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In this study, we report a three-dimensional synthetic aperture imaging system realized by a terahertz time-domain spectrometer. The temporal waveforms of the terahertz pulses scattered by the object to be imaged are Fourier transformed to the frequency domain before image reconstruction by the back-projection algorithm. The resolution of the imaging system is close to the center wavelength of the terahertz pulses, as tested by a resolution test chart. We also demonstrate the ability of this imaging method in non-destructive evaluation applications by measuring the internal structures of a university badge. A three-dimensional terahertz image and a series of slice views of the badge are acquired. The terahertz images clearly show the surface shape and internal structures of the badge, and the logo of the university on the badge is successfully extracted from the three-dimensional terahertz image. [ABSTRACT FROM AUTHOR]

Terahertz imaging technology has been widely used in security inspections due to its ability to detect various concealed hazardous materials and the advantage of being harmless to the human body. However, limited by the terahertz imaging system, it is challenging to detect concealed objects due to hard samples and imbalanced categories caused by terahertz image quality. To solve these issues, we propose a hybrid network with difficult–easy learning (DEL) for concealed object detection in the imbalanced activated terahertz image dataset. Based on the one-stage framework YOLOv5m, a path aggregation hybrid structure (PAHS) is proposed to improve the performance of the proposed network while maintaining real-time detection. Specifically, PAHS with transformer block (TB) and a fine-tuned global context attention (GCA) are designed to fully exploit and fuse the multi-scale information by path aggregation, which improves the detection accuracy of low contrast and noise-interfered objects. To solve the problem of imbalanced categories in the activated terahertz dataset, a DELoss is developed to guide the network classification. Moreover, EIOU is adopted to boost the network training, and a modified B-Ocl loss is used to discriminate the positive and negative samples. Experiments are conducted on a public imbalanced activate terahertz image dataset. The experimental results illustrate that the proposed network achieves competitive performance compared with recently reported state-of-the-art detection methods. Moreover, the proposed method improves the balanced detection ability of different categories. [ABSTRACT FROM AUTHOR]

Terahertz-band squint synthetic aperture radars (SARs) can obtain high-resolution images and have application potential in airborne radar systems. However, airborne radars usually have a large squint, which has led to traditional SAR algorithms no longer being applicable to airborne SARs. Additionally, terahertz radar imaging systems are more susceptible to the error induced by the platform's motion. This paper proposes a motion compensation method for terahertz SAR imaging with a large squint angle. First, the signal model of motion compensation is derived, and the processing flow of imaging and motion compensation is detailed. Second, some simulations and experiments are conducted, and the results are reported. The results indicate that the proposed method can effectively correct the motion errors, and the signal model and processing flow are verified. [ABSTRACT FROM AUTHOR]

Terahertz (THz) single-pixel imaging has received major research attention because of the lack of a suitable high-resolution array detector for THz imaging applications. Improving both imaging speed and quality has become a research hotspot for this field in recent years. In this study, a terahertz single-pixel imaging system with Hadamard spatial encoding was constructed by using optically induced semiconductor materials to perform THz wave modulation. Sparse coding was added to the system's reconstruction algorithm to enhance imaging quality. Numerous image patches were then collected from a natural image set to train an overcomplete dictionary and each patch in the measured image was reconstructed through sparse representation. To validate the effectiveness of the proposed algorithm, the reconstruction performances of different algorithms were compared under various conditions (i.e., with sampling rates varying from 5 to 100% and with noise levels within a signal-to-noise ratio range of 10–50 dB). The proposed algorithm, in combination with sparse representation of an overcomplete dictionary, showed a higher peak signal-to-noise ratio and a lower mean square error than both the inverse Hadamard transform (IHT) and TVAL3 algorithms. Finally, THz imaging experiments were performed to validate the algorithm's reconstruction performance at sub-Nyquist sampling rates. The experimental and simulation results coincided closely, thus indicating that the use of the proposed algorithm enhances the signal-to-noise ratio of the reconstructed image, reduces its mean square error, and retains greater image detail. The proposed algorithm was demonstrated to be the preferred choice for THz single-pixel imaging applications. [ABSTRACT FROM AUTHOR]

The cultivation processes of watermelon seed are often affected by issues such as empty shells and defects, resulting in significant losses. To obtain high‐quality seeds, the terahertz imaging technology combined with image smoothing and enhancement algorithm was proposed to reduce the noise and non‐obvious features caused by the influence in the imaging process and realize the non‐destructive, efficient, and accurate detection of the internal quality of watermelon seeds. Initially, a terahertz imaging system with a spatial resolution of 0.4 mm was used to acquire images of watermelon seeds with varying levels of fullness. Subsequently, denoising techniques, including Gaussian filtering, median filtering, bilateral filtering, discrete wavelet transformation denoising, wavelet denoising, and principal component analysis denoising, were used to handle the terahertz spectral images of watermelon seeds in the frequency range of 1–1.5 THz, respectively. Image enhancement operations, involving segmented linear gray‐level transformation and fractional‐order differentiation, were performed on the terahertz images of watermelon seeds after denoising. The optimal image processing approach was determined based on defect assessment through threshold segmentation. Finally, the validation was conducted at a spatial resolution of 0.2 mm. The images at a spatial resolution of 0.4 mm were subjected to wavelet denoising and window slicing in segmented linear gray‐level transformation (WS‐SLT) enhancement; the results exhibited the following improvements in defect accuracy compared with untreated THz images. A 7.74% increase in accuracy was observed for empty seeds, along with a 6.29% increase in the defect ratio for defective seeds 1. The defect ratio for intact seeds was 0, and there was no significant difference in defect ratio accuracy for defective seeds 2. At a spatial resolution of 0.2 mm, the average defect ratio error of THz imaging handled by wavelet denoising and WS‐SLT was approximately 5.04%. In conclusion, the terahertz imaging technology coupled with wavelet denoising and WS‐SLT methods can be used to enhance the accuracy of internal defect detection in watermelon seeds, and it provides a technical foundation and reference for assessing watermelon seed fullness. Agricultural economic efficiency is enhanced by mitigating empty husks and defects in watermelon seeds. Terahertz imaging results are optimized through comparative analysis of various image processing techniques. Defect detection accuracy is significantly improved by combining terahertz imaging with wavelet denoising and window slicing techniques. Within the validation, the average defect ratio error is a mere 5.04%, with all defect ratio errors falling within 8.06%. A rapid, non‐destructive, and highly accurate assessment of the internal quality of watermelon seeds is achieved. [ABSTRACT FROM AUTHOR]

