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Biological applications predominantly rely upon terahertz (THz) frequency range for segregation of abnormal tissues from normal ones using polarization imaging. Achromatic THz phase retarders are an essential part of THz polarization imaging system. For the first time, design and simulation of a terahertz achromatic quarter-wave plate using two different birefringent materials, i.e. crystalline quartz and lithium niobate (LiNbO3), are demonstrated in the frequency band ranging from 0.8 to 1.3 THz. Phase deviation of the proposed system is within 90° ± 2.2°, which is achieved with only two crystals having different thicknesses without varying their orientation angles. The proposed system exhibits promising behaviour in terms of thickness variation and transmittance. Distinguishable retardance stability in terms of phase deviation and remarkable reduction in overall system thickness has been achieved over the said THz frequency range as compared to other reported systems. This designed achromatic phase retarder finds potential application in THz imaging system used for early detection of colon and gastric cancers in this particular frequency band. [ABSTRACT FROM AUTHOR]

Terahertz (THz) computed tomography (THz CT) exhibits the potential to provide a wealth of data, surpassing that of THz tomographic imaging in applications such as detecting embedded defects, particularly defect evolution within a glass fiber-reinforced polymer. To realize high-resolution THz CT, a systematic approach guided by wave propagation simulation was employed. First, the front wave of the THz beam was fine-tuned to realize a beam diameter of

Here rapid and non-destructive detection of tomato root phenotypes was realized using THz (Terahertz) imaging and ensemble learning. Firstly, three groups of tomato seedlings were grown in three sandy soil substrates watered with different concentrations of nitrogen nutrient solution. 12 groups of tomato root seedlings were collected for THz imaging during 20 days of growth. Secondly, the THz pseudo-color map was imaged on the root system after the optimal reconstruction of time-domain peaks. Color value of pseudo-color map was directly related to the intensity of the root system's THz absorption. The noisy data were removed from the overlapping and main root region in the THz maps of the root system, according to the color value of pseudo-color map. Again, the Rosenfeld refinement was used to obtain the skeleton map of tomato root system. The lengths of the root system pixel points were calculated using the sliding window traversal method. Finally, THz time-domain data and refractive index data were extracted from the effective feature region of the root system. The tomato root length was predicted by the Stacking ensemble model. Among them, the first layer of Stacking ensemble model was integrated with the four base models, namely, GBDT (gradient boosting decision tree), XGBoost (eXtreme gradient boosting), Catboost (categorical boosting), and Adaboost (adaptive boosting). The second layer was employed the linear regression as a meta-learner, in order to prevent the over data fitting. A 5-fold cross-validation was used to train the base models. The extraction of root skeleton showed that the RGB three-channel separation was effectively removed the overlapping roots and the spectral data containing noise, in order to fully display the root framework. Therefore, the calculation error of root length was reduced significantly. Only 4.16% was found in the average relative error of tomato root length value calculated by between the THz false-color image and Image-J software. The linear fitting determination coefficient of two types was 0.967. The THz time domain and refractive index of Stacking model were effectively predicted the root length, indicating the better performance than that of the sub-models. The optimal prediction of tomato root length was obtained using the THz data after WD denoising. The optimal determination coefficient of THz time-domain data prediction was 0.999, and the minimum root-mean-square error was 0.743. The optimal determination coefficient of THz refractive index data prediction was 0.998, and the root-mean-square error was 0.976. The length of tomato root system was accurately calculated to predict the root length phenotypes using THz spectra images. The finding can provide a theoretical basis for the rapid and nondestructive detection of root system phenotypes. Further research can be required to nondestructively detect the complete parameters of root phenotype, and then detect the root internal characteristics using THz spectral data. [ABSTRACT FROM AUTHOR]

