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Flexible and miniaturized photodetectors, offering a fast response across the ultraviolet (UV) to millimeter (MM) wave spectrum, are crucial for applications like healthcare monitoring and wearable optoelectronics. Despite their potential, developing such photodetectors faces challenges due to the lack of suitable materials and operational mechanisms. Here, the study proposes a flexible photodetector composed of a monolayer graphene connected by two distinct metal electrodes. Through the photothermoelectric effect, these asymmetric electrodes induce electron flow within the graphene channel upon electromagnetic wave illumination, resulting in a compact device with ultra‐broadband and rapid photoresponse. The devices, with footprints ranging from 3 × 20 µm2 to 50 × 20 µm2, operate across a spectrum from 325 nm (UV) to 1.19 mm (MM) wave. They demonstrate a responsivity (RV) of up to 396.4 ± 5.1 mV W−1, a noise‐equivalent power (NEP) of 8.6 ± 0.1 nW Hz−0.5, and a response time as small as 0.8 ± 0.1 ms. This device facilitates direct imaging of shielded objects and material differentiation under simulated human body‐wearing conditions. The straightforward device architecture, aligned with its ultra‐broadband operational frequency range, is anticipated to hold significant implications for the development of miniaturized, wearable, and portable photodetectors. [ABSTRACT FROM AUTHOR]

Terahertz imaging is one of the most promising approaches for non-destructive control. An interesting approach to having cost-effective systems is to use frequency-modulated continuous wave (FMCW) radars with a raster scan configuration. Nevertheless, current systems using linear stages or robotic arms have the disadvantage of being heavy, requiring a long scan and not allowing a direct visualization of the result being measured. In addition, it is complex to evaluate the position of the measuring point on the real object, particularly if it is not flat. Here, we propose to solve these previous challenges with a portable system combining an FMCW radar with an augmented reality (AR) interface using a smartphone. This system achieves two goals: (i) first is to achieve data acquisition in the AR environment and (ii) the second is to make possible the visualization of data, even after post-processing, in the AR environment. [ABSTRACT FROM AUTHOR]

Cork phenolic sheets with thermal protection paints are used as thermal protection system in launch vehicles. Detecting debonds and inclusions at the interface of cork phenolic-composite structure and thickness measurement of multi-layer TPS is highly challenging with single-side access. In this study, Terahertz imaging in reflection mode is used to detect the defects. The sample is raster scanned in X-Y directions, and the reflected signal is measured at each pixel and consolidated to obtain a Terahertz C-scan image of the entire area. The image obtained using Terahertz time-domain spectroscopy imaging system can distinguish between the healthy and debond/inclusion areas. Terahertz time-domain spectroscopy in transmission mode is carried out to obtain the refractive index and absorption coefficient of individual layers, which is a prerequisite for thickness measurement. The time-of-flight principle is used for thickness estimation of multi-layer thermal protection systems over carbon fibre reinforced plastic samples. The Sparse deconvolution technique is used to resolve the overlapping echoes to obtain the time delay for each layer. The thickness values obtained for cork phenolic sheet and red and white thermal protection paints over carbon fibre-reinforced plastic samples using Terahertz-time domain spectroscopy are in close agreement with the actual values. [ABSTRACT FROM AUTHOR]

This letter was to present an attempt of large‐wavelength Gaussian deconvolution phase‐contrast computed tomography (LW‐GD‐PCCT) for promotion of image quality reconstructed in low‐frequency band of terahertz (THz) spectrum at 0.11 THz. The interaction between the imaging samples and the THz Gaussian beam were formulated firstly in this paper, where the unwrapped phase was extracted specifically to portray the spatial structure distribution of the samples. Additionally, a Gaussian deconvolution was employed for the further reduction of spatial distortions. Moreover, an image reconstruction was carried out with the obtained phase sinograms based on phase‐used inverse Radon transform from the different positions on the sample. For an experimental assessment of the concept of LW‐GD‐PCCT, a single ellipsoid reflector‐based THz Gaussian beam generating system was established and samples such as polystyrene (PS) foam cuboid (Sample 1), and cylinder (Sample 2) with hollow defects (air holes and triangles) were prepared carefully in this work. To experimentally evaluate the performance of the contributing to the structural imaging over soft samples. Two‐dimensional topographies of each sample were reconstructed successfully, and the obtained cross root‐mean‐square error (cross‐RMSE), cross peak signal‐to‐noise ratio (cross‐PSNR), and cross structural similarity (cross‐SSIM) were 151.6451, 26.3225, and 0.9616 for Sample 1 with a high dose of 180 projections respectively, as well as 30.3242, 33.3129, and 0.9711 for Sample 2 with a low dose of 36 projections, respectively. The obtained imaging indicators of this work showed a superiority of imaging quality over those of recent works. Furthermore, the investigation of the bearing capacity has shown promise in enhancing image quality even in low‐dose conditions. The presented results suggest that the unwrapped phase combining with Gaussian deconvolution in low‐frequency band of THz imaging would be useful to improve the reconstructed image quality, potential to highly feasible non‐destructive testing of polymer foam via large wavelength. [ABSTRACT FROM AUTHOR]

Utilizing a terahertz（THz）linear array detector and a 0.3 THz radiation source, a reflective THz continuous wave linear array scanning imaging system was built. Using the imaging system, THz images reflected by samples could be accurately measured with double scan measurements and an image linear superposition. The imaging resolution and the measurement region could approach 2 mm and 40 mm×40 mm with 3.0 min of the time consumption. Taking advantage of this system, the metallic samples hidden by envelopes and textiles were successively identified. In addition, the foreign matters embedded in polyethylene and plastic foam plates were exactly discerned. The experimental results demonstrated that objects concealed by dielectric materials can be quickly and precisely inspected by the imaging system, which indicated its application values in security inspection and nondestructive testing. [ABSTRACT FROM AUTHOR]

