced resolution was obtained when the mesoscale cube was introduced in the optical path. The terajet effect made possible to resolve a separation between holes of around 0.5 mm (lower than the wavelength, 1 mm), that is, below the diffraction limit. The method described is easy to implement, cost effective and could be used to improve the resolution of any real imaging system.
... Compact THz sources with up to milliwatts of emitted power [9][10][11] and sensitive semiconductor-based receivers, offering systems with 120 − 140 dB peak dynamic range [12], have facilitated development of modular and portable or breadboard-sized THz systems [13], e.g., photonic spectrum analysers [14,15], and photonic vector network analysers (VNAs) [16,17]. Applications pertaining to material characterisation [18,19] and thereby, imaging [7,20,21] using THz radiation has gained pace due to its industrial applicability where it offers solutions that established X-ray or ultrasound solutions face challenges. Examples of such challenges are low density materials that offer little X-ray contrast or layer stacks thereof, or materials or assemblies with air inclusions where ultra-sound can hardly be used [22][23][24]. ...
Material characterisation and imaging applications using terahertz radiation have gained interest in the past few years due to their enormous potential for industrial applications. The availability of fast terahertz spectrometers or multi-pixel terahertz cameras has accelerated research in this domain. In this work, we present a novel vector-based implementation of the gradient descent algorithm to fit the measured transmission and reflection coefficients of multilayered objects to a scattering parameter-based model, without requiring any analytical formulation of the error function. We thereby extract thicknesses and refractive indices of the layers within a maximum 2% error margin. Using the precise thickness estimates, we further image a 50 nm-thick Siemens star deposited on a silicon substrate using wavelengths larger than 300 µm. The vector-based algorithm heuristically finds the error minimum where the optimisation problem cannot be analytically formulated, which can be utilised also for applications outside the terahertz domain.
... Techniques for using phase characteristics, broadband transmitters and receivers, or heterodyne configurations have been proposed to increase the image resolution and quality; however, there is a limitation in obtaining a large-scale image, because the resolution is only enhanced in a small area, the location of the target is fixed, or the complexity of the implementation is increased [17][18][19]. Techniques for increasing the image resolution of terahertz spectroscopy have been proposed using the correlation from adjacent pixels in the detector array; however, these are not suitable for sub-terahertz real-time imaging, because a complex mathematical process is required for the implementation [20][21][22]. In the resolution enhancement technique using the correlation of adjacent pixels, an image error can significantly increase when there is a malfunction or bad pixel in the sub-terahertz detector array. ...
A complementary metal-oxide-semiconductor (CMOS) detector array is proposed to improve the sub-terahertz imaging resolution for objects in the conveyor belt system. The image resolution is limited to the implemented configuration, such as the wide spacing in the detector array, the high conveyor belt speed, and the slow response of the signal conditioning block. The proposed array can improve the image resolution in the direction perpendicular to the movement of the belt, which is determined by the size and interval of the detector pixel, by configuring the array into two replaceable columns located at the misaligned horizontal positions. Replaceable detector unit pixels are individually attached to the motherboard after measuring and evaluating the detection performance to construct the proposed array. The intensities of 32 detector pixels placed under the con-veyor belt with a width of 160 mm were initially calibrated in every image, including the beam pattern of 0.2 THz signals generated from the gyrotron. The image resolution of the perpendicular direction obtained from the proposed array was measured to be approximately 5 mm at a conveyor belt speed of 16 mm/s, demonstrating a 200% improvement in resolution compared to the conventional linear array under the same conditions.
... Therefore, THz image denoising has an important value in practical applications. Various conventional algorithms have been applied to THz image enhancement such as adaptive filtering 20-22 and deconvolution methods [23][24][25] . Adaptive filtering filters out high-frequency noise while preserving the sharpness of edges. ...
Historical documents contain essential information about the past, including places, people, or events. Many of these valuable cultural artifacts cannot be further examined due to aging or external influences, as they are too fragile to be opened or turned over, so their rich contents remain hidden. Terahertz (THz) imaging is a nondestructive 3D imaging technique that can be used to reveal the hidden contents without damaging the documents. As noise or imaging artifacts are predominantly present in reconstructed images processed by standard THz reconstruction algorithms, this work intends to improve THz image quality with deep learning. To overcome the data scarcity problem in training a supervised deep learning model, an unsupervised deep learning network (CycleGAN) is first applied to generate paired noisy THz images from clean images (clean images are generated by a handwriting generator). With such synthetic noisy-to-clean paired images, a supervised deep learning model using Pix2pixGAN is trained, which is effective to enhance real noisy THz images. After Pix2pixGAN denoising, 99% characters written on one-side of the Xuan paper can be clearly recognized, while 61% characters written on one-side of the standard paper are sufficiently recognized. The average perceptual indices of Pix2pixGAN processed images are 16.83, which is very close to the average perceptual index 16.19 of clean handwriting images. Our work has important value for THz-imaging-based nondestructive historical document analysis.
... Although pulsed THz-TDS shows promising results for the inspection of artworks, its raster-scanning working mode still remains timeconsuming, since the technique can only provide point-to-point physical signal emitting and receiving [11]. In our opinion, it is attractive to apply continuous-wave (CW) THz inspection system which is speedy and low-cost, thanks to the recent breakthroughs in CW THz lined imaging [12]. ...
The investigation of cultural heritage objects is commonly carried out by using non-destructive inspection techniques. In fact, due to the artistic peculiarities of cultural heritage objects, conventional contact-type inspection techniques such as ultrasonic scan cannot always be applied. The objective of this study is to develop a new autonomous dynamic line-scan continuous-wave terahertz (CW THz) non-destructive inspection system combined with long-wave infrared (LWIR) thermography. The newly developed system is aimed at producing clear external and internal maps for wooden objects inherent to the cultural patrimony. Additionally, a new unsupervised fusion algorithm is also proposed for multi-energy density data fusion to correct the inaccurate image acquisition caused by unbalanced line-scan exposure source. The algorithm is designed in an encoder-decoder deep learning structure with dense blocks. Finally, it is worth mentioning that the newly developed system has the capability of speedy detection (up to 49.2 mm/s) and fast in-line processing (of the order of seconds).
... Recently, terahertz (THz) technology has attracted much attention in the fields of communication [1], imaging [2], detection [3], and wireless communications [4,5], providing broad application prospects. The limitation of natural materials hindered the fast development of THz functional devices [6][7][8]. ...
In this paper, a hybrid vanadium dioxide ( ${{\rm VO}_2}$ V O 2 )-graphene-based bifunctional metamaterial is proposed. The realization of the different functions of perfect transmission and high absorption is based on the insulator-metal phase transition of ${{\rm VO}_2}$ V O 2 material. The Fermi energy level of graphene can be treated to dynamically tune the absorption and transmission rates of the metamaterial structure. As a result, when ${{\rm VO}_2}$ V O 2 is in the insulating state, the designed metamaterial can be used as a filter providing three adjustable passbands with center frequencies of 1.892 THz, 1.124 THz, and 0.94 THz, and the corresponding transmittances reach 93.11%, 98.62%, and 90.01%, respectively. The filter also shows good stopband characteristics and exhibits good sensing performance at the resonant frequencies of 1.992 THz and 2.276 THz. When ${{\rm VO}_2}$ V O 2 is in metal state, the metamaterial structure acts as a double-band absorber, with three absorption peaks ( ${\gt}{90}\%$ > 90 % ) in the range of 0.684 THz to 0.924 THz, 2.86 THz to 3.04 THz, and 3.28 THz to 3.372 THz, respectively. The designed structure is insensitive to the polarization of vertically incident terahertz waves and still maintains good absorption performances over a large range of incidence angles. Finally, the effects of geometric parameters on the absorption and transmission properties of the hybrid bifunctional metamaterials have also been discussed. The switchable metamaterial structures proposed in this paper provide great potential in terahertz application fields, such as filtering, smart sensing, switching, tunable absorbers, and so on.
... Therefore, the imaging results can be easily optimized according to individual application requirements without increasing the imaging system cost, and improvements can be implemented at any time [8]. These efforts have led to the development of super-resolution image reconstruction methods that seek to Research Article reconstruct very high-resolution (HR) images accurately based on the details available within the original low-resolution (LR) images [9,10]. The significant benefits of this approach have generated considerable interest in the development of various super-resolution algorithms. ...
To date, the existing terahertz super-resolution reconstruction methods based on deep-learning networks have achieved noteworthy success. However, the terahertz image degradation process needs to fully consider the blur and noise of the high-frequency part of the image during the network training process, and cannot be replaced simply by interpolation, which has high complexity. The terahertz degradation model is systematically investigated, and effectively solves the above problems by introducing the remaining channel mechanism into the deep-learning network. On the one hand, an image degradation model suitable for the terahertz imaging process is adopted for the images in the training dataset, which improves the accuracy of network training. On the other hand, the residual channel attention mechanism is introduced to realize the adaptive adjustment of the dependence between network channels, which results in the network being more focused on the restoration of high-frequency information, thereby supporting the extraction of high-frequency edge details in the image. In addition, experimental results demonstrate that this method successfully improves the peak signal-to-noise ratios, and offers clearer edge details and a better overall reconstruction effect. We believe that this work may provide a new possibility to improve the resolution of terahertz images.
... Ces mesures permettent ainsi d'identifier différentes substances par leur signature spectrale [5,6]. L'identification de substances par spectroscopie THz est Ahi (2020) [4] avec permission © Kiarash Ahi. particulièrement intéressante pour les applications dans le domaine agroalimentaire. ...