Abstract Plastic foams, near-ubiquitous in everyday life and industry, show properties that depend primarily on density. Density measurement, although straightforward in principle, is not always easy. As such, while several methods are available, plastic foam industry is not yet supported with a standard technique that effectively enables to control density maps. To overcome this issue, this paper proposes Terahertz (THz) time-of-flight imaging using normal reflection measurements as a fast, relatively cheap, contactless, non-destructive and non-dangerous way to map plastic foam density, based on the expected relationship between density and refractive index. The approach is demonstrated in the case of polypropylene foams. First, the relationship between the estimated effective refractive index and the polypropylene foam density is derived by characterizing a set of carefully crafted samples having uniform density in the range 70–900 kg/m3. The obtained calibration curve subtends a linear relationship between the density and the refractive index in the range of interest. This relationship is validated against a set of test samples, whose estimated average densities are consistent with the nominal ones, with an absolute error lower than 10 kg/m3 and a percentage error on the estimate of 5%. Exploiting the calibration curve, it is possible to build quantitative images depicting the spatial distribution of the sample density. THz images are able to reveal the non-uniform density distribution of some samples, which cannot be appreciated from visual inspection. Finally, the complex spatial density pattern of a graded foam sample is characterized and quantitatively compared with the density map obtained via X-ray microscopy. The comparison confirms that the proposed THz approach successfully determines the density pattern with an accuracy and a spatial scale variability compliant with those commonly required for plastic foam density estimate.

Plastic foams, near-ubiquitous in everyday life and industry, show properties that depend primarily on density. Density measurement, although straightforward in principle, is not always easy. As such, while several methods are available, plastic foam industry is not yet supported with a standard technique that effectively enables to control density maps. To overcome this issue, this paper proposes Terahertz (THz) time-of-flight imaging using normal reflection measurements as a fast, relatively cheap, contactless, non-destructive and non-dangerous way to map plastic foam density, based on the expected relationship between density and refractive index. The approach is demonstrated in the case of polypropylene foams. First, the relationship between the estimated effective refractive index and the polypropylene foam density is derived by characterizing a set of carefully crafted samples having uniform density in the range 70–900 kg/m3. The obtained calibration curve subtends a linear relationship between the density and the refractive index in the range of interest. This relationship is validated against a set of test samples, whose estimated average densities are consistent with the nominal ones, with an absolute error lower than 10 kg/m3 and a percentage error on the estimate of 5%. Exploiting the calibration curve, it is possible to build quantitative images depicting the spatial distribution of the sample density. THz images are able to reveal the non-uniform density distribution of some samples, which cannot be appreciated from visual inspection. Finally, the complex spatial density pattern of a graded foam sample is characterized and quantitatively compared with the density map obtained via X-ray microscopy. The comparison confirms that the proposed THz approach successfully determines the density pattern with an accuracy and a spatial scale variability compliant with those commonly required for plastic foam density estimate. [ABSTRACT FROM AUTHOR]

Terahertz frequency range imaging has become more and more attractive for a wide range of practical applications; however, further component optimization is still required. The presented research introduces 3D-printed high-impact polystyrene (HIPS) beam-shaping components for the terahertz range. Gaussian, Bessel, and Airy beam-shaping structures are fabricated, and different combinations are employed to evaluate imaging system performance. The combination of the Gaussian element as focusing and the Bessel element as collecting is revealed to be similarly efficient and less sensitive to misalignment than the classical Gaussian–Gaussian element setup. The presented research paves the way for introducing cost-effective structured light beam-shaping elements into THz imaging systems.