Probing the time evolution of the terahertz electric field within subwavelength dimensions plays a crucial role in observing the nanoscale lightwave interactions with fundamental excitations in condensed-matter systems and in artificial structures, such as metamaterials. Here, we propose a novel probing method for measuring terahertz electric potentials across nanogaps using a combination of optical and terahertz pulse excitations. To achieve this, we employ ring-shaped nanogaps that enclose a metallic island, allowing us to capture tunneling charges when subjected to terahertz electromagnetic pulse illumination. By controlling and manipulating the terahertz tunneling charges through a focused optical gate pulse, we can obtain the terahertz potential strength as a function of spatial coordinates and time delays between pulses. To accurately quantify the time evolution of terahertz electric potential across quantum barriers, we carefully calibrate the recorded nonlinear tunneling current. Its on-resonance and off-resonance behaviors are also discussed, providing valuable insights into the antenna's characteristics and performance. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Test & Measurement Technology is the property of Publishing Center of North University of China and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Journal of Test & Measurement Technology is the property of Publishing Center of North University of China and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract Flexible and miniaturized photodetectors, offering a fast response across the ultraviolet (UV) to millimeter (MM) wave spectrum, are crucial for applications like healthcare monitoring and wearable optoelectronics. Despite their potential, developing such photodetectors faces challenges due to the lack of suitable materials and operational mechanisms. Here, the study proposes a flexible photodetector composed of a monolayer graphene connected by two distinct metal electrodes. Through the photothermoelectric effect, these asymmetric electrodes induce electron flow within the graphene channel upon electromagnetic wave illumination, resulting in a compact device with ultra‐broadband and rapid photoresponse. The devices, with footprints ranging from 3 × 20 µm2 to 50 × 20 µm2, operate across a spectrum from 325 nm (UV) to 1.19 mm (MM) wave. They demonstrate a responsivity (RV) of up to 396.4 ± 5.1 mV W−1, a noise‐equivalent power (NEP) of 8.6 ± 0.1 nW Hz−0.5, and a response time as small as 0.8 ± 0.1 ms. This device facilitates direct imaging of shielded objects and material differentiation under simulated human body‐wearing conditions. The straightforward device architecture, aligned with its ultra‐broadband operational frequency range, is anticipated to hold significant implications for the development of miniaturized, wearable, and portable photodetectors.

The terahertz range, as a type of electromagnetic wave with wavelengths between microwaves and the infrared band, has the characteristics of penetration, low energy and a stable absorption spectrum of specific substances, and is widely used in non-destructive testing, human security inspections, biological tissue diagnoses and military detection. In particular, terahertz wave 3D imaging technology can detect the internal information of the target of detection, and it has become the focus of current research. This study carried out research on 3D reconstruction and object detection algorithms based on terahertz images. In view of the problem that the MVS (Multi-ViewStereo) series of 3D reconstruction algorithms ignore the context information between the cost layers and have unsatisfactory reconstruction effects when used on complex regions, an improved MVSNet 3D reconstruction algorithm FCTMVSNet(Feature and Cost Transformer Depth Inference for Unstructured Multi-view Stereo) based on Transformer is proposed here. A structured object recognition algorithm was designed to provide theoretical support for subsequent terahertz image-based object detection algorithms.

The terahertz (THz) frequency range is widely considered the most challenging and underdeveloped frequency range due to the lack of technologies to effectively bridge the transition region between microwaves (below 100 GHz) and optics (above 10 000 GHz). Although THz radiation would be perfect for material identification and as a safe alternative to X-rays for producing high-resolution images of the interior of opaque objects, first a fundamentally new approach is needed to establish novel devices and techniques.

A terahertz time domain pulsed spectroscopy system is modified to provide fully polarimetric radiation and analysis. The operation of this polarimetry system is characterized using a birefringent, x-cut quartz crystal. The modification is based on rotating the photoconductive antennas such that both the emitted and detected polarizations are out of the plane of incidence. Subsequently, broadband wire grid polarizers are used to select the incident and detected direction of linear polarization to be either parallel with (vertical) or perpendicular to (horizontal) the plane of incidence with the sample surface. The experiments are conducted in both transmission and reflection. Depending on the frequency, the phase retardation of the incoming electric field components along the two perpendicular optical axes of the quartz crystal changes differently. This results in the polarization of the light exiting the crystal changing with frequency. As a result, multiple frequencies are identified where the crystal behaves as a near ideal quarter-, half-, or full-wave retarder. Additionally, due to the time-domain nature of the experiment, transmitted and reflected electric fields are measured after multiple consecutive reflections within the crystal. This leads to a further, complex control over the final polarization state of the signal. Finally, images of a resolution standard are obtained demonstrating the characteristics of the polarimetry system. [ABSTRACT FROM AUTHOR]

Various methods are employed for food inspection; however, the persistent demand from both manufacturers and consumers remains due to the limitations of existing inspection technologies. This study introduces a novel technique for enhancing food safety and conducting large-area quality assessments using terahertz (THz) waves. This approach enables the classification of quality grades and the safe detection of soft matter, facilitated by high-speed mechanical raster scan imaging. Leveraging a two-dimensional complex refractive index map, we address the challenges posed by the uniformity in optical properties within food materials and foreign bodies, attributed to high water absorption and the presence of diverse compounds. Consequently, we anticipate improved foreign substance inspection and detection. These cutting-edge THz spectroscopic and imaging techniques, representing nondestructive inspection approaches, have the potential to elevate food safety and quality standards.