La gamme THz du spectre électromagnétique est un domaine historiquement difficile d'accès. Durant les trois dernières décennies de nombreux progrès techniques ont été réalisés dans la fabrication de sources et de détecteurs de rayonnement THz. Ces avancées ont permis de découvrir tout le potentiel applicatif des fréquences THz mais on manque encore de sources compactes et performantes. Cette thèse est consacrée à l'étude de la génération d'ondes THz par deux nanostructures lamellaires.La première structure étudiée est une microcavité réalisée à l'aide de couches nanométriques d'alliage d'AlGaAs. Nous montrons que la formation de niveaux polaritonique par couplage fort entre des puits quantiques asymétriques et une microcavité double permet de concevoir une transition interpolaritons optimisée dans le domaine THz. Nous détaillons alors la caractérisation expérimentale d'un échantillon et nous montrons la présence d'une émission THz d'origine thermique sans lien avec les transitions interpolaritons et nous présentons des pistes pour l'amélioration des échantillons.La seconde structure étudiée est le PtSe2 en couches minces, un nouveau matériau 2D appartenant à la famille des TMD. Nous caractérisons expérimentalement la transition entre semi-conducteur et semi-métal avec l'augmentation d'épaisseur des échantillons à l'aide de mesure du gap optique et de conductivité. Nous montrons la présence d'une émission THz issue d'un processus non linéaire du second ordre. Finalement nous montrons en développant un formalisme adapté que cette émission est due principalement aux effets photogalvanique et d'entrainement par les photons linéaires.
... The ACI offers new opportunities for not only conventional imaging systems and superlenses, but also various other linear systems. One can envision its potential generalization and ubiquity to encompass imaging through random media (e.g.., turbulent and scattering atmosphere) [67][68][69][70][71], spectroscopy [72,73], PT -symmetry [33,74,75], photolithography [12,13], and quantum information and image processing [76][77][78][79]. The virtual gain mechanism employed in ACI can alleviate the stringent requirement of balance between loss and gain in PT -symmetric systems [33]. ...
Imaging is indispensable for nearly every field of science, engineering, technology, and medicine. However, measurement noise and stochastic distortions pose fundamental limits to accessible spatiotemporal information despite impressive tools such as SIM, PALM/STORM, and STED microscopy. How to combat this challenge ideally has been an open question for decades. Inspired by a "virtual gain" technique to compensate losses in metamaterials, "active convolved illumination" has been recently proposed to significantly improve the signal-to-noise ratio, hence data acquisition. In this technique, the light pattern of the object is superimposed with a correlated auxiliary pattern, the function of which is to reverse the adverse effect of noise and random distortion based on their spectral characteristics. Despite enormous implications in statistics, an experimental realization of this novel technique has been lacking to date. Here, we present the first experimental demonstration. We find that the active convolved illumination does not only boost the resolution limit and image contrast, but also the resistance to pixel saturation. The results confirm the previous theories and opens up new horizons in a wide range of disciplines from atmospheric sciences, seismology, biology, statistical learning, and information processing to quantum noise beyond the fundamental boundaries.
... Future works will be focused on investigating other configurations and geometries, assessing the sensitivity of detecting the LP technique, as well as using additional nondestructive testing techniques for comparative purposes [45,46]. Author Contributions: Conceptualization, S.P., U.G., S.S. and D.P.; methodology, S.P., U.G., S.S. and D.P.; software, S.P., U.G., S.S. and D.P.; validation, S.P., U.G., S.S. and D.P.; formal analysis, S.P. and D.P.; investigation, S.P., U.G., S.S. and D.P.; resources, S.P., U.G., S.S. and D.P.; data curation, S.P., U.G., S.S. and D.P.; writing-original draft preparation, S.P. and D.P.; writing-review and editing, S.P., U.G., S.S. and D.P.; visualization, S.P., U.G., S.S. and D.P.; supervision, U.G. and S.S.; project administration, S.P., U.G., S.S. and D.P.; funding acquisition, U.G. and S.S.. ...
In building construction, it is very important to reduce energy consumption and provide thermal comfort. In this regard, defects in insulating panels can compromise the capability of these panels of reducing the heat flow by conduction with the surroundings. In recent years, both experimental techniques and numerical methods have been used for investigating the effect of defects on the thermal behavior of building panels. The main novelty of this work regards the application of both numerical and experimental approaches based on infrared thermography techniques for studying the effects of defects such as debonding on the insulation properties of cork panels. In particular, the effects of defects were investigated by using the Long Pulse Thermography technique and then by analyzing the thermal behavior of the panel during the cooling phase. Results show the capability of the proposed approaches in describing the effects of defects in cork panels such as detachments and the benefit effect of a shield coating in improving the insulation properties of the panel.
... The blur kernel used in the deconvolution calculation-i.e., the optical system's point spread function (PSF)-is a key factor in determining the image reconstruction's quality. If the PSF is not obtained accurately, the reconstructed images are most likely to have severe artifacts and ringing effects [3,4], affecting the quality of the image reconstruction. ...
The image deconvolution technique can recover potential sharp images from blurred images affected by aberrations. Obtaining the point spread function (PSF) of the imaging system accurately is a prerequisite for robust deconvolution. In this paper, a computational imaging method based on wavefront coding is proposed to reconstruct the wavefront aberration of a photographic system. Firstly, a group of images affected by local aberration is obtained by applying wavefront coding on the optical system’s spectral plane. Then, the PSF is recovered accurately by pupil function synthesis, and finally, the aberration-affected images are recovered by image deconvolution. After aberration correction, the image’s coefficient of variation and mean relative deviation are improved by 60% and 30%, respectively, and the image can reach the limit of resolution of the sensor, as proved by the resolution test board. Meanwhile, the method’s robust anti-noise capability is confirmed through simulation experiments. Through the conversion of the complexity of optical design to a post-processing algorithm, this method offers an economical and efficient strategy for obtaining high-resolution and high-quality images using a simple large-field lens.
... It is because THz wave possesses insignificant skin depth and photonic energy, which can penetrate non-metallic materials without ionizing and is harmless for organic tissue. Therefore, THz optics earns many potential applications, such as security scanning [1], medical diagnostic, THz imaging [2][3][4] and biological sensing [5][6][7]. The limitation of THz optics is that most natural materials do not have usable electronic response in the THz frequency range and the flexibility of metamaterial design provides an effective approach to deal with it [8]. ...
We present two types of tunable terahertz metamaterial (TTM) resonator. They are denoted as TTM-1 and TTM-2 for TTM resonators composed of a moveable metal frame carrying with inner disc and inner ring on Si substrates, respectively. The electromagnetic responses of TTM resonators are caused by the inductance-capacitance coupling effect between the incident THz wave into TTM structure. There is a Fano-resonance with a reflection intensity of 99% excited by electromagnetic wave within TTM-1 at 2.62 THz. By actively moving the metal frame in a specific direction, it enables a modulation of resonant frequency. The tuning ranges of TTM-1 and TTM-2 are 0.19 THz and 0.79 THz, respectively. Furthermore, TTM devices have the ability of anti-inference by moving metal frame transversely. It means TTM devices exhibit stable characteristic used in dynamic motion. Such designs of TTM configurations provide a high-efficiency THz resonator in the THz-waves applications such as filtering, switching, polarizing, sensing, and imaging fields.
... Esta provee más información que la espectroscopía por transformada de Fourier, que es solo sensible a la amplitud. Debido a que se conoce el tiempo-dominio, la frecuencia-dominio de la señal de THz está disponible, así el efecto de distorsión de la difracción se puede mitigar y la resolución de las imágenes de THz puede ser mejorada (Ahi 2018). ...
El quitosano es uno de los polímeros de origen natural más interesantes de las últimas décadas. Es la forma desacetilada de la quitina, el segundo polisacárido natural más abundante en el planeta. Su actividad antimicrobiana, además de su naturaleza biodegradable, biocompatible y no tóxica le ha conferido un extenso número de aplicaciones en distintas áreas, incluida la agricultura. La versatilidad de su estructura, así como sus propiedades de sorción de macromoléculas y quelación de partículas metálicas, gracias a su carácter catiónico derivado del grupo amino, le otorgan un mayor interés y la facilidad de generar nuevos materiales que potencien su actividad. No obstante, las extensas aportaciones de la investigación sobre los productos derivados de quitosano, en sus formas, síntesis, aplicaciones, incluso a nivel teórico en la descripción de su comportamiento químico y de los mecanismos de acción que ejercen, no cesan, por el contrario su desarrollo continúa en auge. En la presente tesis doctoral se realiza en primer lugar una revisión del estado del arte de la síntesis de nuevos compuestos basados en quitosano, que incluyen desde extractos naturales hasta nanopartículas metálicas, mediante la utilización de nuevas tecnologías. Se revisan también sus aplicaciones como antimicrobiano para diversos materiales que deban mantenerse y conservarse de microorganismos y patógenos. Posteriormente, se realiza por primera vez un análisis molecular de la N-acetil-D-glucosamina (la molécula constituyente de la quitina y el quitosano) por combinación de técnicas de espectroscopía de THz-TDS y FTIR con estudios teóricos basados en métodos semiempíricos de química cuántica.
... However, the image resolution is limited by the size of the lens, thus the radiation pattern of the THz beam should be taken into account as well. The authors of [5,6] used a de-convolution method to improve the resolution in THz imaging. Since the pulsed THz-TDS system can record both amplitude and phase of THz signal so that many parameters can be extracted and thus many different images can be reconstructed by choosing different parameters. ...
A terahertz (THz) imaging enhancement approach is presented by using multi‐features of images. First, a set of THz images are reconstructed from a single data collection using THz time‐domain spectroscopy system by selecting different features. Second, the THz beam model is considered to improve image resolution, which is realised via a Wiener de‐convolution operation. Third, an approach to suppress the noise and enhance image feature is adopted to achieve a better image quality. Finally, three high‐quality images are selected from the new image set as red, green, and blue channels, respectively, to form a pseudo‐colour image. The experimental results suggest that the pseudo‐colour images show multiple features of the sample and achieve a better image quality.