In this paper, the design and optimization of a wideband THz metamaterial absorber (MMA) are proposed. By simulation, we reached four structures with absorptions higher than 50%, 70%, 80%, and 90%, with relative absorption bandwidths (RABWs) of 1.43, 1.29, 0.93, and 0.72, respectively. Terahertz absorbers can be used in many potential applications, such as in imaging, energy harvesting, scattering reduction, and thermal sensing. Our intended application was to use the optimal absorber on a thermal detector for detectivity over a wide THz range. Since broadband absorption in the range of 0.3 to 2 terahertz is considered for use in medical imaging, the MMA with more than 50% absorption in the range of 0.35-2.1 THz was selected. The designs were also intended to have the capability of being implemented on different devices, such as bolometers. The cost of the fabrication of the proposed absorber was also low because of the implementation of a single-layer MMA design and the utilization of affordable and more accessible materials and techniques. Our proposed structure had a minimum feature size of 3 μm, making the fabrication process convenient using the standard photolithography method as well. We used thin layers of nickel as the metal for both the single-layer pattern and ground layer, which were placed on the front and back sides of the structure, respectively. The nickel thin film layers were deposited using the sputtering technique and separated by a dielectric layer. The material chosen for the dielectric layer was SU8, which has proper electromagnetic properties and also good adhesion to nickel. Characterization of the fabricated absorber was performed using a terahertz spectroscopy system, and the experimental results verified the high absorption of the sample. [ABSTRACT FROM AUTHOR]

The ever-increasing popularity of the terahertz (THz) frequency range reveals the increasingly obvious applicability limiting factor - the price of the final setup. This study is intended to contribute to the solution by evaluating the THz frequency range suitability of the optical components, fabricated using two easily accessible materials - high impact polystyrene (HIPS) and paraffin. The primary analysis using time-domain spectroscopy (TDS) revealed promising results, as both materials had sufficient refractive indexes of n ≈ 1.55 and a high transmittance. That allowed one to assume the feasibility of creating a low-cost THz frequency range lens. Lenses of focal lengths of f = 20, 30, 40 mm were fabricated using extrusion 3D printing and paraffin moulding. The produced lenses showcased the satisfactory beam focusing ability, comparable to that of already existing much less cost-efficient solutions. The THz imaging using the fabricated lenses has successfully been realized, proving the applicational possibilities of the imaging system with the proposed low-cost components employed. [ABSTRACT FROM AUTHOR]

Organelles play core roles in living beings, especially in internal cellular actions, but the hidden information inside the cell is difficult to extract in a label-free manner. In recent years, terahertz (THz) imaging has attracted much attention because of its penetration depth in nonpolar and non-metallic materials and label-free, non-invasive and non-ionizing ability to obtain the interior information of bio-samples. However, the low spatial resolution of traditional far-field THz imaging systems and the weak dielectric contrast of biological samples hinder the application of this technology in the biological field. In this paper, we used an advanced THz scattering near-field imaging method for detecting chloroplasts on gold substrate with nano-flatness combined with an image processing method to remove the background noise and successfully obtained the subcellular-grade internal reticular structure from an Arabidopsis chloroplast THz image. In contrast, little inner information could be observed in the tea chloroplast in similar THz images. Further, transmission electron microscopy (TEM) and mass spectroscopy (MS) were also used to detect structural and chemical differences inside the chloroplasts of Arabidopsis and tea plants. The preliminary results suggested that the interspecific different THz information is related to the internal spatial structures of chloroplasts and metabolite differences among species. Therefore, this method could open a new way to study the structure of individual organelles. [ABSTRACT FROM AUTHOR]

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

The characterization of the layered structure is of paramount importance for the study of maiolica wares (tin-glazed pottery). This paper presents the potentialities of Terahertz Time-Domain Imaging (THz-TDI) as a tool to perform non-invasive stratigraphic analysis of the maiolica objects under test. Samples with different types of stratigraphy, as testified by preliminary SEM-EDS analysis, were investigated by THz-TDI in laboratory conditions. The collected THz data were processed by means of noise filtering procedures and a time-of-flight-based imaging approach and the achieved results corroborate the ability to identify glaze layers, whose electromagnetic properties, i.e., the refraction index and the dielectric permittivity value, were estimated by taking into account both THz-TDI and SEM-EDS data. However, layers applied over the white glaze (namely, a transparent overglaze and a luster decoration) were not detected, probably since their thickness is below the range resolution of the adopted THz-TDI system. Morphological features hidden under the surface were also identified and they provided information about the manufacturing technique. [ABSTRACT FROM AUTHOR]

Abstract 6G is believed to go beyond communication and provide integrated sensing and computing capabilities for a vision of Connected Intelligence with everything connected, everything sensed, and everything intelligent. Integrated sensing and communication will play a vital role for the fusion of physical and cyber worlds. The exploration of higher frequency bands, larger bandwidth, and more advanced large antenna technologies is paving the way towards the goal. In particular, the study of THz opens the possibility to have high resolution sensing and imaging capability on a communication mobile device. In this paper, we take a step along this direction and justify such possibility by building a THz sensing prototype with millimeter level imaging resolution while considering the physical aperture constraint of typical mobile device.