Probing the time evolution of the terahertz electric field within subwavelength dimensions plays a crucial role in observing the nanoscale lightwave interactions with fundamental excitations in condensed-matter systems and in artificial structures, such as metamaterials. Here, we propose a novel probing method for measuring terahertz electric potentials across nanogaps using a combination of optical and terahertz pulse excitations. To achieve this, we employ ring-shaped nanogaps that enclose a metallic island, allowing us to capture tunneling charges when subjected to terahertz electromagnetic pulse illumination. By controlling and manipulating the terahertz tunneling charges through a focused optical gate pulse, we can obtain the terahertz potential strength as a function of spatial coordinates and time delays between pulses. To accurately quantify the time evolution of terahertz electric potential across quantum barriers, we carefully calibrate the recorded nonlinear tunneling current. Its on-resonance and off-resonance behaviors are also discussed, providing valuable insights into the antenna’s characteristics and performance.

Terahertz time-domain spectroscopy (THz-TDS) system is a powerful tool in material spectral analysis and non-destructive testing (NDT). However, the data acquisition is time-consuming due to the point-by-point scanning process, which leads to limitations in THz imaging applications. In this paper, a robust adaptive-thresholding primal-dual sparse recovery (APSR) method is proposed to reconstruct high-resolution THz images and reduce the acquisition time. A sparsity averaging dictionary, which is a concatenate of multiple bases, is applied to improve the robustness and generalization. A reweighted ℓ 1 scheme is also adopted to enhance the sparse solution. As a crucial consideration in the sparse recovery method, we propose a robust adaptive threshold estimator based on the median absolute deviation in the sense that the final threshold determines the reconstruction quality and the convergence rate. Meanwhile, together with the Nesterov acceleration technique, a large threshold can be applied at the beginning of the iteration to help accelerate the convergence. Numerical experiments demonstrate that the APSR method can reconstruct THz images with robustness even with low SNR (of 30 dB) and low sampling rate (of 30%) compared to conventional methods. In the real THz-TDS system, our proposed sparse imaging method can successfully recover a composite object with much fewer THz spectra and reduces the acquisition time to 30% at the cost of a relative error of 2.5%, demonstrating the efficiency of our proposed method and the penetrating ability of THz technology in the NDT applications. [ABSTRACT FROM AUTHOR]

Cryogenic fluid storage tanks of launch vehicles use polyurethane-based foam with multi-layered coating as an insulation system. Detection of metal-to-foam debonds in cryofoam with the multi-layered coating is challenging due to the high attenuation of ultrasonic waves in cryofoam, the porous nature of the foam, varying substrate and foam thickness, etc. In this study a novel approach using dynamic depth focusing technique with Terahertz waves has been employed. Cryofoam sample with artificial metal-to-foam debonds is scanned with a continuous wave terahertz imaging system in reflection mode with a frequency range of 0.1 to 0.4 THz. The sample is raster scanned point by point in X-Y directions using a stepper motor. The reflected terahertz signal is received with a photomixer and Schottky detector to obtain a complete C-scan image. The debonds are not detectable by continuous-wave terahertz imaging due to the surface unevenness since the focal distance of the operating system is not variable. This report proposes a three-step procedure to perform dynamic depth focusing on multi-layer coated cryofoam samples. The first step involves the quantification of surface roughness at distinct points using a digital depth gauge to acquire the depth profile of the sample initially with a step size of 2 mm. A depth profile has been generated by interpolating the intermediate depth values as a second step. Finally, the focus of imaging is dynamically varied using a motorized stage at each imaging point based on the initially generated depth profile. This approach gives a better resolution at all depths without using geometry description tools like computer-aided design or other software tools. [ABSTRACT FROM AUTHOR]

Terahertz time-domain spectroscopic imaging has become a common and useful technique for various interdisciplinary studies. Raster scan is still the most general and reliable imaging modality due to the lack of matrix detectors. In terahertz raster scanning imaging, row displacements occur due to the use of a two-way continuous scanning mode, the lack of delay line triggering, or other technical factors. The displacements result in image distortion which significantly affects the subsequent data analysis. In this article, a row displacement correction algorithm based on a genetic algorithm is proposed. The algorithm corrects images based on the principle that row displacements result in an increase of high-frequency components of the Fourier spectrum transformed along the image column direction. The algorithm performance is comprehensively verified by both numerical simulation and experimental data, showing a promising effectiveness and robustness on different sample types, shapes, orientations, and spatial distributions. The algorithm corrects the displacements numerically regardless of the causing factors, offering a wide adaptability for various THz systems to improve the scanning efficiency and image quality. [ABSTRACT FROM AUTHOR]

Measuring mill‐scale thickness in a nondestructive and noncontact fashion in a production environment remains a challenge, as knowledge of the thickness is central to processing of the steel strip. Terahertz time‐of‐flight tomography is demonstrated here to enable nondestructive and noncontact measurements of tertiary mill‐scale thickness down to ≈10 μm on production hot‐rolled steel, building on our recent work on thickness evaluation of laboratory‐produced scale on steel coupons. X‐ray diffraction (XRD) results show that the compositions of the mill‐scale films manufactured under production conditions are slightly different from the laboratory‐prepared steel coupons, resulting from the different compositions of phases present in the two types of samples. The mill‐scale thickness is obtained after advanced signal processing of the raw data and shows good agreement with values obtained from destructive measurements. Compared to conventional techniques, such as scanning electron microscopy which is destructive, eddy current testing, which requires direct contact with the surface of samples investigated and is not suitable for large areas, or elevated temperatures, and acoustic inspection, whose spatial resolution is largely limited by its relatively long‐wavelength, terahertz time‐of‐flight tomography may be implemented for in‐line nondestructive thickness characterization and quality control of optically thin tertiary mill scale on steel products, even in a production environment. [ABSTRACT FROM AUTHOR]