The performance of CMOS transmitters and receivers operating at the submillimeter electromagnetic wave frequencies have sufficiently improved for use in active transmission and reflection-mode imaging applications that have the potential for broad deployment and utilization. Imaging integrated circuits have the potential to be large in area to support a high number of pixels along with digital backend processing circuits. For high volume imaging applications that may eventually be included in automobiles, smartphones, laptops, tablets, and others, a large manufacturing capacity to support the volume of large area ICs is necessary. For this, the use of CMOS technologies with a much larger manufacturing capacity is favored. It should be possible to improve the performance of CMOS circuits to increase the range, and operation margin and frequency. The electronically steerable submillimeter-wave reflector technology holds the promise for improving the performance and energy efficiency of submillimeter-wave imaging systems by multiple orders of magnitude, and it is a critical research area. Increasing the operating frequency from 430 to 850 GHz using CMOS integrated circuits to improve the angular resolution by 2X at a given form factor (∼0.15° for a reflector diameter of 15 cm) can make the submillimeter-wave imaging competitive to the LIDAR angular-resolution performance, while providing superior capabilities in visually impaired conditions and making the imaging devices more affordable. [ABSTRACT FROM AUTHOR]

Terahertz imaging has significant development prospects in terahertz technology due to its strong penetration and high spatial resolution. Currently, optical system designs are generally limited to the visible light range. To design an optical system suitable for terahertz imaging, an analysis of optical materials and system structures has been conducted. TPX (4-methylpentene-1) material and inverse telephoto structure were selected as the design basis. A terahertz optical system with a full field angle of 50°, a relative aperture of 1/4 and a focal length of 100 mm is designed using ZEMAX. The optimization results show that the root mean square radius of the spot in each field of view is smaller than the airy disk radius, indicating good imaging quality of the system. [ABSTRACT FROM AUTHOR]

We established a design method for a Magic-T with a single-layer dielectric/metal structure suitable for both wideband and multi-element applications for millimeter and submillimeter wave imaging observations. The design method was applied to a Magic-T with a coupled-line, stubs, and single-stage impedance transformers in a frequency-scaled model (6–14 GHz) that is relatively easy to demonstrate through manufacturing and evaluation. The major problem is that using the conventional perfect matching condition for a coupled-line alone produces an impractically large width coplanar coupled-line (CPCL) to satisfy the desired bandwidth ratio. In our study, by removing this constraint and optimizing impedances utilizing a circuit simulator with high computation speed, we found a solution with a ∼ 180 μm wide CPCL, which is approximately an order of magnitude smaller than the conventional analytical solution. Furthermore, considering the effect of transition discontinuities in the transmission lines, we optimized the line length and obtained a design solution with return loss < − 20 dB, amplitude imbalance < 0.1 dB, and phase imbalance < 0.5 ∘ from 6.1 to 14.1 GHz. [ABSTRACT FROM AUTHOR]

A reliable and effective diagnosis of coating structure is important for further maintenance. For the problem of slow detection and identification using terahertz non-destructive testing technology in the industrial inspection, a rapid diagnosis algorithm based on the wavelet packet energy and support vector machine method was developed for quick evaluation of the epoxy protective coating. The process mainly included time domain signal acquisition of various epoxy protective coating samples detected by a terahertz pulse imaging system, wavelet packet energy parameters extraction as the diagnosis feature vectors, classification model establishment based on the support vector machine algorithm and coating status evaluation using a three-class classifier. The influence on classification accuracy by the various feature vectors inputs with the support vector machine classifier was analysed. Satisfying results were achieved when the relative wavelet packet energy was taken as diagnostic features. A strong defective area could be quickly identified and more detail targeted analysis could be implemented as needed. The time spent was significantly reduced compared to the terahertz imaging of the whole area along with the manual judgement. The analysis indicated that the proposed method would be very useful and can be effectively employed for the coating monitoring application. [ABSTRACT FROM AUTHOR]

The shift of an image of a dielectric sphere asymmetrically irradiated by a terahertz Gaussian beam in terahertz imaging systems has been demonstrated experimentally. Numerical calculations using the operator scattering theory have shown the bending of the localization region of the electromagnetic field near the shadow surface of the dielectric sphere when it is shifted with respect to the waist center of the Gaussian beam, leading to the formation of the so-called "photonic hook" with the properties depending on the polarization and frequency of the incident radiation. The possibility of reaching a spatial resolution of 0.38λ in the studied range has been shown experimentally. The obtained results can be used to increase the resolution of commercial scanning terahertz systems. [ABSTRACT FROM AUTHOR]