... The second challenge for evaluating the PT technique involves the effective decision-making criteria for judging the detectability of mould compound material and die structure from PT, which is still unclear. The use of a machine learning classification algorithm to characterise the damage has been attracting increasing attention in maintenance industries [28]; studies that have been conducted to automatically determine the defects in material [29] and deterioration in infrastructure [30] where traditional NDE techniques such as ultrasonic testing, radar, acoustic emission, infrared thermography and Terahertz spectroscopy have been used [31,32]. In material identification, Aujeszky et al. ...
Counterfeit Electronic Components (CECs) pose a serious threat to all intellectual properties and bring fatal failure to the key industrial systems. This paper initiates the exploration of the prospect of CEC detection using pulsed thermography (PT) by proposing a detectability evaluation method for material and structural anomalies in CECs. Firstly, a numerical Finite Element Modelling (FEM) simulation approach of CEC detection using PT was established to predict the thermal response of electronic components under the heat excitation. Then, by experimental validation, FEM simulates multiple models with attribute deviations in mould compound conductivity, mould compound volumetric heat capacity and die size respectively considering experimental noise. Secondly, based on principal components analysis (PCA), the gradients of the 1 st and 2 nd principal components are extracted and identified as two promising classification features of distinguishing the deviation models. Thirdly, a supervised machine learning-based method was applied to classify the features to identify the range of detectability. By defining the 90% of classification accuracy as the detectable threshold, the detectability ranges of deviation in three attributes have been quantitively evaluated respectively. The promising results suggest that PT can act as a concise, operable and cost-efficient tool for CECs screening which has the potential to be embedded in the initial large scale screening stage for anti-counterfeit.
... Jepsen proved that the beam is approximated by Gaussian illumination distribution [26]. As Kiarash reported, the two-dimensional THz beam scan has been mathematically modeled by the convolution of the PSF and the object function [3,27,28]. In order to mathematically model the PSF, the system spectrum and transmission parameters are combined into the Gaussian beam theory, such as correlated spectrum intensity I(x, y, z), the amplitude extinction coefficient α(z, f ) of the object, the beam divergence ρ and the depth of focus z. ...
High-quality terahertz (THz) images are vital to integrated circuit (IC) manufacturing. Due to the unique sensitivity of THz waves to different materials, the images obtained from the point-spread function (PSF) model have fewer image details and less texture information in some frequency bands. This paper presents an image fusion technique to enhance the resolution of THz IC images. The source images obtained from the PSF model are processed by a fusion method combining a multiscale transform (MST) and sparse representation (SR). The low-pass band is handled by sparse representation, and the high-pass band is fused by the conventional “max-absolute” rule. From both objective and visual perspectives, four popular multiscale transforms—the Laplacian pyramid, the ratio of low-pass pyramids, the dual-tree complex wavelet transform and the curvelet transform—are thoroughly compared at different decomposition levels ranging from one to four. This work demonstrates the feasibility of using image fusion to enhance the resolution of THz IC images.
... Due to its longer wavelength, images made using THz waves have lower resolution than X-rays and need to be enhanced. 1 The THz band is a virgin between occupied other sides. So numerous applications could be allocated to the band. ...
Utilizing a double bias scheme for graphene patterns as a single layer, a THz meta‐surface structure is proposed. The structure includes two dual bias layers and a graphene continuous sheet on top. So five possible biases are available which lead to a highly tunable absorption response. All consisting parts are modeled via passive circuit elements to obtain whole device input impedance. Then impedance matching theory is used to investigate potential frequencies for perfect absorption. Also, ample simulations are performed to show the validity and accuracy of theoretical descriptions. According to the simulation results in the conventional finite element method as a reference approach is in acceptable agreement with the developed circuit model approach. The proposed structure shows nearly perfect absorption in 5, 6, 7, and 8 THz with absorption rate over 90%. Such a tunable reaction is in incredible requests in optical systems and has various applications in structuring sensors, modulators, and photonic functions.
... Terahertz (THz) imaging and spectroscopy are among the promising NDT techniques as well [26][27][28]. Unfortunately, THz equipment is not available in our respective laboratories. ...
The relationship between wood and its degree of humidity is one of the most important aspects of its use in construction and restoration. The wood presents a behavior similar to a sponge, therefore, moisture is related to its expansion and contraction. The nondestructive evaluation (NDE) of the amount of moisture in wood materials allows to define, e.g., the restoration procedures of buildings or artworks. In this work, an integrated study of two non-contact techniques is presented. Infrared thermography (IRT) was able to retrieve thermal parameters of the wood related to the amount of water added to the samples, while the interference pattern generated by speckles was used to quantify the expansion and contraction of wood that can be related to the amount of water. In twenty-seven wooded samples, a known quantity of water was added in a controlled manner. By applying advanced image processing to thermograms and specklegrams, it was possible to determine fundamental values controlling both the absorption of water and the main thermophysical parameters that link the samples. On the one hand, results here shown should be considered preliminary because the experimental values obtained by IRT need to be optimized for low water contents introduced into the samples. On the other hand, speckle interferometry by applying an innovative procedure provided robust results for both high and low water contents.
... Based on THz TDS, the phase-imaging approach was further developed by several groups of researchers [1][2][3][4][5][6][7][8][9][10]. THz pulsed time-domain holography (THz PTDH) described in [1] received its further development in different aspects: holography in dispersive objects [11], biomedical applications [12][13][14], wavefront sensing [15,16]. ...
We investigated the peculiarities of the terahertz pulse time-domain holography principle in the case of raster scanning with the balance detection system. The noise in this system represents a Skellam distribution model, which differentiates it from systems based on a photoconductive antenna. We analyzed this Skellam model and provided both numerical and experimental investigations. We found that the variance of the noise in the balance detection system does not depend on the true signal. Complex-domain images obtained in this model are filtered by block-matching algorithms adapted for spatio-temporal and spatiospectral volumetric data. We presented a new cube complex-domain filter algorithm that uses block matching in all 3D data sets simultaneously in spatial and frequency coordinates. A combination of temporal and complex-domain filters allows us to expand the dynamic range of terahertz frequencies for which we can obtain amplitude/phase information. Experimental data demonstrate an improvement in the quality of the resultant images both in the time domain and complex-spectral domain. The simulation and experimental results are in good agreement.
In the present manuscript, we have presented a novel theoretical model to study the monochromatic terahertz (THz) generation by beating two spatial-Gaussian (SG) laser beams in the array of anharmonic upright carbon nanotubes (CNTs) present in the form of weakly coupled plasma under the influence of static magnetic and electric fields, well supported by numerical simulation and physical interpretation. We have chosen SG laser beams because these have steep gradients in their intensity, which results in stronger ponderomotive force. The nonlinear restoration force along with the strong ponderomotive, magnetic, and electric forces with surface plasmon resonance (SPR) condition \(\left( {\omega /\omega_{{\text{P}}} = \sqrt {0.5\left( {1 + i\upsilon /\omega } \right)\rho /\varepsilon_{{\text{g}}} + (\omega_{{\text{c}}}^{2} /\omega_{{\text{P}}}^{2} ) - \left( {\upsilon^{2} /\omega_{{\text{P}}}^{2} } \right) + 2i\upsilon \omega /\omega_{{\text{P}}}^{2} } } \right)\) facilitates the enhancement of nonlinear current. To evaluate the SPR condition, we have assumed that all the electrons of upright CNTs in the array are oscillating in the same phase. As a result, monochromatic THz waves are expected to be emitted by the system. Most of the THz sensors and instruments are employed in the frequency range of 0.1–10 THz to use for the nondestructive evaluation technique (NDET) with the purpose of detecting structural defects in ceramics and imaging the physical structure of old heritage paintings and manuscripts. In the proposed scheme of monochromatic THz generation, we can obtain the THz radiations in a specific frequency range of 3.15–3.25 THz and hence can play a key role in NDET. The emitted monochromatic THz radiations can also be used to perform THz pump/probe experiments in biology and nanotechnology, where output THz radiation is sensitive to the phase. In addition to that, monochromatic THz radiations with symmetrical profiles have immense potential in communication services. According to the International Telecommunication Union, the monochromaticity of THz radiations plays a significant role in communication services because of higher band utilization and lower interference.
Fourier imaging is an indirect imaging method which records the diffraction pattern of the object scene coherently in the focal plane of the imaging system and reconstructs the image using computational resources. The spatial resolution, which can be reached, depends on one hand on the wavelength of the radiation, but also on the capability to measure – in the focal plane – Fourier components with high spatial wave-vectors. This leads to a conflicting situation at THz frequencies, because choosing a shorter wavelength for better resolution usually comes at the cost of less radiation power, concomitant with a loss of dynamic range, which limits the detection of higher Fourier components. Here, aiming at maintaining a high dynamic range and limiting the system costs, we adopt heterodyne detection at the 2nd sub-harmonic, working with continuous-wave (CW) radiation for object illumination at 600 GHz and local-oscillator (LO) radiation at 300 GHz. The detector is a single-pixel broad-band Si CMOS TeraFET equipped with substrate lenses on both the front- and backside for separate in-coupling of the waves. The entire scene is illuminated by the object wave, and the Fourier spectrum is recorded by raster scanning of the single-detector unit through the focal plane. With only 56 µW of power of the 600-GHz radiation, a dynamic range of 60 dB is reached, sufficient to detect the entire accessible Fourier space spectrum in the test measurements. We present a detailed comparison between plane-to-plane imaging and Fourier imaging, and show that, with both, a lateral spatial resolution of better than 0.5 mm, at the diffraction limit, is reached.