A terahertz time domain pulsed spectroscopy system is modified to provide fully polarimetric radiation and analysis. The operation of this polarimetry system is characterized using a birefringent, x-cut quartz crystal. The modification is based on rotating the photoconductive antennas such that both the emitted and detected polarizations are out of the plane of incidence. Subsequently, broadband wire grid polarizers are used to select the incident and detected direction of linear polarization to be either parallel with (vertical) or perpendicular to (horizontal) the plane of incidence with the sample surface. The experiments are conducted in both transmission and reflection. Depending on the frequency, the phase retardation of the incoming electric field components along the two perpendicular optical axes of the quartz crystal changes differently. This results in the polarization of the light exiting the crystal changing with frequency. As a result, multiple frequencies are identified where the crystal behaves as a near ideal quarter-, half-, or full-wave retarder. Additionally, due to the time-domain nature of the experiment, transmitted and reflected electric fields are measured after multiple consecutive reflections within the crystal. This leads to a further, complex control over the final polarization state of the signal. Finally, images of a resolution standard are obtained demonstrating the characteristics of the polarimetry system.

Terahertz biotechnology has been increasingly applied in various biomedical fields and has especially shown great potential for application in brain sciences. In this article, we review the development of terahertz biotechnology and its applications in the field of neuropsychiatry. Available evidence indicates promising prospects for the use of terahertz spectroscopy and terahertz imaging techniques in the diagnosis of amyloid disease, cerebrovascular disease, glioma, psychiatric disease, traumatic brain injury, and myelin deficit. In vitro and animal experiments have also demonstrated the potential therapeutic value of terahertz technology in some neuropsychiatric diseases. Although the precise underlying mechanism of the interactions between terahertz electromagnetic waves and the biosystem is not yet fully understood, the research progress in this field shows great potential for biomedical noninvasive diagnostic and therapeutic applications. However, the biosafety of terahertz radiation requires further exploration regarding its two-sided efficacy in practical applications. This review demonstrates that terahertz biotechnology has the potential to be a promising method in the field of neuropsychiatry based on its unique advantages.

Terahertz (THz) tomographic imaging has recently attracted significant attention thanks to its non-invasive, non-destructive, non-ionizing, material-classification, and ultra-fast nature for object exploration and inspection. However, its strong water absorption nature and low noise tolerance lead to undesired blurs and distortions of reconstructed THz images. The diffraction-limited THz signals highly constrain the performances of existing restoration methods. To address the problem, we propose a novel multi-view Subspace-Attention-guided Restoration Network (SARNet) that fuses multi-view and multi-spectral features of THz images for effective image restoration and 3D tomographic reconstruction. To this end, SARNet uses multi-scale branches to extract intra-view spatio-spectral amplitude and phase features and fuse them via shared subspace projection and self-attention guidance. We then perform inter-view fusion to further improve the restoration of individual views by leveraging the redundancies between neighboring views. Here, we experimentally construct a THz time-domain spectroscopy (THz-TDS) system covering a broad frequency range from 0.1 to 4 THz for building up a temporal/spectral/spatial/material THz database of hidden 3D objects. Complementary to a quantitative evaluation, we demonstrate the effectiveness of our SARNet model on 3D THz tomographic reconstruction applications. [ABSTRACT FROM AUTHOR]

The paper reports the art of recording a real-time imaging of concealed metallic objects, paracetamol drug, wood and TNT explosive using 0.11 Terahertz(THz) IMPATT diode laser source of 60 mW power and a 320 × 260 pixels plane focal array (PFA) uncooled microbolometer-based camera coupled with indigenously designed Teflon lenses and metallic mirrors. The noise equivalent power (NEP) and noise equivalent temperature difference (NETD) of microbolometer were ascertained of the order of 0.9pW/ Hz and 1004.5 mK, respectively. The image of the object was recorded using a X–Y motorized stage. The microbolometer camera with each pixel size of 150 µm combined with FLIR software for the recording the raw imaging data of the metallic sample. In the next step, raw data were transferred to python software for generating terahertz images. Finally, the quality of images is compared with the terahertz pulsed imaging system in reflection mode, which consist of an optical fiber-coupled femtosecond laser of 60 fs pulse width and repetition rate of 100 MHz for the generation and detection of THz radiation. [ABSTRACT FROM AUTHOR]

In order to meet the increasing demand of non-destructive testing (NDT) in engineering practice, a continuous terahertz NDT platform based on linear scanning has been developed, with a center frequency of 154 GHz and a bandwidth of 56 GHz. This system combines frequency modulation continuous wave (FMCW) radar technology with a continuous scanning structure, as well as a data acquisition platform to provide a non-contact detection method; this is highly efficient and compensates for the shortcomings of traditional methods such as microwave, X-ray, ultrasonic, and others in safety inspection and special detection. In addition, a signal processing method of spectral refinement and correction is proposed in this paper for accurate thickness measurement. The results show that the method has a high accuracy for ABS, PVC, and ceramic matrix composites. By extracting the characteristic parameters, the detection and imaging of prefabricated defects, such as debonding and bubbles in composite materials, have been successfully achieved. This helps to evaluate the internal state of the inspected object more intuitively and further meets the requirements of industrial NDT. [ABSTRACT FROM AUTHOR]