Random media pose limitations on the imaging capability of photoelectric detection devices. Currently, imaging techniques employed through random media primarily operate within the laser wavelength range, leaving the imaging potential of terahertz waves unexplored. In this study, we present an approach for terahertz bistatic three-dimensional imaging (TBTCI) of hidden objects through random media. By deducing the field distribution of bistatic terahertz time-domain spectroscopy system, and proposing an explicit point spread function of the random media, we conducted three-dimensional imaging of hidden objects obscured by the random media. Our proposed method exhibits promising applications in imaging scenarios with millimeter-wave radar, including non-invasive testing and biological imaging. [ABSTRACT FROM AUTHOR]

An improved design of a controlled cylindrical hyperlens based on alternating layers of indium antimonide and silicon for imaging with a subwave resolution in the terahertz range is proposed. A possibility of dynamic tuning of hyperlens in a wide frequency range at small variations is temperature is demonstrated by numerical simulation. Distinctive features of this structure are small sizes, low loss in the insulator, and the ability of forming images not only in the near-field zone but also in the far-field zone, which was confirmed by plotting directional patterns. Based on the proposed computer model, a cylindrical hyperlens can be fabricated using conventional methods of deposition or electron-beam evaporation. The results obtained may contribute to improvement of the resolution of imaging systems in the terahertz range and development of modulators of hyperbolic metamaterials and sensors in the terahertz range. [ABSTRACT FROM AUTHOR]

Off-axis parabolic mirrors (OAPMs) are widely used in the THz and mm-wave communities for spectroscopy and imaging applications, as a result of their broadband, low-loss operation and high numerical apertures. However, the aspherical shape of an OAPM creates significant geometric aberrations: these make achieving diffraction-limited performance a challenge, and lower the peak electric field strength in the focal plane. Here, we quantify the impact of geometric aberrations on the performance of the most widely used spectrometer designs, by using ray tracing and physical optics calculations to investigate whether diffraction-limited performance can be achieved in both the sample and the detector plane. We identify simple rules, based on marginal ray propagation, that allow spectrometers to be designed that are more robust to misalignment errors, and which have minimal aberrations for THz beams. For a given source, this allows the design of optical paths that give the smallest THz beam focal spot, with the highest THz electric field strength possible. This is desirable for improved THz imaging, for better signal-to-noise ratios in linear THz spectroscopy and optical-pump THz-probe spectroscopy, and to achieve higher electric field strengths in non-linear THz spectroscopy. [ABSTRACT FROM AUTHOR]

Methods of reconstructing holographic images of objects made of the composite based on carbon fibermodified acrylonitrile-butadiene styrene and a thin electrically conductive film are presented. Holograms are obtained in the axial scheme when the collimating lens is placed on the source axis. Results of comparison of the calculated frequency response of the model transmission coefficient with measurements on an experimental setup are presented. [ABSTRACT FROM AUTHOR]

In video surveillance, real-time detection of hidden or concealed objects under the clothes of humans is a challenging task. Very few research works have been carried out on this hidden object detection and recognition with the aid of various coherent technologies namely Millimeter wave (MMW) imaging, Infrared (IR) imaging, and Terahertz (THz) imaging systems. Hidden objects that include weapons like axe, knife, gun, bomb, pistol, etc., are a major threat to security surveillance as it needs to be recognized in a few seconds irrespective of their size. This paper proposes an efficient hidden object recognition method that focuses on the detection and recognition of concealed weapons in humans using Modified Weighted You Only Look Once v5 (MWYOLOv5) model. The probable occurrence of inaccurate forecast results due to the sense of a low confidence score is mitigated by the use of the weighted boxes fusion (WBF) method. As a result, higher-confidence boxes contribute more to fused box coordinates than lower-confidence boxes. Meanwhile, it is significant to select optimal hyperparameter values for training YOLOv5 model with CSPDarknet53 based feature extractor and Path Aggregation Network (PANet) based feature aggregation procedures to predict concealed objects. To achieve this, a new crossover salp swarm algorithm (CSSA) is developed to tune the YOLO hyperparameters such as learning rate, momentum, weight decay and batch size. This provides more accuracy in recognizing hidden objects on using THz and MMW images on comparison with existing methods. The proposed hazardous weapon recognition model is trained and tested on both MMW and THz Imagery datasets and it shows that the proposed methodology is showing good results with high mAP@.5 and mAP@.5:95. [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]

In recent years, terahertz imaging technology has been widely used in security, medicine, and other fields. However, the image resolution is low due to the limits of imaging equipment and diffraction. Traditional super‐resolution methods based on machine learning use artificial paired image datasets, and their image degradation process is quite different from the actual terahertz imaging mechanism. In the paper, an unpaired hybrid attention network is proposed, using the real unpaired high‐resolution and low‐resolution terahertz images as the original input, two sub‐networks are trained: degenerate reconstruction network and super‐resolution network. The degenerate reconstruction network is used to learn the degradation process of the real terahertz imaging system, and reconstruct the generated degradation images into the fixed mapping down‐sampled image of high‐resolution terahertz image. The super‐resolution network uses the generated paired image with fixed correspondence to train the super‐resolution network of paired terahertz images, to improve the imaging resolution of the network in the super‐resolution work of real low‐resolution terahertz images. In addition, the no‐reference image quality evaluation system is introduced to objectively evaluate the network performance. The experimental results show that the UHA‐CycleGAN network outperforms the traditional paired super‐resolution network on both open‐source and self‐built datasets. [ABSTRACT FROM AUTHOR]