The terahertz (THz) radiation refers to electromagnetic waves between infrared and millimeter waves. THz technology has shown a significant potential for medical diagnosis and biomedical applications over the past three decades. Therefore, exploring the biological effects of THz waves has become an important new field in life sciences. Specifically, THz radiation has been proved to be able to diagnose and treat several head and neck diseases. In this review, we primarily discuss the biological characteristics of THz waves and clinical applications of THz technology, focusing on the research progress of THz technology in head and neck diseases (brain cancer, hypopharyngeal cancer, oral diseases, thyroid nodules, Alzheimer’s disease, eyes diseases, and otitis). The future application perspectives of THz technologies in head and neck diseases are also highlighted and proposed.
To investigate the effectiveness of selective tree improvement work, an accurate and calibration-free framework based on THz time-domain sensing for intrinsic wood quality traits assessment was specifically designed for tree ring core sample analysis. This study contains a global phase unwrapping strategy, effective medium theory (EMT)-based measurement, combined measurement, and resolution enhancement, all of which aim to solve and discuss several challenges in THz time-domain sensing. Namely, the global phase unwrapping strategy eliminates the global jumps in phase difference by analysing the principle of jump generation; EMT-based THz measurement can assess properties of hundreds of consecutive sampling points on wood cores without model training; the combined measurement adjusts relative permittivity and dielectric loss relative to one other in a coordinate system; the resolution enhancement converts 3-D PSF into 1-D PSF, and extracts attenuation to achieve resolution enhancement of target properties. The results show that the global phase unwrapping strategy has the potential to deal with phase differences of both homogeneous and heterogeneous samples and can be considered a general unwrapping method. All R2 obtained by combined measurement are higher than that obtained by separate relative permittivity measurement. The resolution enhancement can achieve more accurate and faster measurement. The framework presented here offers significant potential in promoting the application of THz in wood quality traits assessment by providing a low-cost and high precision analysis system.
Terahertz imaging is the science of imaging that uses terahertz radiation. Electromagnetic radiation between the frequency of 0.1 and 10 THz is called terahertz radiation. Terahertz imaging has been around for approximately 30 years now, giving us an appropriate moment to review what has been achieved so far in this field and what new prospects lie ahead. This chapter will emphasize the fields that should be using terahertz imaging and will focus upon more new fields that should embrace this technology. Also called T-ray imaging, terahertz imaging has several applications like non-invasive medical imaging, spectrometers, and optics. Beneficial for 3D imaging, it can be used for imaging on a cellular level of biological tissues and submicron semiconductor devices. Researchers have used terahertz imaging on nanoresolution levels. The industry is shifting to use terahertz radiations for real-time imaging. Commercial terahertz imaging applications extend to weld joint inspection, food product examination, and investigation of birefringence. This chapter will also analyze the challenges and opportunities of producing commercially viable equipment using terahertz imaging technology. This chapter will introduce terahertz technology, its importance and relevance, and some successful terahertz imaging applications. Then a review of the timeline of events in the terahertz imaging history will show how much the technology has progressed so far and what more lies ahead. An analysis of how terahertz competes with the existing technology in several fields of applications will be shown. Then a detailed explanation of the terahertz imaging technology will be presented so that the researchers can understand what goes beyond the picture of this imaging technology. Further, a few application ideas will be presented where this technology can be used both from an academic and industrial point of view. Finally, the chapter will conclude with a discussion of challenges for this technology with some insights on how they can be handled.KeywordsTerahertzImagingElectromagneticT-ray imagingSpectroscopy
The low image acquisition speed of terahertz (THz) time-domain imaging systems limits their application in biological products analysis. In the current study, a local pixel graph neural network was built for THz time-domain imaging super-resolution. The method could be applied to the analysis of any heterogeneous biological products as it only required a small number of sample images for training and particularly it focused on THz feature frequencies. The graph network applied the Fourier transform to graphs extracted from low-resolution (LR) images bringing an invariance of rotation and flip for local pixels, and the network then learnt the relationship between the state of graphs and the corresponding pixels to be reconstructed. With wood cores and seeds as examples, the images of these samples were captured by a THz time-domain imaging system for training and analysed by the method, achieving the root mean square error (RMSE) of pixels of 0.0957 and 0.1061 for the wood core and seed images, respectively. In addition, the reconstructed high-resolution (HR) images, LR images and true HR images at several feature frequencies were also compared in the current study. Results indicated that the method could not only reconstruct the spatial details and the useful signals from high noise signals at high feature frequencies but could also operate super-resolution in both spatial and spectral aspects.
Terahertz (THz) waves are a promising candidate for detection, imaging, and advanced communications with ultrafast transmission speed. To develop THz technologies, miniaturized devices for controlling THz waves are essential. In this study, a high-performance vanadium dioxide (VO2)-based switchable band-pass metamaterial integrated with a silicon substrate for THz waves is designed and fabricated. In the “on” state, it offers a passband with a 70% depth and a 0.34 THz bandwidth at 0.56 THz frequency. Additionally, it can be switched thermally to a mirror for 0.1–2.0 THz waves in the “off” state by the phase transition effect of the VO2 film. Moreover, the relationship between the line width and spectra of the device is investigated by simulation. The wheel-shaped gold structure with round corners provides the filter with tractability during fabrication. Furthermore, the silicon substrate allows the metamaterial to be readily miniaturized and integrated into micro-electromechanical systems (MEMS) technology. This filter is expected to be favorable in frequency-selective THz applications.
This book comprises selected peer-reviewed proceedings of the International Conference on Advances in Industrial Automation and Smart Manufacturing (ICAIASM) 2019. The contents focus on innovative manufacturing processes, standards and technologies used to implement Industry 4.0, and industrial IoT based environment for smart manufacturing. The book particularly emphasizes on emerging industrial concepts like industrial IoT and cyber physical systems, advanced simulation and digital twin, wireless instrumentation, rapid prototyping and tooling, augmented reality, analytics and manufacturing operations management. Given the range of topics covered, this book will be useful for students, researchers as well as industry professionals.
Terahertz (THz) radiation lies between the millimeter and infrared region of the electromagnetic spectrum, which is typically defined as the frequency range of 0.1–10 THz and the corresponding wavelength ranges from 30 μm to 3 mm. Terahertz radiation due to wide spectrum, high penetration, low energy, and other important features, has been a valuable tool for imaging and non-destructive testing on a submillimeter scale. Continuous-wave (CW) terahertz ptychography is a type of phase-contrast technique with advantages of simple set-up and large field-of-view. It retrieves the complex-valued transmission function of the specimen and the probe function at the same time. The extended ptychographic iterative engine (ePIE) algorithm is used as the reconstruction algorithm in the field of ptychography, because it is relatively simple, and can use computer memory efficiently. However, the problem of algorithm convergence delay makes us unable to acquire the reconstruction result very quickly. Since the ptychography is a problem of retrieving phase information, physical constraints affect the convergence speed of the algorithm strongly. In this paper, we propose a dual-plane ePIE (dp-ePIE) algorithm for CW THz ptychography. By moving detector along the axis and capturing diffraction patterns of one zone of an object at two recording planes, then, two sets of patterns used as the constraints simultaneously can increase the diversity of experimental parameter. Hence, the convergence rate can be improved. The simulation results suggest better reconstruction fidelity with a faster convergence rate by the dp-ePIE algorithm. The dual-plane terahertz ptychography experimental setup is built based on 2.52 THz optically pumped laser and Pyrocam-III pyroelectric array detector. Compared with other methods to increase the diversity of measurement, the setup of dual-plane ptychography can be compact and simple, thus reducing the terahertz wave transmission loss. A polypropylene sample is adopted and it is approximated as a pure phase object. No-reference structural sharpness (NRSS) is utilized as a quantitative evaluation index. It takes 45.086 s to achieve NRSS value of 0.9831 by using the dp-ePIE algorithm in 10 iterations, while the NRSS value and calculation time for e-PIE algorithm are 0.9531 and 57.117 s (20 loops), respectively. The experimental results show that the dp-ePIE algorithm can obtain high-quality amplitude and phase distribution with less iterations than the traditional ePIE algorithm.
Aluminium can be used to make parts of aircrafts, and it is less in strength. The combination of aluminium with titanium carbide will increase the strength of the material. By varying the composition of titanium carbide mixture, surface properties of the developed material have been analysed. The characteristics and properties of composite material can be analysed by the non-destructive testing method without breaking the composite. The following tests can be performed to analyse the material are penetrant test and radiographic test. Here, Penetrant test is to identify the surface and radiography test is to identify the sub surface of the material; Different types of aluminium alloys are mixed with different composition of titanium carbide to increase the strength.
In recent trends, metal matrix composites (MMC) have attracted the newline potential interest in engineering materials due to their superior mechanical newline properties than the conventional materials. The way of selecting the particular newline properties of the constituent materials is to meet the specific demand. In general, MMCs possess newline increased strength, higher service temperature, decreased part weight and improved newline wear resistance, when compare with materials of monolithic. Aluminium and its alloys are the most widely used AMMC. Matrix reinforced with ceramic particles such as oxides and non-oxides (carbides, nitrides, borides) offers a huge variety of mechanical properties depending on the aluminium and its alloy chemical composition. Among the various ceramics, boron carbide (B4C) possesses excellent physical and newline mechanical properties like high melting point and hardness, good impact and wear resistance. In the present work, a combination of Al6061 + B4C metal matrix composites is fabricated with different proportions (1.5, 3, 4.5%) by stir casting technique. Their structural analysis was done by using SEM analysis; wear resistance was found out by using pin-on-disc method; and surface roughness was found out.
In recent years, terahertz (THz) imaging technology has attracted much attention in the detection of the integrated circuit (IC). However, limited by the hardware of the imaging system, THz images often contain a significant amount of noise, which impairs the quality of the image details. The THz image is also degraded due to the long wavelength. In this study, we propose a novel method for THz image restoration. We first apply a wavelet denoising technique to process the THz time-frequency signal. The point spread function (PSF) is then mathematically modeled to restore the details of the IC image, as the degradation of the THz image can be regarded as the convolution process of the object equation and PSF. Finally, we compare the performance between the restored THz images before and after wavelet denoising. The results demonstrate that the restored image after denoising performs better in peak signal-to-noise ratio and visual improvements, proving the practicability and precision of our proposed method.