The increasing scarcity of natural resources, worsening global climate change, environmental degradation, and rising demand for food are forcing the biotechnology and plastics industries to seek and apply circular economy models that would lead to a sustainable transition in the production and use of bioplastics. Circular economy models can improve the economic productivity of bio-based plastics and have a positive impact on the environment by reducing conventional plastic waste and the consumption of petrochemical feedstocks for plastic production. In addition, some agricultural wastes that have the potential to be used as bioplastics can be reused. Terahertz (THz) systems are already used in the plastics and rubber industries for non-destructive testing, detection, imaging, and quality control. Several reports have highlighted the potential applications of THz spectroscopy and imaging in polymer analysis and plastics characterisation. This potential is even greater with chemometric methods and artificial intelligence algorithms. In this review, we focus on applications that support the transformation of the biotechnology sector to the circular economy, particularly via the transition from conventional plastics to bioplastics. In this review, we discuss the potential of THz systems for the characterisation and analysis of bioplastics and biopolymers. The results of previous studies on biopolymers in the THz frequency range are summarised. Furthermore, the potential of using artificial intelligence approaches such as machine learning as advanced analytical methods in THz spectroscopy and imaging, in addition to the conventionally used chemometric methods, is discussed. The results of this review highlight that THz technology can contribute to closed technological circles in important areas of biotechnology and the related plastics and rubber industries. [ABSTRACT FROM AUTHOR]

The paper discusses issues of digital processing of terahertz images. It is shown that despite the improvement of the hardware part of imaging setups, the acquired images still often have a low resolution and suffer from noise and blurring effects. Thus, to improve their visual quality, it is advisable to use special digital processing methods. While some progress has already been made in terms of denoising of terahertz images, the research of their deblurring is only at the very early stage. Therefore, this paper attempts to analyze the properties of blur functions for real terahertz images to further use them while designing a corresponding deblurring technique. For this purpose, the phase-only image method has been used. A study of blur properties for the three most common blur types (defocus, motion and Gaussian blur) has shown that for test images they can be distinguished and their main parameters can be assessed. However, the application of this method to real terahertz images has shown that the blur characteristics in them are very different from the ones obtained for modelled examples. The real blur demonstrates a quite complex behaviour and estimating its kernel requires additional research. [ABSTRACT FROM AUTHOR]

We report on the development of a handheld three-dimensional (3D) terahertz scanning system with an aspherical telecentric 3D-printed f-θ lens using selective laser sintering. The lens covers a broader scan line of 50 mm with its larger aperture, compared to the 20 mm range in our initial work, which was presented at the European Microwave Week 2021. In order to evaluate the adaptability of the optomechanical components with different sensor units, two different integrated frequency-modulated continuous wave radar modules based on monolithic microwave integrated circuit technology, operating in W- and D-bands are tested within the measurement scheme. The optomechanical part consists of a galvanometer scanner mirror and the f-θ lens. The optical system enables B-scans perpendicular to the manual translational movement of the sensor unit by its user. An integrated guiding wheel system with rotary encoder makes it possible to correlate the measurement points to their respective locations, enabling complete 3D volumetric inspection of the corresponding structures, which is particularly useful for the inspection along cracks and welds. [ABSTRACT FROM AUTHOR]

Terahertz waves, positioned between microwaves and infrared in the electromagnetic spectrum, are distinguished by their exceptional penetration capabilities, minimal energy requirements, and consistent absorption profiles for specific substances. Their versatility in applications such as non-destructive evaluation, human security scans, and biological diagnostics has propelled them to the forefront of scientific inquiry. However, existing terahertz equipment poses limitations in terms of compromised resolution, diffraction-induced blurring, and degradation of clarity due to texture overlaps. Consequently, numerous multi-view 3D reconstruction algorithms struggle to produce high-quality results with terahertz imagery. To address these challenges—particularly the scarcity of terahertz datasets and texture conflation—we integrated X-ray images from the DTU dataset with our collected terahertz projections. By observing inconsistent texture projections in multi-view terahertz images resulting from angle shifts, we developed DETransMVSnet—a state-of-the-art multi-view 3D reconstruction approach based on the Multi-Scale Deep Equilibrium Layer (MDEQ) paradigm. Leveraging equilibrium layers within homography-projected feature maps enables us to extract masks that differentiate different layers within a scene. The Intra-Attention and Mask-Attention Blocks further refine feature selection by preserving relevant terahertz details while suppressing disruptive background elements. As evidence of its effectiveness, DETransMVSnet achieves comparable performance to conventional algorithms on the DTU dataset but notably outperforms them when applied to terahertz datasets by successfully reconstructing images where previous methods have failed.

Much research has been conducted to improve the defect-detection rate and detection accuracy of the imaging technology used in terahertz nondestructive testing. Due to the power limit of light sources and noise interference in terahertz equipment, images have low resolution and fuzzy defect edges. Hence, improving the resolution is crucial for detecting defects. We designed an edge detection network structure based on a traditional deep neural network. Besides, we devised a node-fusing strategy to train the network. It demonstrates significant improvement of the resolution of the terahertz defect contour. A quartz fiber composites with embedded defects was tested with our network. The results showed that the proposed super-resolution reconstruction algorithm improves resolution, particularly on the edges of defect contours.

In this paper, we report our use of a hardware encoder-based synchronization method for a fast terahertz time-domain spectroscopy raster scanner built with the commercially available TeraFlash Smart platform. We describe the principles of our method, including our incorporation of synchronization signals from various devices included in the scanner. We also describe its implementation in a microcontroller with a dedicated counter. By such means, a fast scanning mode was obtained, which was 35 times faster than a traditional step-by-step approach. To validate the proposed synchronization method, we carried out measurements using the USAF 1951 resolution test and a fiberglass plate with a set of intentionally introduced defects. Our results confirmed that the TDS scanner with the developed synchronization method was able to capture high-quality images with resolutions as high as those obtained using traditional step-by-step scanning, but with significantly reduced scanning times.