Imaging using millimeter waves (MMW) and terahertz (THz) waves can help inspect hazardous materials hidden beneath clothing in a non-contact and non-invasive manner. A 3D terahertz imaging system for security gate applications in the 275–305 GHz range was developed and experimentally demonstrated to detect concealed objects carried by pedestrians. This system performs 3D measurements by combining depth detection using frequency-modulated continuous wave (FM-CW) radar, vertical scanning of the detection spot using a 1D high-speed mechanical beam scanner, and horizontal movement of the irradiated area and detection spot as the pedestrian walks. The high-speed beam scanner comprises an F-Theta telecentric lens and a polygon mirror. It has a vertical line scan rate of 142 lines/s and spatial resolution of ~10 mm, consistent with the design value, and a depth resolution of ~7 mm, which is 40% larger than the theoretical value estimated from the FM-CW radar principle. The depth-dependent lateral distortion in 3D images, known as telecentricity, measured using the body scanner imaging system, was also evaluated. Consequently, images with the same magnification were obtained at a range of more than 500 mm of focus depth. Finally, the detection of concealed objects carried by pedestrians was demonstrated, showing that the system can work for a pedestrian walking at speeds from 4 km/h to 7 km/h. [ABSTRACT FROM AUTHOR]

With shorter wavelengths than microwaves and greater penetration depth than infrared light, waves in the terahertz spectrum offer unique material testing opportunities. Terahertz technology offers non-invasive and non-destructive testing in the form of spectroscopy and imaging. The most used systems for terahertz imaging are time-domain spectroscopy systems. However, frequency domain spectroscopy systems could offer excellent frequency resolution and be more suitable for biomedical applications. Terahertz imaging based on frequency domain spectroscopy systems is slow, and suffers from frequency tuning errors. A novel one-dimensional imaging principle is presented in this paper. In addition, frequency range optimization based on convolutional neural networks and occlusion sensitivity is utilized for frequency range optimization. Frequency range optimization is used to determine the optimal frequency range for data acquisition. The optimal frequency range or bandwidth should be wide enough for effective phase detection, and should be at the intersection of several spectral footprints in the observed medium. The intersection of spectral footprints is estimated using the proposed frequency range optimization algorithm based on a convolutional neural network and occlusion sensitivity algorithm. The proposed algorithm selects the most sensitive frequency band of THz spectrum automatically, and enables very fast acquisitions for object inspection and classification. [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]

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]

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]

• The need for the best direction of specular reflection is eliminated to the greatest extent. • Imaging quality and angle compatibility of terahertz fast reflective system are improved. • Design compact element to modulate propagation direction of wide-field illumination light. • A novel application of microlens array in terahertz imaging field. Active imaging can provide significantly larger signal margins in the terahertz (THz) spectral region than passive imaging. However, the imaging effect of THz active imaging is severely constrained by the orientation requirement of target reflection, which limits the application scenarios of this technique. In view of the shortcomings of existing THz reflective imaging systems, this paper proposes a new system optimization scheme from the aspect of hardware - a compact quasi-scattering screen (QSS) is proposed and designed as a beam-shaping element to eliminate the need for the best direction of specular reflection to the greatest extent. The corresponding relationship between the divergent Gaussian beam and target light field is directly established by using the equal-energy mapping method in non-imaging optics, which rejects the collimation process of the THz source. By customizing the optimization function in ZEMAX® to automatically optimize the surface of the QSS, the direction of incident light propagation can be adjusted. A non-coaxial reflective THz imaging system using a 0.1 THz continuous wave source demonstrates the ability of the QSS to improve the angle compatibility of the traditional THz reflective imaging system. The results show that the imaging angle compatibility range of the system can be increased to more than 5 times without changing the original imaging optical path and its resolution. In addition, it can also improve the imaging quality of the original imaging optical path for the object with a smooth surface or certain structure. [ABSTRACT FROM AUTHOR]

• A material pattern reconstruction technique based on terahertz waves is proposed for the first time, and experimental result similar to the ideal simulation is obtained. • A generalized model for general image reconstruction is proposed. • Combined with the deep blind degradation algorithm based on physical features and the random shuffle strategy, completed the establishment of the mapping relationship of the data set in the complex real environment. • Based on the existing technology, a scheme combining game theory and neural network is proposed to effectively improve the quality of material pattern reconstruction of terahertz ptychography. • The inclusion noise and high humidity of the weak terahertz radiation source simulate the working environment of the terahertz optical imaging system under harsh conditions. Finally, good imaging results approximate to the ideal conditions are obtained by our method. Ptychography is an imaging technique that uses the redundancy of information generated by the overlapping of adjacent light regions to calculate the relative phase of adjacent regions and reconstruct the image. In the terahertz domain, in order to make the ptychography technology better serve engineering applications, we propose a set of deep learning terahertz ptychography system that is easier to realize in engineering and plays an outstanding role. To address this issue, we propose to use a powerful deep blind degradation model which uses isotropic and anisotropic Gaussian kernels for random blurring, chooses the downsampling modes from nearest interpolation, bilinear interpolation, bicubic interpolation and down-up-sampling method, and introduces Gaussian noise, JPEG compression noise, and processed detector noise. Additionally, a random shuffle strategy is used to further expand the degradation space of the image. Using paired low/high resolution images generated by the deep blind degradation model, we trained a multi-layer residual network with residual scaling parameters and dense connection structure to achieve the neural network super-resolution of terahertz ptychography for the first time. We use two representative neural networks, SwinIR and RealESRGAN, to compare with our model. Experimental result shows that the proposed method achieved better accuracy and visual improvement than other terahertz ptychographic image super-resolution algorithms. Further quantitative calculation proved that our method has significant advantages in terahertz ptychographic image super-resolution, achieving a resolution of 33.09 dB on the peak signal-to-noise ratio (PSNR) index and 3.05 on the naturalness image quality estimator (NIQE) index. This efficient and engineered approach fills the gap in the improvement of terahertz ptychography by using neural networks. [ABSTRACT FROM AUTHOR]