A356 has its extensive applications in the fabrication of airplane wing parts, turbine blades for jet engine and other complex parts of the airplane which led to lightweight and hard material. TiB2 is a hard ceramic particle which has high hardness is reinforced in aluminium matrix to increase its mechanical properties. The material was fabricated through liquid metallurgy route by reinforcing different weight percentage of TiB2 (4 wt%, 8 wt% and 12 wt%). The mechanical properties and morphological studies are analysed. The result reveals that there is an increase in tensile strength, hardness and impact strength of the composite with the increase in reinforcement. SEM morphology shows the bonding between the reinforcing particle and the matrix alloy.
Terahertz (THz) radiation suffers severe signal loss due to the high absorptivity of water vapor abundant in normal atmosphere, greatly limiting its potential in noninvasive material characterization. We propose a novel THz signal processing method that enables effective extraction of hidden sample information leading to accurate thickness determination. The thicknesses of multiple silicon wafers are measured using only a single THz pulse obtained in ambient atmosphere without any prior sample information. The results verify our proposed approach to achieve accurate and precise characterization of materials in a realistic environment.
In this paper, terahertz time-domain spectroscopy (THz-TDS) is used for the first time to detect fabricated defects in a glass fiber-skinned lightweight honeycomb composite panel. A novel amplitude polynomial regression (APR) algorithm is proposed as a pre-processing method. This method segments the amplitude-frequency curves to simulate the heating and the cooling monotonic behavior as in infrared thermography. Then, the method of empirical orthogonal function (EOF) imaging is applied on the APR pre-processed data as a post-processing algorithm. Signal-to-noise ratio analysis is performed to verify the image improvement of the proposed APR-EOF modality from a quantitative point of view. Finally, the experimental results and the physical analysis show that THz is more suitable with respect to the detection of defects in glass fiber lightweight honeycomb composites.
The process by which art paintings are produced typically involves the successive applications of preparatory and paint layers to a canvas or other support; however, there is an absence of nondestructive modalities to provide a global mapping of the stratigraphy, information that is crucial for evaluation of its authenticity and attribution, for insights into historical or artist-specific techniques, as well as for conservation. We demonstrate sparsity-based terahertz reflectometry can be applied to extract a detailed 3D mapping of the layer structure of the 17th century easel painting Madonna in Preghiera by the workshop of Giovanni Battista Salvi da Sassoferrato, in which the structure of the canvas support, the ground, imprimatura, underpainting, pictorial, and varnish layers are identified quantitatively. In addition, a hitherto unidentified restoration of the varnish has been found. Our approach unlocks the full promise of terahertz reflectometry to provide a global and detailed account of an easel painting’s stratigraphy by exploiting the sparse deconvolution, without which terahertz reflectometry in the past has only provided a meager tool for the characterization of paintings with paint-layer thicknesses smaller than 50 μm. The proposed modality can also be employed across a broad range of applications in nondestructive testing and biomedical imaging.
This paper presents an investigation of terajets formation by dielectric periodic structure at terahertz frequencies (0.1 – 0.2 THz) in effective medium regime (photonic metamaterial). The dispersions of effective permittivity for periodic structures formed by plastics (ABS, black) were analytically obtained for both regimes. The study of the interaction of a plane wave and artificial cubic dielectric on a substrate parallelepiped to form a subwavelength terahertz beams (terajet) was performed. The results of numerical simulation of S-parameters of structure are discussed. The subwavelength beam waist of terajets about 0.39λ – 0.41λ were obtained. The quality factor of terajets are discussed. The results may be applied for creation of methods to obtain subwavelength beams in the terahertz frequency range, and devices based on them. These investigations are opening new research lines such as mesoscale focusing devices for different sensing applications.
GaN provides the highest electron saturation velocity, breakdown voltage, operation temperature, and thus the highest combined frequency-power performance among commonly used semiconductors. The industrial need for compact, economical, high-resolution, and high-power terahertz (THz) imaging and spectroscopy systems are promoting the utilization of GaN for implementing the next generation of THz systems. As it is reviewed, the mentioned characteristics of GaN together with its capabilities of providing high two-dimensional election densities and large longitudinal optical phonon of ∼90meV make it one of the most promising semiconductor materials for the future of the THz emitters, detectors, mixers, and frequency multiplicators. GaN-based devices have shown capabilities of operation in the upper THz frequency band of 5 to 12 THz with relatively high photon densities in room temperature. As a result, THz imaging and spectroscopy systems with high resolution and deep depth of penetration can be realized through utilizing GaN-based devices. A comprehensive review of the history and the state of the art of GaN-based electronic devices, including plasma heterostructure field-effect transistors, negative differential resistances, hetero-dimensional Schottky diodes, impact avalanche transit times, quantum-cascade lasers, high electron mobility transistors, Gunn diodes, and tera field-effect transistors together with their impact on the future of THz imaging and spectroscopy systems is provided. © 2017 Society of Photo-Optical Instrumentation Engineers (SPIE).
Polymer matrix composites (PMC) play important roles in modern industry. Increasing the number of such structures in aerospace, construction, and automotive applications enforces continuous monitoring of their condition. Nondestructive inspection of layered composite materials is much more complicated process than evaluation of homogenous, (mostly metallic) structures. Several nondestructive methods are utilized in this case (ultrasonics, shearography, tap testing, acoustic emission, digital radiography, infrared imaging) but none of them gives full description of evaluated structures. Thus, further development of NDT techniques should be studied. A pulsed terahertz method seems to be a good candidate for layered PMC inspection. It is based on picosecond electromagnetic pulses interacting with the evaluated structure. Differences of dielectric parameters enables detection of a particular layer in a layered material. In the case of multilayered structures, only layers close to surface can be detected. The response of deeper ones is averaged because of multiple reflections. In this paper a novel inspection procedure with a data processing algorithm is introduced. It is based on a double-sided measurement, acquired signal deconvolution, and data combining. In order to verify the application of the algorithm stress-subjected glass fiber-reinforced polymer (GFRP) was evaluated. The obtained results enabled detection and detailed analysis of delaminations introduced by stress treatment and proved the applicability of the proposed algorithm.
The terahertz (THz, 0.1--10 THz) region has been attracting tremendous research interest owing to its potential in practical applications such as biomedical, material inspection, and nondestructive imaging. Those applications require enhancing the spatial resolution at a specific frequency of interest. A variety of resolution-enhancement techniques have been proposed, such as near-field scanning probes, surface plasmons, and aspheric lenses. Here, we demonstrate for the first time that a mesoscale dielectric cube can be exploited as a novel resolution-enhancer by simply placing it at the focused imaging point of a continuous wave THz imaging system. The operating principle of this enhancer is based on the generation---by the dielectric cuboid---of the so-called terajet, a photonic jet in the THz region. A subwavelength hotspot is obtained by placing a Teflon cube (n=1.46) at the imaging point of the imaging system, regardless of the numerical aperture (NA). The generated terajet at 125 GHz is experimentally characterized, using our unique THz-wave visualization system. The full width at half maximum (FWHM) of the hotspot is approximately 0.55λ, which is even better than the FWHM obtained by a conventional focusing device with the ideal maximum NA=1 in air. Nondestructive subwavelength-resolution imaging demonstrations of an integrated circuit card, and an aluminum plate with 0.63λ trenches are presented. The amplitude and phase images obtained with the enhancer at 125 GHz can clearly resolve both the air-trenches on the aluminum plate and the card's inner electronic circuitry, whereas the images obtained without the enhancer are blurred because of insufficient resolution.
The terajet, a jet in the terahertz (THz, 0.1–10 THz) region, has attracted considerable research interest owing to its capability to compress beams to subwavelength hotspots. Here, we report a finding of an abnormal asymmetric Gouy phase anomaly in a terajet generated from a Teflon (n = 1.46) cuboid illuminated by the TM-mode oblique incidences. This asymmetric Gouy phase anomaly causes an interesting deviation angle between the normal of the phase distribution of the Gouy phase anomaly and the propagation direction of the incident beam at the position where the terajet is generated. This deviation angle exponentially decreases as THz waves propagate over several wavelengths. The output angle of the generated terajet, which is evaluated based on the experimental peak values of the amplitude distribution, shows linearity and is identical to the input angle of the incident beam. The full width at half maximum is also maintained within a wavelength for wide angles (45°) of oblique illuminations.
This Letter presents a method for enhancing the depth resolution of terahertz deconvolution based on autoregressive (AR) spectral extrapolation. The terahertz frequency components with a high signal-to-noise ratio (SNR) are modeled with an AR process, and the missing frequency components in the regions with low SNRs are extrapolated based on the AR model. In this way, the entire terahertz frequency spectrum of the impulse response function, corresponding to the material structure, is recovered. This method, which is verified numerically and experimentally, is able to provide a “quasi-ideal” impulse response function and, therefore, greatly enhances the depth resolution for characterizing optically thin layers in the terahertz regime.
Terahertz (THz) imaging is increasingly used in the cultural heritage field. In particular, continuous wave (CW) and low frequency THz is attracting more attention. The first application of the THz technique inherent to the cultural heritage field dates back 10 years ago. Since 2006, tangible improvements have been conducted in the refinement of the technique, with the aim to produce clear maps useful for any art restorer. In this paper, a CW THz (0.1 THz) imaging system was used to inspect paintings on canvas both in reflection and in transmission modes. In particular, two paintings were analyzed: in the first one, similar materials and painting execution of the original artwork were used, while in the second one, the canvas layer is slightly different. Flash thermography was used herein together with the THz method in order to observe the differences in results for the textile support materials. A possible application of this method for the detection of artwork forgery requires some parameterization and analysis of various materials or thickness influence which will be addressed in a future study. In this work, advanced image processing techniques including principal component thermography (PCT) and partial least squares thermography (PLST) were used to process the infrared data. Finally, a comparison of CW THz and thermographic results was conducted.