The development of new non-invasive approaches able to recognize defective food is currently a lively field of research. In particular, a simple and non-destructive method able to recognize defective hazelnuts, such as cimiciato-infected ones, in real-time is still missing. This study has been designed to detect the presence of such damaged hazelnuts. To this aim, a measurement setup based on terahertz (THz) radiation has been developed. Images of a sample of 150 hazelnuts have been acquired in the low THz range by a compact and portable active imaging system equipped with a 0.14 THz source and identified as Healthy Hazelnuts (HH) or Cimiciato Hazelnut (CH) after visual inspection. All images have been analyzed to find the average transmission of the THz radiation within the sample area. The differences in the distribution of the two populations have been used to set up a classification scheme aimed at the discrimination between healthy and injured samples. The performance of the classification scheme has been assessed through the use of the confusion matrix on 50 samples. The False Positive Rate (FPR) and True Negative Rate (TNR) are 0% and 100%, respectively. On the other hand, the True Positive Rate (TPR) and False Negative Rate (FNR) are 75% and 25%, respectively. These results are relevant from the perspective of the development of a simple, automatic, real-time method for the discrimination of cimiciato-infected hazelnuts in the processing industry.

Millimeter wave (MMW) and submillimeter wave (SMW) imaging techniques have a wide range of security applications due to their characteristics such as transmission through barrier materials. Biometric recognition based on MMW and SMW imaging has been attracting attention as an alternative to recognition methods based on imaging human body in visible band, since these systems pose a number of limitations such as being sensitive to illumination conditions and occlusions. This study explores the possibility of biometric recognition based on terahertz (THz) face images. The THz images are acquired by an active imaging system which utilizes a transceiver operating at 340 GHz in frequency modulated continuous wave (FMCW) mode. We examine if these images contain sufficient information on facial characteristics for distinguishing individuals, using a similarity comparison between images of different individuals. Analysis is based on a set of THz face images of 20 individuals. Images of the head areas of the participants have been recorded with and without a concealing clothing, in order to explore the possibility of identifying individuals with concealed faces. Analysis within this closed data set indicates that imaging at 340 GHz has sensitivity to the facial characteristics which are preserved in images through clothing, and can be used for biometric recognition. [ABSTRACT FROM AUTHOR]

The fullness of rice seeds is an important factor affecting the growth and yield of rice. Therefore, it is of great significance to detect fullness of rice seeds in nature state. In this paper, the emerging terahertz imaging detection technology is used to carry out the study of rice seed fullness detection. Firstly, the terahertz spectral images of rice seeds with different fullness are acquired. Secondly, the terahertz spectra of sample free region, empty shell seed region, and full seed region are extracted, respectively. To improve the accuracy of the model and reduce the computational effort, competitive adaptive reweighting sampling (CARS), uninformative variable elimination (UVE), Successive projection algorithm (SPA), and their combination are used to extract the features of terahertz spectral information. The corresponding support vector machine (SVM) and K-nearest neighbor (KNN) qualitative discriminant models are established to detect and identify the full and empty regions of rice seeds. In addition, the binarization of terahertz image is carried out to realize the visual expression of rice seeds. The UVE-SPA-KNN model established after band screening is used for classification, and the accuracy of prediction set reaches 98.33%. The UVE-SPA feature extraction method can reduce the amount of imaging data by 97.5% and realizes the visualization detection of kernels in rice seeds. This research verified that the visual detection of seed kernel fullness inside rice seeds can be well achieved by using terahertz imaging and spectrum fusion, which provides a new method for rapid detection of kernel fullness of rice seeds, and also provides a theoretical reference for terahertz imaging technology to detect the fullness of other thin-shell seeds. Highlights: In this paper, three different spectra of different sample free region, empty shell seed region and full seed region in 0.5–3.0 THz image are extracted, and five band screening methods of CARS, UVE, SPA, CARS-SPA and UVE-SPA are used to extract the characteristic wavelength of the THz spectral. The discriminant model is established to distinguish between background region, empty shell seed region and full seed region. The research shows that the established UVE-SPA-KNN qualitative discriminant model has a recognition rate of up to 98.33%. The fullness of rice seeds is calculated by the ratio of binarization and shell-kernel pixel points of 0.5–3.0 THz images, the 150 threshold is used for shell in channel B, and the 235 threshold is used for kernel in channel R in the process of image binarization. The error between the detection fullness of rice seeds and the actual fullness is less than 10%. In order to reduce the amount of imaging data and remove redundant information, the UVE-SPA feature extraction method is further used to compress the full spectrum of 329 wavelengths into 8 wavelengths, which reduces the amount of imaging data by 97.5% and can also realize the visualization detection of kernels in rice seeds. [ABSTRACT FROM AUTHOR]

As one of the main functional substances, carbohydrates account for a large proportion of the human diet. Conventional analysis and detection methods of dietary carbohydrates and related products are destructive, time-consuming, and labor-intensive. In order to improve the efficiency of measurement and ensure food nutrition and consumer health, rapid and nondestructive quality evaluation techniques are needed. In recent years, terahertz (THz) spectroscopy, as a novel detection technology with dual characteristics of microwave and infrared, has shown great potential in dietary carbohydrate analysis. The current review aims to provide an up-to-date overview of research advances in using the THz spectroscopy technique in analysis and detection applications related to dietary carbohydrates. In the review, the principles of the THz spectroscopy technique are introduced. Advances in THz spectroscopy for quantitative and qualitative analysis and detection in dietary carbohydrate-related research studies from 2013 to 2022 are discussed, which include analysis of carbohydrate concentrations in liquid and powdery foods, detection of foreign body and chemical residues in carbohydrate food products, authentication of natural carbohydrate produce, monitoring of the fermentation process in carbohydrate food production and examination of crystallinity in carbohydrate polymers. In addition, applications in dietary carbohydrate-related detection research using other spectroscopic techniques are also briefed for comparison, and future development trends of THz spectroscopy in this field are finally highlighted. [ABSTRACT FROM AUTHOR]