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Real‐time terahertz passive imaging systems are highly demanded in security checking application. In this paper, two four‐pixel receiver arrays for 340 and 250 GHz passive imaging systems are presented. As the core device of the front‐end of the receiving array, the low noise terahertz mixer is realized based on the enhanced Schottky diode model and low‐loss circuit structure. The minimum double side band (DSB) noise temperature of the separated 340 and 250 GHz mixers is 900 and 580 K, respectively, within the operational band‐width. Without considering fluctuation in the power gain, the calculated temperature sensitivity of 340 GHz receiver is 0.8 K with 40 GHz RF band. Finally, based on all‐solid‐state millimeter‐wave drive front end and reflector optics, a passive terahertz imaging system is completed. Less than 1 cm space resolution is achieved for 1–3 m object distances, and the distinct imaging to hidden threats is produced and made contrast among the two receiver arrays. The perfect imaging performances of the system verify the low noise characteristic of terahertz mixer. The Schottky solid‐state receiver array provides a cheap and feasible option for personal security screening. [ABSTRACT FROM AUTHOR]

Terahertz (THz) imaging has an outstanding advantage of high resolution due to the high frequency and has promising potential in VideoSAR. However, limited to the THz source power and the air absorption, the THz image usually has a low SNR and is susceptible to noise in remote sensing and imaging. In order to improve the quality of THz images, a THz image enhancement method is proposed based on the noise2noise strategy. The THz images are reconstructed using the AFBP algorithm. They are organized as noisy image pairs and filtered with a mask to remove the influence of moving targets. Then, the Noise2Noise network is constructed based on the CNN network and takes the noisy image pair as input and reference. In the training stage, 1000 noisy image pairs are used as the training set and 100 noisy images are used as the test set to verify the performance of the proposed method. The experimental results of real VideoSAR data have demonstrated that the proposed method is capable of suppressing noise and enhancing the THz image. [ABSTRACT FROM AUTHOR]

The article discusses the stretchable broadband photo-sensor sheets for nonsampling, source-free, and label-free chemical monitoring by simple deformable wrapping. It mentions that effective use of omnidirectional and transparent blackbody radiation from free-form targets themselves allows compact measurement configuration and enhances the functionality and simplicity of this scheme, while the presenting technology monitors concentrations of arbitrary water-soluble chemicals.

Terahertz imaging technology has shown great potential in many fields. As the core component of terahertz imaging systems, terahertz detectors have received extensive attention. In this paper, a metasurface-based terahertz optomechanical detector is proposed, which is made of two fabrication-friendly materials: gold and silicon nitride. The optomechanical detector is essentially a thermal detector composed of metasurface absorber, bi-material micro-cantilevers and heat insulation pillars. Compared with traditional thermal terahertz detectors, the optomechanical detector employs a metasurface absorber as the terahertz radiation coupler and obtains an absorptivity higher than 90% from 3.24 to 3.98 THz, which is much higher than that of traditional terahertz detectors with absorbers made from natural materials. Furthermore, the detector is fabricated by MEMS process and its responsivity has been verified by a specifically designed optical read-out system; the measured optomechanical responsivity is 24.8 μm/μW, which agrees well with the multi-physics simulation. These results indicated that the detector can be employed as a pixel to form a terahertz focal plane array in the future, and further realize real-time terahertz imaging at room temperature. [ABSTRACT FROM AUTHOR]

Strict requirements for target detection and identification of the complex background have increased because a single detection method is susceptible to environmental factors. The abilities to use terahertz imaging systems with high photoresponsivity and polarization sensitivity are of central importance to practical photodetectors. The present technology for high‐sensitivity, polarization THz detectors integrated into a compact, common aperture (co‐aperture) system remains largely unexplored challenges. By exploiting the inherent features of graphene/Bi2Te3 Dirac material and InAs nanowire, the plasma‐wave and thermoelectric co‐aperture phototransistors with a selective, synchronous, and controllable operating modes are devised in the same field of view, which provide remarkable efficiencies for THz‐light harvesting on‐chip. The achieved selective detection, better than that of single nanowire phototransistor, exhibits excellent sensitivity of 18.5 mA W−1 (12 V W−1) in graphene/Bi2Te3 channel and 3.5 mA W−1 (400 V W−1) in InAs nanowire channel, corresponding to the noise equivalent power of less than 0.1 nW/Hz0.5. The results are exploited to highlight a novel strategy for the realization of efficient co‐aperture phototransistors toward tunable, multiple modes, polarization THz detection by combining with in situ integration advantages of various materials, compatibility mechanisms, integration structures with metamaterials. [ABSTRACT FROM AUTHOR]

The particular challenges of implementing such established technologies in the millimeter and terahertz frequency range was long associated with the term terahertz gap, which emphasized the lack of application-ready technologies in this spectral region. On the high-frequency end, approaches extending established technologies from the infrared into the terahertz regime are mostly based on opto-electronic system concepts and are commonly summarized as optical terahertz technologies. The field of terahertz and millimeter wave science and technology has evolved in recent years into an area attracting a lot of attention from all sides of science, industry, and the public. The same device type of field-effect transistor-based terahertz detector with integrated receiving antennas (TeraFET) but fabricated using commercial foundry-based silicon CMOS technology is presented by Ikamas et al. [[21]], where the detectors are combined in a free-space transmission system with also silicon CMOS-based emitters at 250 GHz. [Extracted from the article]