Terahertz (THz) imaging is a non-invasive and high spatial-resolution technique that uses non-ionizing electromagnetic signals in a frequency range of 0.1–10 THz. Hence, this article focuses on diverse THz imaging techniques, THz antennas and designing methods, image reconstruction algorithms, and its applications. The antennas include planar patch, photoconductive, dielectric-resonator, substrate-integrated waveguide, wire, and wave guide. In this study, it is noted that antennas with high efficient conducting materials having a compact size, high gain, and high directional properties are required for THz imaging. The image reconstruction algorithms cover back-projection algorithm, range migration algorithm, phase-shift migration algorithm, single-band compressed sensing reconstruction and compressed sensing hyper spectral image reconstruction. High-resolution image reconstruction algorithms and challenges are also reported. From this study, it is noted that a multi–input–multi–output-based phase shift migration algorithm is used for THz imaging due to its high-accuracy and low computation time. It is also noted that the image quality can be enhanced by introducing an inverse fresnel diffraction algorithm into THz images retrieved by compressed sensing. Applications of THz imaging focus on cell detection (tissue-detection, cancer-detection, and bacteria-detection), concealed objects detection, food safety and quality inspection, and monitoring the water level of plant leaves. [ABSTRACT FROM AUTHOR]

In this study, the possibility of the industrial application of terahertz (THz) imaging technology was verified. It was applied to the inspection of voids in multistack semiconductors that require safe inspection and to the high-resolution detection and inspection of foreign substances in tablets in the pharmaceutical field. To acquire a high-resolution THz image, a resonant slit probe operating in the THz region was designed, and a high-speed scanning system was established. For the inspection of a multistack semiconductor, a lateral scan method was proposed, and voids with a diameter of 0.5 mm in the multistack semiconductor were detected. In addition, the proposed probe even enables the distinguishment of the positions of voids in the multistack semiconductor. For pharmaceutical inspection, we investigated the application of THz imaging to detect mixed foreign objects frequently occurring in the tablet manufacturing process. For metals, plastics, and rubber, which are the most frequently mixed materials in the tablet manufacturing process, the foreign objects were identified in tablets using a transmission THz system. The measured THz image was compared with the conventional X-ray test result to confirm the potential of THz inspection. In the X-ray image, only metal and some polymer foreign objects were detected. In contrast, in the THz image, although the materials could not be distinguished, most foreign substances were detected. Consequently, the THz imaging test was verified as a possible new tool in fields where X-ray or existing tests are not possible.

In this paper, the design and optimization of a wideband THz metamaterial absorber (MMA) are proposed. By simulation, we reached four structures with absorptions higher than 50%, 70%, 80%, and 90%, with relative absorption bandwidths (RABWs) of 1.43, 1.29, 0.93, and 0.72, respectively. Terahertz absorbers can be used in many potential applications, such as in imaging, energy harvesting, scattering reduction, and thermal sensing. Our intended application was to use the optimal absorber on a thermal detector for detectivity over a wide THz range. Since broadband absorption in the range of 0.3 to 2 terahertz is considered for use in medical imaging, the MMA with more than 50% absorption in the range of 0.35-2.1 THz was selected. The designs were also intended to have the capability of being implemented on different devices, such as bolometers. The cost of the fabrication of the proposed absorber was also low because of the implementation of a single-layer MMA design and the utilization of affordable and more accessible materials and techniques. Our proposed structure had a minimum feature size of 3 μm, making the fabrication process convenient using the standard photolithography method as well. We used thin layers of nickel as the metal for both the single-layer pattern and ground layer, which were placed on the front and back sides of the structure, respectively. The nickel thin film layers were deposited using the sputtering technique and separated by a dielectric layer. The material chosen for the dielectric layer was SU8, which has proper electromagnetic properties and also good adhesion to nickel. Characterization of the fabricated absorber was performed using a terahertz spectroscopy system, and the experimental results verified the high absorption of the sample.