We present measurements of optical characteristics of sodium polyacrylate in the terahertz frequency range and discuss how material dispersion affects object visualisation in terahertz imaging. We demonstrate how important is to take into account the material dispersion for phase and relief reconstruction of the investigated object.
IC debug and diagnosis techniques like photon emission and FIB circuit edit are well established as powerful ways to attack secret codes in security ICs through chip frontside. But protective additions like interconnect meshes serve as countermeasures. This work shows examples how the risk assessment of contactless fault isolation (CFI) techniques through chip backside has indicated a drastic increase of vulnerability. Acclaimed unclonable functions and keys have been successfully challenged. There is no low-cost electronic backside protection concept available like the frontside meshes, because alignment and contact of backside structures to active IC layers cannot be handled without expensive through-silicon-via (TSV) technologies. But optical interaction can also be used to create backside protection concepts: Such concepts based on electro-optical properties are presented and proven to be operational.
This paper reports our invented methods for detection of counterfeit electronic. These versatile techniques are also handy in quality control applications. Terahertz pulsed laser systems are capable of giving the material characteristics and thus make it possible to distinguish between the materials used in authentic components and their counterfeit clones. Components with material defects can also be distinguished in section in this manner. In this work different refractive indices and absorption coefficients were observed for counterfeit components compared to their authentic counterparts. Existence of unexpected ingredient materials was detected in counterfeit components by Fourier Transform analysis of the transmitted terahertz pulse. Thicknesses of different layers are obtainable by analyzing the reflected terahertz pulse. Existence of unexpected layers is also detectable in this manner. Recycled, sanded and blacktopped counterfeit electronic components were detected as a result of these analyses. Counterfeit ICs with die dislocations were detected by depicting the terahertz raster scanning data in a coordinate plane which gives terahertz images. In the same manner, raster scanning of the reflected pulse gives terahertz images of the surfaces of the components which were used to investigate contaminant materials and sanded points on the surfaces. The results of the later technique, reveals the recycled counterfeit components.
This paper introduces a novel reconstruction approach for enhancing the resolution of the terahertz (THz) images. For this purpose the THz imaging equation is derived. According to our best knowledge we are reporting the first THz imaging equation by this paper. This imaging equation is universal for THz far-field imaging systems and can be used for analyzing, describing and modeling of these systems. The geometry and behavior of Gaussian beams in far-field region imply that the FWHM of the THz beams diverge as the frequencies of the beams decrease. Thus, the resolution of the measurement decreases in lower frequencies. On the other hand, the depth of penetration of THz beams decreases as frequency increases. Roughly speaking beams in sub 1.5 THz, are transmitted into integrated circuit (IC) packages and the similar packaged objects. Thus, it is not possible to use the THz pulse with higher frequencies in order to achieve higher resolution inspection of packaged items. In this paper, after developing the 3-D THz point spread function (PSF) of the scanning THz beam and then the THz imaging equation, THz images are enhanced through deconvolution of the THz PSF and THz images. As a result, the resolution has been improved several times beyond the physical limitations of the THz measurement setup in the far-field region and sub-Nyquist images have been achieved. Particularly, MSE and SSIM´ have been increased by 27% and 50% respectively. Details as small as 0.2 mm were made visible in the THz images which originally reveals no details smaller than 2.2 mm. In other words the resolution of the images has been increased by 10 times. The accuracy of the reconstructed images was proved by high resolution X-ray images.
Terahertz pulse time-domain holography (THz PTDH) is the powerful technique for high-resolution amplitude and phase THz imaging that allows mapping spectroscopic information across the imaged object. In this paper, we consider most sought after applications of phase imaging provided by this technique and experimentally demonstrate the ability of the method to reconstruct smooth and stepped relief features of an object that is transparent in THz region. Unlike the amplitude distribution, which does not contain any significant information in this case, phase distribution not only reveals the object qualitatively, but also allows the reconstruction of the object thicknesses pattern, even in low signal-to-noise registration conditions. Main limitations of the proposed method, such as transverse resolution and low signal-to-noise environment are carefully studied and mitigated.
A method to reconstruct the terahertz (THz) refrac-tive index and absorption coefficient of in vivo tissue using THz pulsed spectroscopy (TPS) has been proposed. The method utilizes a reference THz window to fix the sample of interest during the TPS reflection mode measurements. Satellite pulses caused by multiple THz-wave reflections in the reference window are taken into account to accurately solve the inverse problem. The stability of the proposed method in the presence of various factors, including digital noise in the TPS waveforms and fluctuations of the reference THz window position, has been accurately analyzed. The method has been implemented to study in vivo the THz refractive index and absorption coefficient of the human skin. The skin from three persons has been measured, and the results agree with the well-known data on healthy skin spectroscopy in general, except for several regions of the skin. Thus, for the elbow, the hand, the knee, and the heel the THz refractive index and absorption coefficient considerably differ from the average values. The observed results are of principle importance for further development of novel approaches to skin diagnosis based on THz technologies. Index Terms—Absorption coefficient, inverse problem, material parameters, refractive index, spectroscopy of the skin, terahertz pulsed spectroscopy, THz technology.
Nowadays, composite materials are widely used in building and construction industry, in motor-vehicles, spacecrafts and aircrafts, and in biomedical science due to the ability of combining various consistent components for manufacturing composite materials with physical and chemical properties significantly different from the properties of each component. Polymer composite materials (PCMs) appear to be the most common type of composites. PCMs consist mainly of polymer binder reinforced with the glass-fiber-fabric. Although PCMs are widely applied, PCM manufacturing technology lacks the methods of non-destructive testing. In this paper we demonstrate that terahertz (THz) pulsed spectroscopy (TPS) appears to be a unique instrument for solving important problems of PCM manufacturing control. We experimentally demonstrate the efficiency of TPS for non-destructive control of PCM binder polymeriza-tion, since THz radiation is sensitive to changes of picosecond dynamics in media. Furthermore, we show the ability to detect the internal non-impregnated voids inside the PCM structure by means of THz time-of-flight tomography. These results highlight the potentials of TPS applications for non-destructive control of PCM manufacturing process. Index Terms—Non-destructive evaluation, polymer composite materials (PCMs), Terahertz (THz) pulsed spectroscopy (TPS), THz technology, THz time-of-flight tomography.
THz radiation is capable of penetrating most of nonmetallic materials and allows THz spectroscopy to be used to image the interior structures and constituent materials of wide variety of objects including Integrated circuits (ICs). The fact that many materials in THz spectral region have unique spectral fingerprints provides an authentication platform to distinguish between authentic and counterfeit electronic components. Counterfeit and authentic ICs are investigated using a high-speed terahertz spectrometer with laser pulse duration of 90 fs and repetition rate of 250 MHz with spectral range up to 3 THz. Time delays, refractive indices and absorption characteristics are extracted to distinguish between authentic and counterfeit parts. Spot measurements are used to develop THz imaging techniques. In this work it was observed that the packaging of counterfeit ICs, compared to their authentic counterparts, are not made from homogeneous materials. Moreover, THz techniques were used to observe different layers of the electronic components to inspect die and lead geometries. Considerable differences between the geometries of the dies/leads of the counterfeit ICs and their authentic counterparts were observed. Observing the different layers made it possible to distinguish blacktopped counterfeit ICs precisely. According to the best knowledge of authors the reported THz inspection techniques in this paper are reported for the first time for authentication of electronic components.
Wide varieties of techniques such as X-ray tomography, scanning electron microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and optical inspections using a high resolution microscope have also been being employed for detection of counterfeit ICs. In this paper, the achieved data from THz material inspections/ THz imaging are compared to the obtained results from other techniques to show excellent correlation. Compared to other techniques, THz inspection techniques have the privilege to be nondestructive, nonhazardous, less human dependent and fast.
Terahertz and infrared radiation have unique properties applicable to the field of surveillance and security systems. We investigated the possibility of detecting potentially dangerous objects covered by various types of clothing using passive imagers operating at 1.2 mm (250 GHz) and long-wavelength infrared at 6–15 μm (20–50 THz). We developed a measurement methodology that assumes to investigate theoretical limitations, performance of imagers, and physical properties of fabrics. To evaluate stability of the detection capabilities of imagers, we performed measurement sessions each lasting 30 min. We present a theoretical comparison of the two spectra and results of experiments using state-of-the-art equipment.
In this paper we consider using the terahertz (THz) time domain spectroscopy (TDS) for non destructive testing and determining the chemical composition of the vanes and rotor-blade spars. A versatile terahertz spectrometer for reflection and transmission has been used for experiments. We consider the features of measured terahertz signal in temporal and spectral domains during propagation through and reflecting from various defects in investigated objects, such as voids and foliation. We discuss requirements are applicable to the setup and are necessary to produce an image of these defects, such as signal-to-noise ratio and a method for registration THz radiation. Obtained results indicated the prospects of the THz TDS method for the inspection of defects and determination of the particularities of chemical composition of aircraft parts.
In this paper, we propose to use terahertz pulsed imaging (TPI) as a novel tool to measure the thickness and quality of up to four layers of car paint on both metallic and non-metallic substrates. Using a rigorous one-dimensional electromagnetic model for terahertz propagation in a multi-layered medium combined with a numerical fitting method, the refractive index, extinction coefficient, and thickness of individual paint layers were determined. This proposed method was shown to be able to resolve coating layers down to a thickness of 18 $mu{hbox{m}}$ and was validated for both single- and multi-layer automobile paint samples. Results of the terahertz measurements were benchmarked against other techniques that are currently used for non-destructive testing during car manufacture: ultrasound and eddy current measurements, as well as two reference techniques, X-ray microcomputed tomography and surface profilometry. Good consistency was found between the techniques. Compared to conventional techniques, TPI has the advantage that it is a non-contact method and that it is able to spatially resolve the thickness uniformity distribution information by two-dimensional mapping.