The passive terahertz (THz) detection on objects concealed under clothing is quite challenging due to the heavy noise interference. In this paper, an active‐contour‐based self‐adaptive DBSCAN (AC‐SDBSCAN) detection algorithm is proposed. The core of AC‐SDBSCAN algorithm lies in that the object contours are first extracted by AC method in a noise scenario and then the statistical features of the contours are used to motivate a SDBSCAN to complete clustering without initialization. Benefiting from the strong robustness of AC and DBSCAN to the noise, the concealed objects in the noisy THz images can be detected accurately. Extensive simulations are verified on four passive THz image datasets. The results indicate that the AC method in our solution can achieve over 87% accuracy for contour extraction of passive THz images, while the classical methods achieve less than 77%; in addition, the SDBSCAN can achieve over 90% clustering accuracy without manual initialization which is significantly superior to the conventional DBSCAN. Eventually, the proposed method completes the object detection of passive THz images with a maximum recall of 90.38% and a maximum precision of 94%. [ABSTRACT FROM AUTHOR]

Ultrafast reconfigurable devices with high‐speed responses, high‐resolution capabilities, and ultracompact sizes will be essential for future real‐time terahertz imaging, chemical detection, nondestructive biosensing, and communication systems. Multifunctional terahertz metasurfaces with active, programmable controls have enabled conspicuous functionalities of subwavelength planar devices and components for manipulating electromagnetic (EM) waves such as beam focusing, polarization modification, generation of exotic EM modes, or multibeam scanning applications. However, active metasurface technologies for EM beam manipulation, capable of dynamical topology change, or induced‐phase reconfiguration are usually requiring various semiconductor‐based controlling elements (e.g., PN diodes and transistors), which are difficult to implement in the high‐frequency spectrum of terahertz waves. In this work, a coding metasurface is introduced integrating patterned GeTe phase‐change materials as command elements, which allows the optical control of terahertz wave propagation. The suggested metasurface design brings plentiful of remarkable functionalities through different coding patterns and is highly effective to control beam tilting, directivity, and splitting of terahertz beams. The proposed concepts of coding metasurfaces integrating optically active phase‐change materials are successfully confirmed by experimental demonstration and can be extended to more complex terahertz systems for imaging, tomography, sensing, and 6G communication applications. [ABSTRACT FROM AUTHOR]

Clarity of Terahertz image is essential at various security checkpoints to avoid life's dangers and treats. However, Terahertz images are distorted by noise. Noise is frequently present in digital images during the image collection, coding, delivery, and processing phases. It is extremely difficult to remove noise from digital images without prior knowledge of the noise model. Wavelet transforms have gained popularity as a tool for image denoising. In this paper, we advance a solution to this challenge using Global Threshold selection as well as wavelet transform filters. When compared to denoising Gaussian noise at the same percentage induced, biorthogonal is the most effective denoising filter for salt and pepper noise. As the salt and pepper noise increases from 20% to 60%, the hidden security image as our target varnishes or is overpowered by the induced salt and pepper noise. We discover that despite the fact that the bior 4.4 and sym4.0 wavelet transform filters prove powerful in denoising the image, it is still not clearer and that when an image is tainted by Gaussian noise, wavelet shrinkage denoising is nearly perfect in both bior4.4 and sym 4.0, whereas when the image is tainted by salt & pepper noise, wavelet shrinkage denoising is nearly perfect in both bior4.4 and sym 4.0. [ABSTRACT FROM AUTHOR]

A CMOS plasmon detector with a low‐gain buffer amplifier, which enhances the characteristics of the detector core, is proposed for improving the performance of THz imaging systems. A high voltage gain (AV) of the main amplifier integrated with the detector improves the voltage responsivity (RV) but not the performances of the detector core itself. The increase in the ratio of the RV to the noise‐equivalent power (NEP) in the proposed detector with a low‐gain buffer can improve the signal‐to‐noise ratio (SNR) of the THz images. The proposed detector with a − 1.5 dB AV buffer amplifier is fabricated by TSMC 0.25 μm CMOS technology and measured to have the RV of 357.1 kV/W and NEP of 57.3 pW/√Hz at the gate bias of 0.17 V for 0.2 THz input signals. The measurement results indicate that the proposed detector has an RV lower than that of a detector with an AV of 11.2 dB exhibiting a similar NEP. The obtained 0.2 THz raster scanning images show that the image SNR can be improved by 14.8 dB using in the proposed detector. [ABSTRACT FROM AUTHOR]

Absorption contrast between the terahertz (THz) frequency range of fatty and cancer tissues allows cancer diagnosis by THz imaging. We successfully demonstrated the ability of THz imaging to measure small breast cancer volume in the subcutaneous xenotransplantation mouse models even without external comparison. We estimated the volume detection limitation of the fiber-based THz scanning imaging system using a highly sensitive cryogenic-temperature-operated Schottky diode detector to be smaller than 1 mm3, thus showing the potential application of this technique in preliminary early cancer diagnosis. [ABSTRACT FROM AUTHOR]

This paper proposes a novel terahertz (THz) image recovery algorithm and a new THz image dataset is publicly available. Because of transmission noise, artificial errors and problems with diffraction phenomena are among the main problems that cause degradation of THz images. The point spread function (PSF) in real transmission is first obtained using a terahertz imaging system. Experiments can generate degraded images and clean image datasets for supervised training of neural networks via terahertz PSF. A Depthwise Dense Instantiation Normalization Block (DwDIN Block) has been proposed to recover the datasets. We constructed a powerful multi-stage network (DwDINet) at each scale by utilizing the DwDIN module. For input and output, the results of each stage of the network and the original images are taken as input into the next layer in order to enhance the background information. DwDINet achieves state-of-the-art (SOTA) results on the two main THz datasets. To verify the generality of the model, we performed experiments on various image recovery tasks. According to the experimental results, the proposed model is of great potential for the field of low-level image clarification. [ABSTRACT FROM AUTHOR]