Retention of rice freshness is highly dependent on storage temperature. Timely and accurate identification of rice freshness is critical to ensure food security. Here, we characterize the freshness of rice in reference to storage temperature. Terahertz reflection imaging is a non-destructive and deeply penetrating technique that can be used for detecting rice freshness. Due to the shortcomings of traditional machine learning, such as limited processing of nonlinear problems and insufficient computing power. Deep learning has the advantages of strong learning ability and high portability. Therefore, for rice freshness identification, the VGG19 network and the Inception-ResNet-v2 network were used in this paper. Moreover, we propose an improved 1D-VGG19-Inception-ResNet-A network. This network possesses the advantages of low time consumption from the 1D-VGG19 network and high classification accuracy from the 1D-Inception-ResNet-V2 network. Compared with the traditional algorithms, the accuracy of the proposed network is significantly improved, with the rice freshness recognition accuracy of 99.80%. The experimental results indicate that terahertz spectral imaging and deep learning algorithms are viable tools for monitoring rice freshness. [ABSTRACT FROM AUTHOR]

To reduce the water footprint in agriculture, the recent push toward precision irrigation management has initiated a sharp rise in photonics-based hydration sensing in plants in a non-contact, non-invasive manner. Here, this aspect of sensing was employed in the terahertz (THz) range for mapping liquid water in the plucked leaves of Bambusa vulgaris and Celtis sinensis. Two complementary techniques, broadband THz time-domain spectroscopic imaging and THz quantum cascade laser-based imaging, were utilized. The resulting hydration maps capture the spatial variations within the leaves as well as the hydration dynamics in various time scales. Although both techniques employed raster scanning to acquire the THz image, the results provide very distinct and different information. Terahertz time-domain spectroscopy provides rich spectral and phase information detailing the dehydration effects on the leaf structure, while THz quantum cascade laser-based laser feedback interferometry gives insight into the fast dynamic variation in dehydration patterns. [ABSTRACT FROM AUTHOR]

In this study, we develop a 3D THz time-of-flight (TOF) imaging technique by using reflective optics to preserve the high-frequency components from a THz antenna. We use an Fe:InGaAs/InAlAs emitter containing relatively high-frequency components. THz-TOF imaging with asynchronous optical sampling (ASOPS) enables the rapid scanning of 100 Hz/scan with a time delay span of 100 ps. We characterize the transverse resolution using knife edge tests for a focal length of 5; the Rayleigh resolution has been measured at 1.0 mm at the focal plane. Conversely, the longitudinal resolution is determined by the temporal pulse width, confirmed with various gap structures enclosed by a quartz substrate. The phase analysis reveals that reflected waves from the top interface exhibit a phase shift when the gap is filled by high-indexed materials such as water but shows in-phase behavior when it is filled with air and low-indexed material. Our imaging tool was effective for inspecting the packaged chip with high lateral and longitudinal resolution. Importantly, the phase information in 2D and 3D images is shown to be a powerful tool in identifying the defect—in particular, delamination in the chip—which tends to be detrimental to the packaged chip's stability. [ABSTRACT FROM AUTHOR]

For decades, terahertz (THz) microscopic imaging has been limited by the resolution of the system due to the larger wavelength, the power of the source, and the equivalent noise power of the detector, so a lot of research has focused on single-point scanning imaging. With the development of hardware, full-field THz imaging based on high-power continuous-wave THz sources have been developed such as the direct intensity imaging method and lensless coherent imaging. In particular, the THz direct intensity imaging method requires no complicated computational reconstruction, while the high resolution, as a key issue, still needs to be improved. In this paper, the rotating coherent scattering microscopy was applied to THz imaging for the first time. Here, we designed and fabricated a hemisphere lens with high-resistance silicon. The tilted hemisphere lens transformed the incident divergent beam into a plane wave, and the total internal reflection occurred in the planar surface within the hemispherical lens, and generated evanescent waves in the rare medium. At the same time, the sample was placed very close to the plane of the hemispherical lens, so that the sample was illuminated by the evanescent waves. The scattered waves carried high frequency information to the far field, and thus through an objective, the super-resolution imaging was achieved along a single direction. Then, the hemispherical lens was rotated to obtain coherent scattering microscopic images under different evanescent wave illumination angles. Finally, the full-field super-resolution imaging results were obtained through incoherent superposition. [ABSTRACT FROM AUTHOR]

This paper presents the development, performance, integration, and implementation of a 150 GHz FMCW radar based on a homodyne harmonic mixing scheme for noncontact, nondestructive testing. This system offers high-dynamic-range measurement capabilities up to 100 dB and measurement rates up to 7.62 kHz. Such interesting characteristics make this system attractive for imaging applications or contactless sensing. Numerous samples of different materials and geometries were imaged by taking advantage of the radar's performance. By taking into account the nonionizing capability of the system, new applicative fields such as food industry and pharmaceutical packaging were explored. [ABSTRACT FROM AUTHOR]

Lenses/mirrors with fast data acquisition speeds and extended focal depths have practical importance in terahertz (THz) imaging systems. Thus, a high-speed 600 GHz-band THz imaging scanner system with enhanced focal depth is presented in this work. A polygon mirror with a 250 Hz scanning frequency and an integrated off-axis parabolic (OAP) mirror with an ~170 mm focal depth were employed for 2D imaging. The simulation and experimental results show that a spatial resolution of ~2 mm can be achieved as the imaging distance varies from ~85 to 255 mm. The proposed system was applied to image a hidden metal object as a potential security application, demonstrating that this system can image targets with an enhanced focal depth. [ABSTRACT FROM AUTHOR]