Abstract Background The terahertz radiation is a specific part of the electromagnetic radiation spectrum and has multiple significant applications in multiple scientific researches such as the applications in the medicine. An important application of the terahertz is its use in tumor imaging which is very important in the tumor surgery; however, lots of physicians and workers in the medical field have little information or having no information at all, dealing with this significant part of the electromagnetic spectrum. Results In this work, we interviewed a number of local surgeons in Syrian Arab Republic, who reported that they visually delineate the contour of tumors to be removed, and in order to reduce the number of future possible interventions, a large margin of healthy tissue is often excised. Furthermore, a number of pathologists who reported that preparing samples of excised tissues for examination takes a long period of time which may extend to several days, and that the results of histopathology indicate in some cases the integrity of removed tissues. Conclusion We have found that a significant number of participants in the survey demonstrated that the importance of dealing with terahertz imaging and terahertz spectroscopy, encouraging to implement the technique in the Syrian Arab Republic.

The fourth industrial revolution (4IR or Industry 4.0), its communication and information enabling technologies are revolutionizing the production and services applications. In the healthcare domain, electronic health (eHealth) is evolving towards healthcare 4.0 through the Internet of things, big data analytics, Cloud and Fog computing technologies. There is need for early cancer detection, which can be achieved by new imaging technologies for example Terahertz imaging and Infrared thermography. The integration of such new imaging modalities and wearable sensor networks with computer aided diagnosis systems (CAD) and IoT systems with interconnectivity and interoperability could enable continuous, remote observation of cancer diagnosed patients by experts to monitor the success of treatment procedures, side effects of drugs, statistical analysis and making predictions of cancer based on age and demographics etc. for timely intervention and personalized care. In the healthcare 4.0 paradigm, there is need not only for smart diagnosis but also care through remote patient monitoring. The vital signs associated with cancer and clinical information can be integrated to make the system culminate into a smart system with health records. This work proposes an IoT enabled deep learning framework for classification of breast images; we propose the development of both custom CNN model and fine-tuned, pretrained CNN models capable of multimodal image classification. IoT simulation and cloud processing is performed in the ThingSpeak cloud platform with email alert capability. The performance of the CNN models has been evaluated using the accuracy, sensitivity and specificity metrics which achieved 98.44%, 98.44% and 98.45% respectively for the custom CNN model together with the receiver operating characteristics curves and confusion matrices. [ABSTRACT FROM AUTHOR]

In this work, terahertz time‐domain spectroscopy (THz TDS) was applied to monitor strain distribution as a function of mechanical loading through a passive composite sensor. The composite sensor was made up of strontium titanate (STO) particles. Strain distribution was measured by analysing the change in THz pulse amplitude while it passes through the composite sensor. The change in THz pulse amplitude is related to change in dielectric permittivity. The change in dielectric permittivity induced in the STO composite layer due to deformation leads to a change in the time of arrival (ToA) of the electromagnetic pulse (EMP) in THz band. This change in the sub‐millimetre domain is correlated to the strain and can be mapped for reproducing strain distribution around different geometries. The measurements of strain mapping are compared with finite element simulations and digital image correlation (DIC) measurements, showing similar strain contours. [ABSTRACT FROM AUTHOR]