The supply chain today spreads all around the globe. For assembling a system, several components, each of which may be produced in a different country and by a different manufacturer, need to travel the globe to get to the assembly sites. In such a global supply chain, tracing the components could be a great challenge and counterfeiters can find open doors to infiltrate the markets. Several billions of dollars of counterfeit integrated circuits (IC)s are sold each year. Low-quality authentic ICs might also pass the quality control sections and enter the supply chain occasionally. The endurance and reliability of the counterfeit and low-quality ICs are less than those of their authentic counterparts, and besides economical damage, unexpected system failures as a result of the failure of a counterfeit component can lead to loss of lives. As a result, quality management, authentication and counterfeit prevention in today`s electronic industry play a crucial role. The semiconductor business is constantly expanding and as a result, more counterfeiters become attracted to the semiconductor device manufacturing. Moreover, counterfeiters are constantly trying more complicated techniques to avoid being exposed. As a result, the counter-counterfeit industry needs to constantly develop new techniques for revealing the counterfeiters. From the quality management point of view, as the technology becomes more advanced, more advanced techniques are needed to control the quality of the ICs before entering the supply chain and prior to being selected in the assembly sites. In this work, we develop innovative techniques for quality management, authentication, and counterfeit detection. In the first part of this work, encrypted micro-signatures are designed and fabricated by electron beam lithography (EBL) on the ICs. These signatures travel on the ICs through the global supply chain. Once the component is received at the assembly site, the signature is decrypted and authenticity is proved. In the second part of this work, we developed advanced innovative techniques for physical inspection of the ICs by the use of terahertz (THz) spectroscopy and imaging. Recycled, blacktopped, and reverse engineered ICs are all detected by our THz authentication system. In the third part, we realized a resolution enhancement technique (RET) for developing super-resolution THz images. This RET enhances the resolution of the THz images by more than 10 times, enabling the THz imaging systems to image and measure the dimensions of the features by accuracies beyond Abbe`s diffraction limit.
Carbon fibres have become the natural choice as reinforcements for polymer composite materials (PMCs). The non-destructive inspection of dry carbon fibre preforms has the potential to increase the reproducibility and reduce the cost of PMC manufacturing, by identifying defects in dry multilayer preforms prior to resin injection. However, use of optical excitation thermography for inspecting dry carbon fibre preforms that constitute the structural reinforcement precursor in the manufacturing of PMCs is poorly documented in the open literature. In this work, optical excitation thermography was used for inspecting six dry multilayer carbon fibre preforms featuring different textile structures, thicknesses and defects, for the first time. Advanced image processing techniques were used in processing the thermographic data for comparative purposes. In particular, partial least square thermography, as a recently proposed technique, was studied in detail. Finally, the performance of different thermography techniques was analysed in terms of: 1) summarizing the capabilities of image diagnosis/processing techniques by signal-to-noise ratio analysis, and 2) identifying the monitoring modalities most suitable to industrial manufacturing.
The paper deals with the experimental performance assessment of Compressive Sampling (CS) based Terahertz (THz) Imaging systems, an emerging approach for carrying out non-destructive tests of materials with the aim of detecting defects and flaws. Differently from traditional methods based on raster scan, CS approach allows to reconstruct the image of interest through a reduced number of measurements, with a notable reduction of the time of investigation.
Although both simulated and experimental results concerning the performance assessment of THz Imaging technique are available in literature, the additional uncertainty due to the application of CS approach has never been in-depth taken into account, since the step of CS processing has been considered as ideal. Due to the success of CS-based Imaging THz technique and the promising performance of its exploitation also in industrial applications, the considered assumption is no more acceptable. Therefore, the authors focused their attention on the uncertainty sources associated with the experimental application of CS to THz Imaging systems and on their impact on the overall quality of the reconstructed image.
Several numerical tests, conducted by means of an optimized design of experiments, allow to (i) assess the sensitivity to relevant uncertainty sources of the reconstructed image quality and (ii) define a suitable performance factor capable of driving experimenters towards a proper configuration of the measurement station. In particular, misalignment of the CS masks turns out to be the most impacting uncertainty source, as confirmed by experimental tests carried out through an actual THz Imaging system. Nevertheless, the performance factor estimated on the reconstructed image of a reference target is capable of highlighting the presence of an incorrect configured Imaging system, thus making it possible to remedy and provide accurate and reliable THz images.
This paper presents a comprehensive theory for cohesive mathematical modeling and simulation of THz imaging systems. For mathematical modeling of the point spread function (PSF), system and transmission variables such as spectrum, absorption coefficient, beam divergence, and depth of focus are incorporated into the Gaussian beam distribution. The raster scanning process is mathematically modeled as the convolution of the object function and the PSF. Simulated transmission THz images are achieved as a result. The simulated THz images, compared to the experimental THz images, show great accuracy in terms of the location of the details and structural similarity.
In this paper, optical and mechanical excitation thermography were used to investigate basalt fiber reinforced polymer (BFRP), carbon fiber reinforced polymer (CFRP) and basalt-carbon fiber hybrid specimens subjected to impact loading. Interestingly, two different hybrid structures including sandwich-like and intercalated stacking sequence were used. Pulsed phase thermography (PPT), principal component thermography (PCT) and partial least squares thermography (PLST) were used to process the thermographic data. X-ray computed tomography (CT) was used for validation. In addition, signal-to-noise ratio (SNR) analysis was used as a means of quantitatively comparing the thermographic results. Of particular interest, the depth information linked to Loadings in PLST was estimated for the first time. Finally, a reference was provided for taking advantage of different hybrids in view of special industrial applications.
In this paper, a comprehensive set of techniques for quality control and authentication of packaged integrated circuits (IC) using terahertz (THz) time-domain spectroscopy (TDS) is developed. By material characterization, the presence of unexpected materials in counterfeit components is revealed. Blacktopping layers are detected using THz time-of-flight tomography, and thickness of hidden layers is measured. Sanded and contaminated components are detected by THz reflection-mode imaging. Differences between inside structures of counterfeit and authentic components are revealed through developing THz transmission imaging. For enabling accurate measurement of features by THz transmission imaging, a novel resolution enhancement technique (RET) has been developed. This RET is based on deconvolution of the THz image and the THz point spread function (PSF). The THz PSF is mathematically modeled through incorporating the spectrum of the THz imaging system, the axis of propagation of the beam, and the intensity extinction coefficient of the object into a Gaussian beam distribution. As a result of implementing this RET, the accuracy of the measurements on THz images has been improved from 2.4 mm to 0.1 mm and bond wires as small as 550 µm inside the packaging of the ICs are imaged.
The application of the thermal and infrared technology in different areas of research is considerably increasing. These applications involve Non-destructive Testing (NDT), Medical analysis (Computer Aid Diagnosis/Detection- CAD), Arts and Archaeology among many others. In the arts and archaeology field, infrared technology provides significant contributions in term of finding defects of possible impaired regions. This has been done through a wide range of different thermographic experiments and infrared methods. The proposed approach here focuses on application of some known factor analysis methods such as standard Non-Negative Matrix Factorization (NMF) optimized by gradient-descent-based multiplicative rules (SNMF1) and standard NMF optimized by Non-negative least squares (NNLS) active-set algorithm (SNMF2) and eigen decomposition approaches such as Principal Component Thermography (PCT), Candid Covariance-Free Incremental Principal Component Thermography (CCIPCT) to obtain the thermal features. On one hand, these methods are usually applied as preprocessing before clustering for the purpose of segmentation of possible defects. On the other hand, a wavelet based data fusion combines the data of each method with PCT to increase the accuracy of the algorithm. The quantitative assessment of these approaches indicates considerable segmentation along with the reasonable computational complexity. It shows the promising performance and demonstrated a confirmation for the outlined properties. In particular, a polychromatic wooden statue and a fresco were analyzed using the above mentioned methods and interesting results were obtained.
Thermal and infrared imagery creates considerable developments in Non-Destructive Testing (NDT) area. Here, a thermography method for NDT specimens inspection is addressed by applying a technique for computation of eigen-decomposition which refers as Candid Covariance-Free Incremental Principal Component Thermography (CCIPCT). The proposed approach uses a shorter computational alternative to estimate covariance matrix and Singular Value Decomposition(SVD) to obtain the result of Principal Component Thermography(PCT) and ultimately segments the defects in the specimens applying color based K-medoids clustering approach. The problem of computational expenses for high-dimensional thermal image acquisition is also investigated. Three types of specimens (CFRP, Plexiglas and Aluminium) have been used for comparative benchmarking. The results conclusively indicate the promising performance and demonstrate a confirmation for the outlined properties.
We have developed a method of solid immersion THz imaging—a non-contact technique employing the THz beam focused into evanescent-field volume and allowing strong reduction in the dimensions of THz caustic. We have combined numerical simulations and experimental studies to demonstrate a sub-wavelength 0.35λ0-resolution of the solid immersion THz imagingsystem compared to 0.85λ0-resolution of a standard imagingsystem, employing only an aspherical singlet. We have discussed the prospective of using the developed technique in various branches of THz science and technology, namely, for THz measurements of solid-state materials featuring sub-wavelength variations of physical properties, for highly accurate mapping of healthy and pathological tissues in THz medical diagnosis, for detection of sub-wavelength defects in THz non-destructive sensing, and for enhancement of THz nonlinear effects.
In this paper, we introduce wide-aperture aspherical lens for high-resolution terahertz (THz) imaging. The lens has been designed and analyzed by numerical methods of geometrical optics and electrodynamics. It has been made of high-density polyethylene by shaping at computer-controlled lathe and characterized using a continuous-wave THz imaging setup based on a backward-wave oscillator and Golay detector. The concept of image contrast has been implemented to estimate image quality. According to the experimental data, the lens allows resolving two points spaced at 0.95λ distance with a contrast of 15%. To highlight high resolution in the THz images, the wide-aperture lens has been employed for studying printed electronic circuit board containing sub-wavelength-scale elements. The observed results justify the high efficiency of the proposed lens design.