Successful water management is critical to achieving high performance in proton exchange membrane fuel cells (PEMFCs). The relatively high sensitivity of terahertz radiation to liquid water is promising as a complementary alternative for flooding detection in PEMFCs. Using a novel, commercially available terahertz source and camera, this paper investigates the feasibility of a compact and low‐cost terahertz imaging system for visualizing water build‐up inside an operating PEMFC, supported by simultaneous high‐resolution optical imaging. Several phenomena of water accumulation and transport, such as membrane hydration, main droplet appearance, water pool formation, growth, and eventual flush out by gases are imaged by terahertz imaging. These results agree with optical imaging and cell voltage readings. The proposed setup is also able to distinguish the effect of different air flow rates. [ABSTRACT FROM AUTHOR]

We built a high-speed TDS setup with the use of electronically controlled optical sampling (ECOPS), which can measure up to 1600 terahertz pulses per second. The movement of the sample was provided by two fast-speed motorized linear stages constituting the gantry. We developed a flat-bar-based metal marker approach for the synchronization of continuous line-by-line scans. We carefully compared the performance of the terahertz reflection time-domain spectroscopy (TDS) scanner operating in a slow point-by-point and a one-hundred-times faster line-by-line imaging scheme. We analyzed images obtained for both schemes for a uniform metallic breadboard with holes, as well as a glass composite sample with defects. Although the measurement time was reduced by 100 times in terms of the line-by-line scheme, the overall performance in both schemes was almost identical in terms of the defects' sizes, shapes and locations. The results proved that the proposed ECOPS TDS system can provide uniform and extremely fast scanning without any deterioration in image quality. [ABSTRACT FROM AUTHOR]

Terahertz (THz) imaging is a powerful technique allowing us to explore non-conducting materials or their arrangements such as envelopes, packaging substances, and clothing materials in a nondestructive way. The direct implementation of THz imaging systems relies, on the one hand, on their convenience of use and compactness, minimized optical alignment, and low power consumption; on the other hand, an important issue remains the system cost and its figure of merit with respect to the image quality and recording parameters. In this paper, we report on the design and performance of an extraordinary low-cost THz imaging system relying on a InP Gunn diode emitter, paraffin wax optics, and commercially available GaAs high-electron-mobility transistors (HEMTs) with a gate length of 200 nm as the sensing elements in a room temperature environment. The design and imaging performance of the system at 94 GHz is presented, and the spatial resolution in the range of the illumination wavelength (∼3 mm) and contrast of nearly two orders of magnitude is determined. The operation of two models of the HEMTs of the same nominal 20 GHz cut-off frequency, but placed in different packages and printed circuit board layouts was evaluated at 94 GHz and 0.307 THz. The presence of two competing contributions—self-resistive mixing and radiation coupling through the antenna effects of the printed circuit boards—to the detected signal is revealed by the signal dependence on the gate-to-source voltage, resulting in a cross-sectional responsivity of 27 V/W and noise-equivalent power of 510 pW/ Hz at 94 GHz. Further routes in the development of low-cost THz imaging systems in the range of EUR 100 are considered. [ABSTRACT FROM AUTHOR]

Due to certain hardware limitations the quality of terahertz images is often lower than desired, which makes it difficult to extract valuable information from them. The goal of this paper is to investigate possibilities to overcome some of these limitations by means of digital image processing. The research is held on a set of images obtained at different distances from the source of terahertz radiation at 0.1 THz frequency. It is shown that the noise in these images is mixed and has a significant level of spatial correlation. For image quality enhancement a fully automatic denoising method based on the use of a discrete cosine transform with a spatially adapted spectrum is proposed. It is shown that despite an initially low spatial resolution of terahertz images and intensive noise, it provides a good noise reduction with a good preservation of edges, which allows one to noticeably improve the quality of these images and make them more convenient for visual analysis carried out by a human operator. [ABSTRACT FROM AUTHOR]

Photodynamic therapy (PDT) is considered a promising noninvasive therapeutic strategy in biomedicine, especially by utilizing low-level laser therapy (LLLT) in visible and near-infrared spectra to trigger biological responses. The major challenge of PDT in applications is the complicated and time-consuming biological methodological measurements in identification of light formulas for different diseases. Here, we demonstrate a rapid and label-free identification method based on artificial intelligence (AI)-assisted terahertz imaging for efficient light formulas in LLLT of acute lung injury (ALI). The gray histogram of terahertz images is developed as the biophysical characteristics to identify the therapeutic effect. Label-free terahertz imaging is sequentially performed using rapid super-resolution imaging reconstruction and automatic identification algorithm based on a voting classifier. The results indicate that the therapeutic effect of LLLT with different light wavelengths and irradiation times for ALI can be identified using this method with a high accuracy of 91.22% in 33 s, which is more than 400 times faster than the biological methodology and more than 200 times faster than the scanning terahertz imaging technology. It may serve as a new tool for the development and application of PDT. [ABSTRACT FROM AUTHOR]

Strong terahertz (THz) emission from the methylammonium lead iodide (MAPbI3) perovskite semiconductors has been observed following above-bandgap photoexcitation, yet local THz responses of crystalline microstructures are absent. We implement laser THz emission microscope (LTEM), yet-to-be applied to the perovskite semiconductors, as a novel and complementary tool to evaluate the electronic and grain heterogeneity of MAPbI3 thin films. Two MAPbI3 samples with different grain sizes are studied. Using this approach, we show that the one with a larger grain size gives more uniform THz radiation. More significant spatial THz intensity fluctuation is observed for the sample with a smaller grain size. [ABSTRACT FROM AUTHOR]