6G is believed to go beyond communication and provide integrated sensing and computing capabilities for a vision of Connected Intelligence with everything connected, everything sensed, and everything intelligent. Integrated sensing and communication will play a vital role for the fusion of physical and cyber worlds. The exploration of higher frequency bands, larger bandwidth, and more advanced large antenna technologies is paving the way towards the goal. In particular, the study of THz opens the possibility to have high resolution sensing and imaging capability on a communication mobile device. In this paper, we take a step along this direction and justify such possibility by building a THz sensing prototype with millimeter level imaging resolution while considering the physical aperture constraint of typical mobile device. [ABSTRACT FROM AUTHOR]

A fully integrated 490-GHz receiver (RX) adopting a dual-locking receiver-based FLL (DL-RBFLL) is presented. The proposed RBFLL structure saves the power consumption by reusing the existing blocks in RX instead of the power-hungry blocks such as dividers and buffers operating at sub-THz. Contrary to the single-loop implementation, the dual-loop RBFLL, which consists of the coarse and fine locking loops, extends the locking range by six times with negligible additional power dissipation. In the RF front-end (FE), the proposed 2nd-order sub-harmonic mixer (SHM) enhances interport isolation and suppresses the undesired LO leakage using a simple passive network. In the IF path, a 2-stage low-noise amplifier (LNA) followed by a 10-stage programmable gain amplifier provides a controllable gain of 0–80 dB with 4-dB step. An N-path filter, which serves as a high-Q bandpass noise filter, improves SNR in the IF path and thus RX sensitivity. Implemented in a 65-nm CMOS, the 490-GHz RX achieves the measured noise figure of 51.1 dB and noise equivalent power (NEP) of 0.85 pW/Hz $^{0.5}$ for the noise bandwidth of 17 MHz. The estimated sensitivity of the proposed RX is −92.9 dBm for a 1 kHz noise bandwidth, which dissipates 31.8 mW from a 1.2-V supply. [ABSTRACT FROM AUTHOR]

Chestnut crops are threatened by fungal pathogens such as Gnomoniopsis castaneae , which cause significant degradation of quality. Early detection of such infections is crucial to maintain the quality of chestnuts in the food industry. This study explores the application of Terahertz Time-Domain Hyperspectral Imaging (THz-TDHIS) combined with unsupervised learning techniques to identify fungal infections in chestnuts. Unlike conventional methods that rely on light attenuation, this approach leverages the unique spectral signatures of infected tissues. By employing Principal Component Analysis, K-Means Clustering, and Agglomerative Clustering, we effectively differentiate between healthy and infected portions of chestnuts. Our findings indicate that spectral features, rather than just intensity variations, provide more reliable markers for infection. In addition, we demonstrate that these methods enable the quantification of the degree of infection in chestnuts. The robustness of these unsupervised learning methods in handling large and heterogeneous data sets further underscores their potential in agricultural applications. This integrated THz-TDHIS and machine learning approach presents a promising solution to ensure chestnut quality and safety. [Display omitted] • THz Spectroscopy and Imaging combined with unsupervised machine learning technique enables a correct classification of the quality of chestnut; • A quantitative assessment of the percentage of healthy chestnut portions is demonstrated; • Our experiment is a proof-of-concept for a non-invasive and real-time monitoring of chestnut quality. [ABSTRACT FROM AUTHOR]

Terahertz (THz) pulse/time-domain imaging attracted increased interest in recent years mostly due to its ability to extract dielectric properties of sample materials (i.e., absorption coefficient and the refraction index) from the amplitude and phase of each spectral component of the THz pulse. The resulting data from a THz time-domain system represents a 3-dimensional (3D) hyperspectral cube which contains several 2D images corresponding to different frequencies or bands. Due to a frequency-dependent non-zero THz beam waist, these 2D images are corrupted by blurring artifacts: a THz beam waist is wider on lower frequencies leading to more blurry corresponding 2D images. At higher frequencies, the beam waist is smaller resulting in sharper, but noisier images due to the decrease in the THz signal amplitude. The main focus of this work is the joint reduction of blur and noise from THz time-domain images. We propose two instances of a fast joint deblurring and denoising approach which is able to deal with THz time-domain images corrupted by different noise types and frequency-dependent blur. The experiments performed on synthetic and real THz time-domain images show that the proposed approach outperforms conventional 2D deblurring approaches and methods tailored to remote sensing hyperspectral images. To the best of our knowledge, this is the first time that a joint deblurring and denoising approach tailored to THz time-domain images is proposed taking into consideration band-dependent blur and different noise types.