During the fabrication process of the factory joint in submarine cables, manual cutting and polishing are required, which can easily result in scratches, pits, and other defects. Meanwhile, due to the limitations of insulation recovery and injection molding technology, air gap defects are prone to occur inside the joints, posing significant safety hazards. In order to preemptively detect such defects before the cable is put into operation, this study mimics the insulation recovery process of the cable joint to create micron air gaps inside the insulation and uses terahertz imaging technology to visualize them. The results indicate that 153 μm and 76 μm arc-shaped air gaps can be clearly recorded by terahertz imaging systems. Meanwhile, terahertz time-domain signals clearly show that the electromagnetic waves experience a significant attenuation of up to 45 % after passing through the arc-shaped air gap, and their energy exhibits an uneven distribution. To explain the propagation phenomena of terahertz waves in micron air gaps, this study proposes a propagation model based on absorption effects, reflection effects, partial interference effects, and off-axis Gaussian scattering effects. This paper proved the potential of terahertz waves in detecting air gap defects in cables joint. The associated theoretical models can be utilized to develop targeted imaging algorithms. • This study created153um and 76um arc-shaped micron air gaps at homogeneous interfaces, visualized using terahertz imaging. • Terahertz time-domain signals experienced significant attenuation (up to 45 %). • The energy of terahertz waves show uneven distribution after passing through arc-shaped air gaps. • Proposed a propagation model for terahertz waves in micrometer scale arc-shaped air gaps. [ABSTRACT FROM AUTHOR]

We present a photoconductive terahertz detector that employs a plasmonic nanocavity to offer high-sensitivity and broadband operation when used in a terahertz time-domain spectroscopy system even at very low optical pump power levels. By employing a plasmonic nanocavity, all of the photocarriers are generated within a 100 nm distance from the photoconductor contact electrodes, enabling a short transport time for almost all of the photogenerated carriers. As a result, the photoconductive detector maintains high quantum efficiency and ultrafast operation simultaneously, enabling high-sensitivity and broadband operation at very low optical pump power levels. We utilize a photoconductive detector based on a plasmonic nanocavity optimized for operation at a 770 nm optical wavelength in a terahertz time-domain spectroscopy system and demonstrate a 100 dB signal-to-noise ratio and a 0.1–6 THz noise-equivalent bandwidth at a record-low average optical pump power of 0.1 mW, compared to the state-of-the-art photoconductive terahertz detectors. The extremely low optical power budget of the demonstrated photoconductive detector makes this detector attractive for multi-pixel terahertz imaging systems. [ABSTRACT FROM AUTHOR]

Fine‐scale (submillimeter to centimeter) depositional and diagenetic features encountered during the Curiosity rover's traverse in Gale crater provide a means to understand the geologic history of Vera Rubin ridge (VRR). VRR is a topographically high feature on the lower north slope of Aeolis Mons, a 5‐km high stratified mound within Gale crater. We use high‐spatial resolution images from the Mars Hand Lens Imager (MAHLI) as well as grain sizes estimated with the Gini index mean score technique that uses ChemCam Laser‐Induced Breakdown Spectroscopy (LIBS) chemical data to constrain the postdepositional history of the strata exposed on this ridge. MAHLI images were used to examine the color, grain size, and style of lamination of the host rocks, as well as to explore the occurrence of nodules, diagenetic crystals, pits, and a variety of dark‐gray iron‐rich features. This survey revealed abundant and widespread diagenetic features within the rocks exposed on VRR and demonstrated that rock targets estimated to be coarser generally contain more diagenetic features than those estimated to have finer grains, which indicate that grain size may have influenced the degree and type of diagenesis. A subset of rocks within VRR are gray in color and exhibit the highest proportion of diagenetic features. We suggest that these targets experienced a different diagenetic history than the other rocks on VRR and hypothesize that redistribution and recrystallization of iron within specific intervals may have resulted in both the gray color and the abundance of dark‐gray iron‐rich diagenetic features. Plain Language Summary: We use high resolution images to identify submillimeter‐to‐centimeter scale features that resulted from either primary deposition or alteration of the rocks after the sediment was deposited at Vera Rubin ridge (VRR). VRR is located on the flank of Aeolis Mons, the 5‐km high mound that occupies the center of Gale crater on Mars. We find that the majority of these fine‐scale features were created after the initial deposition of sediment, when water existed in the subsurface and interacted with and altered the rocks. A subset of these features is dark gray in color and have iron‐rich compositions that suggest iron remobilization and recrystallization within the host rock. We find that rocks with larger grain sizes also show more postdepositional alteration features, suggesting that grain size influenced where water moved and interacted with rocks in the subsurface. Key Points: We investigate depositional and diagenetic features on Vera Rubin ridge using high‐spatial resolution imagesObservations of submillimeter‐to‐centimeter scale features indicate extensive diagenesis occurred in this areaSubtle deviations in grain size likely contributed to the distribution and variability of diagenesis [ABSTRACT FROM AUTHOR]