Nondestructive imaging of packaged silicon carbide power transistors was performed using terahertz (THz) time-domain reflection imaging. Techniques were developed to process the data gathered by these measurements and enhance the quality of the THz time-domain images. It was found that applying high-pass error function filters in the frequency domain provided the best balance of improving image clarity and minimizing distortion of the time-domain signal. The results indicate that with proper data processing, THz imaging can be a viable nondestructive method for inspecting packaged power electronic devices.
In this work, terahertz images have been simulated from X-ray images. For this aim the terahertz raster scanning process is modeled by a two dimensional convolution of the modelled THz beam and the X-ray image. The mathematical model of the terahertz beam has been modeled by a Gaussian function. The variables in this function are frequency of the beam, lateral location and absorption coefficient of the object. The accuracy of the proposed approach has been verified by comparing the results with the actual terahertz images.
We compared the possibility of detecting hidden objects covered with various types of clothing by using passive imagers operating in a terahertz (THz) range at 1.2 mm (250 GHz) and a mid-wavelength infrared at 3-6 μm (50-100 THz). We investigated theoretical limitations, performance of imagers, and physical properties of fabrics in both the regions. In order to investigate the time stability of detection, we performed measurements in sessions each lasting 30 min. We present a theoretical comparison of two spectra, as well as the results of experiments. In order to compare the capabilities of passive imaging of hidden objects, we combined the properties of textiles, performance of imagers, and properties of radiation in both spectral ranges. The paper presents the comparison of the original results of measurement sessions for the two spectrums with analysis.
We propose a new approach for THz image quality enhancing using correlation function between the image under consideration and a standard image. The standard image moves in two directions along a image under analysis. As a result, 2 D correlation function is obtained. Multiplying this function by color number belonging to a grey scale, we restore the image under the analysis. This allows to suppress a noise on a new image. This method allows to see the person clothes details that it means multi-times increasing of the passive THz camera temperature resolution. We discuss a choice of standard image characteristics for an achievement of correlation function for high contrast. Other feature of our approach arises from a possibility of a person image coming to the THz camera by using a computer processing of the image only. It means that we can “decrease” a distance between a person and the passive THz camera. This algorithm is very convenient for using and has a high performance.
Stitching is used to reduce incomplete infusion of T-joint core (dry-core) and reinforce T-joint structure. However, it might cause new types of flaws, especially micro-sized flaws. In this paper, a new micro-laser line thermography (micro-LLT) is presented. X-ray micro-computed tomography (micro-CT) was used to validate the infrared results. The micro-LLT and micro-CT inspection are compared. Then, a finite element analysis (FEA) is performed. The geometrical model needed for finite element discretization was developed from micro-CT measurements. The model is validated for the experimental results. Finally a comparison of the experiments and simulation is conducted. The infrared experimental phenomenon and results are explained based on the FEA results.
Pulsed terahertz imaging is very effective tool in nondestructive evaluation of layered composite materials. In this paper an experiment and an inverse scheme based on numerical or analytical model and hybrid optimization algorithm are presented and verified in order to obtain dielectric parameters profile of layered composite structures.
Due to high benefits, counterfeiters are continually making their counterfeiting techniques more sophisticated. It becomes more and more difficult to distinguish authentic components from the counterfeit ones. More and more counterfeit electronic components are injected to the global market as well. Consequently, new authentication techniques should be developed day by day to distinguish counterfeit components. These techniques need to be fast, economically reasonable, reliable, accessible for wide variety of consumers, nonhazardous and nondestructive. THz inspection techniques have the entire mentioned requirements. In this work it is showed that time-domain THz techniques have the ability to distinguish authentic components from the wide variety of counterfeit ones. In transmission experimental setup mode, THz pulse laser systems are capable to give the material characteristics such as refractive index and absorption coefficient. Obtaining the refractive index and absorption coefficient of each of the electronic components makes it possible to distinguish between the authentic components and their counterfeit counterparts. In this work different refractive indices and absorption coefficients were observed for counterfeit components compared to their authentic counterparts. In reflection experimental setup mode, thicknesses of different layers of objects are obtainable. In this work by developing THz techniques distance of the leads and die from the surface of the integrated circuits (IC) are calculated. The calculated values are confirmed by the results obtained from the X-ray images. Moreover, blacktopped counterfeit electronic components are distinguished by THz techniques. It is notable that blacktopped counterfeit components are not distinguishable by X-ray imaging. Moreover, conventional techniques for distinguishing blacktopped components are time consuming, random, destructive and hazardous for personnel. Capabilities of THz systems for producing tomography images are discussed as well. This work shows that by THz imaging techniques, counterfeit ICs with die and lead dislocations are distinguishable. In addition, images from the surfaces of the ICs can give an authentication platform for investigation of the materials on the surfaces of the components. In this work presence of foreign materials on the surfaces of the counterfeit electronic components was observed by THz imaging technique.
In vivo terahertz (THz) spectroscopy of pigmentary skin nevi is performed. The in vivo THz dielectric characteristics of healthy skin and dysplastic and non-dysplastic skin nevi are reconstructed and analyzed. The dielectric
permittivity curves of these samples in the THz range exhibit significant differences that could allow non-invasive early diagnosis of dysplastic nevi, which are melanoma precursors. An approach for differentiating dysplastic and non-dysplastic skin nevi using the THz dielectric
permittivity is proposed. The results demonstrate that THz pulsed spectroscopy is potentially an effective tool for non-invasive early diagnosis of dysplastic nevi and melanomas of the skin.
Reconstructing the dielectric permittivity profile (depth dependence of sample dielectric permittivity) is an important inverse problem. We present a new method for permittivity profile reconstruction based on terahertz time-domain spectroscopy signal processing. Reconstruction is accomplished in two steps. First, the sample pulse function is reconstructed using sample time-domain reflection data. Low-and high-frequency noise filtering and the interpolation of the pulse function at low frequencies are then applied. Second, an invariant embedding technique is used to calculate the dielectric permittivity profile based on the sample pulse function. Samples with known permittivity profiles have been studied experimentally using this procedure in order to verify this algorithm. This algorithm is stable to additive Gaussian white noise as shown using mathematical modeling based on the finite-difference time-domain technique. Possible applications of this permittivity profile reconstruction technique are discussed. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
As it is well-known, application of the passive THz camera for the security problems is very promising way. It allows seeing concealed object without contact with a person and this camera is non-dangerous for a person. Efficiency of using the passive THz camera depends on its temperature resolution. This characteristic specifies possibilities of the detection of concealed object: minimal size of the object, maximal distance of the detection, image detail. One of probable ways for a quality image enhancing consists in computer processing of image. Using computer processing of the THz image of objects concealed on the human body, one may improve it many times. Consequently, the instrumental resolution of such device may be increased without any additional engineering efforts. We demonstrate new possibilities for seeing the clothes details, which raw images, produced by the THz cameras, do not allow to see. We achieve good quality of the image due to applying various spatial filters with the aim to demonstrate independence of processed images on math operations. This result demonstrates a feasibility of objects seeing. We consider images produced by THz passive cameras manufactured by Microsemi Corp., and ThruVision Corp., and Capital Normal University (Beijing, China).
Modern industry makes more and more extensive use of various composite materials. In this paper, for the purposes of various composite materials evaluation, the terahertz imaging method is presented. Basalt fibre-reinforced composites and polymeric anticorrosion coatings are considered. Basalt fibre composites are the innovative materials that are being increasingly used in modern industry. The paper also briefly introduces a specific type of complex coating of steel applied in the industry (e.g. oil or chemical). Two methods of defects detection in the mentioned structures are presented. The first method is based on a system identification, whereas the second one is on the estimation of time-domain signal parameters. Finally, the results achieved during terahertz inspection of coatings are compared with those obtained using active thermography.
Terahertz (THz) imaging which is a new technology for material classification and nondestructive detection has been extensively investigated in the past decade. The time-domain waveform acquired at each point of the object by using the THz time-domain spectroscopy contains much information about the object. Processing this waveform will present the characters of the object. Several methods are adopted to generate the image of the explosive samples and results are compared and discussed. Experiment results indicate that this new imaging technology can be used for explosive detection.
In terahertz reflection imaging, a deconvolution process is often employed to extract the impulse function of the sample of interest. A band-pass filter such as a double Gaussian filter is typically incorporated into the inverse filtering to suppress the noise, but this can result in over-smoothing due to the loss of useful information. In this paper, with a view to improving the calculation of terahertz impulse response functions for systems with a low signal to noise ratio, we propose a hybrid Frequency-Wavelet Domain Deconvolution (FWDD) for terahertz reflection imaging. Our approach works well; it retrieves more accurate impulse response functions than existing approaches and these impulse functions can then also be used to better extract the terahertz spectroscopic properties of the sample.
We present what is to our knowledge the first imaging system based on optoelectronic terahertz time-domain spectroscopy. Terahertz time-domain waveforms are downconverted from the terahertz to the kilohertz frequency range, and the waveform for each pixel is frequency analyzed in real time with a digital signal processor to extract compositional information at that point. We demonstrate applications to package inspection and chemical content mapping in biological objects. (C) 1995 Optical Society of America
- M Stecher
M. Stecher et al., Towards Industrial Inspection with THz, Systems (2016) 311-335.
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- G Cavallo
- A Liccardo
- G P Papari
- A Andreone
L. Angrisani, G. Cavallo, A. Liccardo, G.P. Papari, A. Andreone, THz
Measurement Systems, in: New Trends and Developments in Metrology,
InTech, 2016.