Among image reconstruction methods, Fourier transform-based techniques provide computationally better performance. However, conventional Fourier-based reconstruction techniques require uniform data sampling at the radar aperture. In this paper, a multiple-input multiple-output (MIMO) scenario for near-field (NF) terahertz imaging systems is considered. A compressive-sensing-based method compatible with efficient fast Fourier-based techniques for image reconstruction is proposed. To reduce the error due to the multistatic array topology in the NF, a multistatic-to-monostatic conversion is used. Employing the proposed method significantly reduces the number of antennas and channels. This, in addition to saving hardware resources, can improve the overall performance of the system depending on the type of channel access scheme. The results based on both numerical and electromagnetic data, presented as reconstructed images of the scene, confirm the performance of the proposed method.

The benefits of terahertz (THz) radiation have increased its use, especially in imaging systems. Recently, the use of a linear sparse periodic array (SPA) has been proposed as an effective solution for two-dimensional (2D) scanning in THz imaging systems. However, the special multistatic structure of the SPA is such that it is not possible to apply fast Fourier transform-based techniques directly in the near-field (NF). Therefore, in this paper, a fast processing approach based on two Fourier techniques compatible with linear SPA is presented for NF THz imaging. In this approach, we first employ a multistatic-to-monostatic conversion to reduce phase errors due to NF multistatic imaging. Then, to improve the quality of the results, we mathematically derive an interpolation formula to counteract the non-uniform spacing of the virtual array. The modified data is then processed by three rapid techniques (fast Fourier transform (FFT)-inverse fast Fourier transform, matched filtering and a novel 1D FFT-based technique with low computational complexity) to obtain reconstructed images of the scene. Numerical and experimental results confirm the satisfactory performance of the proposed approach in terms of both the computational time and the quality of the reconstructed images. [ABSTRACT FROM AUTHOR]

Ultrabroadband photodetection is of great significance in numerous cutting‐edge technologies including imaging, communications, and medicine. However, since photon detectors are selective in wavelength and thermal detectors are slow in response, developing high performance and ultrabroadband photodetectors is extremely difficult. Herein, one demonstrates an ultrabroadband photoelectric detector covering visible, infrared, terahertz, and millimeter wave simultaneously based on single metal–Te–metal structure. Through the two kinds of photoelectric effect synergy of photoexcited electron–hole pairs and electromagnetic induced well effect, the detector achieves the responsivities of 0.793 A W−1 at 635 nm, 9.38 A W−1 at 1550 nm, 9.83 A W−1 at 0.305 THz, 24.8 A W−1 at 0.250 THz, 87.8 A W−1 at 0.172 THz, and 986 A W−1 at 0.022 THz, respectively. It also exhibits excellent polarization detection with a dichroic ratio of 468. The excellent performance of the detector is further verified by high‐resolution imaging experiments. Finally, the high stability of the detector is tested by long‐term deposition in air and high‐temperature aging. The strategy provides a recipe to achieve ultrabroadband photodetection with high sensitivity and fast response utilizing full photoelectric effect. [ABSTRACT FROM AUTHOR]

Terahertz (THz) imaging techniques have attracted significant attention and have developed rapidly in recent years. However, despite several advances, these techniques are still not mature, and their high cost and system complexity continue to limit their applications. In this article, the techniques for achieving a practical imaging system with a compact THz transceiver are addressed, while considering the limitations of the current technique. The aim is to provide a brief review of related topics, while also covering our recent progress, which can provide some general perspectives and contrasting approaches for realizing a practical THz imaging system. The continuous wave devices are mainly focused for their flexibility of balancing the imaging resolution and data acquisition time. The importance of transceiver integration is also discussed and illustrated by introducing a 600-GHz band micro-photonic interface for integrating a THz source and detector, with a single resonant tunneling diode as a transceiver. With regard to system issues, spatial sampling with mechanical beam-scanning is discussed as an intermediate approach for moving stage and array technology. The potential and limitations of this approach are evaluated, along with an elliptical reflector as an alternative to an f-theta lens owing to its low cost and simplicity. The combination of integrated devices, along with the mechanical beam-scanning, is also discussed for demonstrating our current concept of realizing a practical THz imaging system. [ABSTRACT FROM AUTHOR]