A high numerical aperture (NA = 0.92) square all-silicon metalens is designed for a wavelength of λ= 0.11883 mm with a focal length of 16.8λ and a side length of 104.4λ. Based on the waveguide effect, anisotropic rectangular silicon pillars with different dimensions are arranged on the silicon substrate according to the conical phase distribution, and then the rotation angle of each rectangular pillar is adjusted to realize the polarization conversion. Furthermore, a polarization-independent square antireflection silicon pillar array is proposed to be patterned at the other side of the substrate. The metalens can efficiently convert the normally incident x- (or y-) polarized plane wave into the radially (or azimuthally) polarized beam and conduct the sub-diffraction tight-focusing simultaneously, thus the shape of the focus can be flexibly switched. The numerical simulation results show that, under high-NA conditions, our metalens can obtain a tighter focus compared with linearly polarized metalens and a longer depth of focus (DOF) compared with metalens based on the hyperboloidal phase distribution. Under x- (or y-) polarized plane wave incidence, a circular-spot-shaped (or doughnut-shaped) focus with a full width at half maximum of 0.4λ (or 0.3λ) is obtained and the DOF is 26.509λ (or 18.935λ). In addition, under x-polarized beam incidence, the influence of the incident beam intensity distribution on the focusing results is also discussed. The use of a single material makes our metalens more favorable in fabrication and it is expected that our study is of great significance for applying terahertz wave in imaging and communication systems, high-resolution microscopy, optical coherence tomography and manipulation of particles. [ABSTRACT FROM AUTHOR]

Fingerprints are unique to primates and koalas but what advantages do these features of our hands and feet provide us compared with the smooth pads of carnivorans, e.g., feline or ursine species? It has been argued that the epidermal ridges on finger pads decrease friction when in contact with smooth surfaces, promote interlocking with rough surfaces, channel excess water, prevent blistering, and enhance tactile sensitivity. Here, we found that they were at the origin of a moisture-regulating mechanism, which ensures an optimal hydration of the keratin layer of the skin for maximizing the friction and reducing the probability of catastrophic slip due to the hydrodynamic formation of a fluid layer. When in contact with impermeable surfaces, the occlusion of the sweat from the pores in the ridges promotes plasticization of the skin, dramatically increasing friction. Occlusion and external moisture could cause an excess of water that would defeat the natural hydration balance. However, we have demonstrated using femtosecond laser-based polarization-tunable terahertz wave spectroscopic imaging and infrared optical coherence tomography that the moisture regulation may be explained by a combination of a microfluidic capillary evaporation mechanism and a sweat pore blocking mechanism. This results in maintaining an optimal amount of moisture in the furrows that maximizes the friction irrespective of whether a finger pad is initially wet or dry. Thus, abundant low-flow sweat glands and epidermal furrows have provided primates with the evolutionary advantage in dry and wet conditions of manipulative and locomotive abilities not available to other animals. [ABSTRACT FROM AUTHOR]

This letter experimentally demonstrates 220 GHz transmission‐based terahertz imaging based on an integrated terahertz 1 × 16 array detector. The array detector consists of a 3D‐printed lens array, 16 detector chips and a chip board. Each detector chip contains a planar bowtie antenna printed on a 50 μm‐thick quartz laminate, where a GaAs Schottky barrier diode is loaded in the center of the antenna. The detector shows excellent performance with an average responsivity of 1650 V/W and an average noise equivalent power of 3.6 pW/√Hz at 220 GHz. Based on the proposed array detector, a transmission‐based terahertz imaging system is built, including a 220 GHz illumination source, a servo system, a signal processing unit and a 280 mm‐diameter doubly‐hyperbolic Polytetrafluorethylen lens. A lateral resolution of 8 mm at a standing‐off distance of 1.5 m is achieved, approaching to the theoretical result. [ABSTRACT FROM AUTHOR]

Sensing systems that can ensure product reliability in a broad range of environments are urgently required for broader application of the internet of things. Imaging technologies based on the terahertz (THz) frequency are considered a promising solution to the challenge of inspecting industrial products in a nondestructive manner. However, current THz imaging systems require bulky and complicated components that hamper their practical application. Therefore, we herein present flexible THz imaging systems based on single-walled carbon nanotube (CNT) films that leverage the material's advantages of mechanical strength, broad THz absorption, high thermoelectric power, and flexibility. This work investigates and optimizes the physical parameters that govern the detection sensitivity of the proposed CNT THz detector, and then tests a flexible THz imaging system based on the optimization. These imaging systems can be used in a wide range of industrial sensing applications, including inline pharmaceutical quality screening, multi-view imaging, and portable THz imaging for nondestructive quality testing of industrial products, with no bulky measurement components. Image 1 [ABSTRACT FROM AUTHOR]

Featured Application: Convenient and easy-to-use detectors for accurate adjustment and control of spatial mode profiles without additional focusing components for various weak power THz sources operating either in time-domain or continuous wave regimes. Terahertz (THz) imaging and spectroscopy set-ups require fine optical alignment or precise control of spatial mode profile. We demonstrate universal, convenient and easy-to-use imaging—resonant and broadband antenna coupled ultrasensitive titanium-based—dedicated to accurately adjust and control spatial mode profiles without additional focusing optical components of weak power THz sources. Versatile operation of the devices is shown using different kinds of THz—electronic multiplier sources, optical THz mixer-based frequency domain and femtosecond optoelectronic THz time-domain spectrometers as well as optically pumped molecular THz laser. Features of the microbolometers within 0.15–0.6 THz range are exposed and discussed, their ability to detect spatial mode profiles beyond the antennas resonances, up to 2.52 THz, are explored. Polarization-sensitive mode control possibilities are examined in details. The suitability of the resonant antenna-coupled microbolometers to resolve low-absorbing objects at 0.3 THz is revealed via direct, dark field and phase contrast imaging techniques as well. [ABSTRACT FROM AUTHOR]