This study describes the design and characterization of a highly sensitive THz refractive index-based metamaterial sensor for the detection of biological samples. The proposed ultra-miniaturized THz sensor is constructed using a fan-shaped resonator enclosed within a square loop. The proposed sensor has a footprint of 0.353 $\lambda$ eff × 0.353 $\lambda$ eff where $\lambda$ eff is the wavelength calculated at the operating frequency of 4.87 THz. The sensitivity of the THz sensor is estimated using the absorption characteristics of the metamaterial. The estimated free space absorptivity is 99.9%. The proposed sensor is polarization insensitive due to the rotational symmetry and it is angularly stable up to 60°. The performance of the sensor is evaluated for different materials and biomedical samples. The results indicate that the proposed sensor has an average sensitivity of 1936 GHz/ Refractive Index Unit (RIU). Furthermore, the influence of the sample thickness is analysed, and the results are presented. From the results, it is inferred that the proposed THz sensor has a good Figure of Merit (FoM) and is suitable for chemical sensing, including the nucleic acids and other components present in the biological samples. [ABSTRACT FROM AUTHOR]

In this study, we demonstrate an Nb5N6 terahertz (THz) detector with radio frequency (RF) choke-enhanced dipole antenna structure for 0.3 THz detection. The maximum electric field intensity of 218 V/m is obtained by optimizing the parameters of the dipole antenna with RF choke. Compared to a dipole antenna without RF choke, the electric field intensity of that with RF choke is improved by 2.6 times. The RF choke-enhanced dipole antenna-coupled Nb5N6 THz detector is fabricated and characterized. The measured maximum responsivity of the detector is 1100 V/W at 0.308 THz, and the corresponding noise equivalent power (NEP) is 6.4 × 10–12 W/Hz1/2. The measured response time of the Nb5N6 THz detector is as low as 8.46 μs. Furthermore, the Nb5N6 THz detector is applied to a homemade THz transmission imaging system for demonstrating its performance. The THz imaging results of a blade and access card show that the contrast of the blade image is sharp and the components hidden within the access card are clearly visible. This indicates that the Nb5N6 THz detector can be used in THz imaging, particularly in THz active imaging, which will have greater application prospects.

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]

Recent theoretical and experimental research is triggering interest to technologies based on radiation in the region from ~0.1 to 20 Terahertz (THz). Today, this region of the electromagnetic (e.m.) spectrum is a frontier area for research in many disciplines. The technological roadmap of the THz radiation considers outdoor and indoor communications, security, drug detection, biometrics, food quality control, agriculture, medicine, semiconductors, and air pollution, and demands high-power and sub-ps compact sources, modern detectors, and new integrated systems. There are still many open questions regarding working at THz frequencies and with THz radiation. In particular, we need to invest in new methodologies and in materials exhibiting the unusual or exotic properties of THz. This book contains original papers dealing with some emerging THz applications, new devices, sources and detectors, and materials with advanced properties for applications in biomedicine, cultural heritage, technology, and space. [ABSTRACT FROM AUTHOR]

Material characterization in the 0.1 - 10 THz range has been a major topic of research since its first demonstration 30 years ago. Advances in terahertz generation, detection, and data acquisition have contributed to improved bandwidth, signal power and signal-to-noise ratio. However, material characterization is still performed using conventional spectroscopic measurement schemes which require detailed information about the test object's shape, location, orientation relative to the measurement system, and a reference measurement. Here, we present a method for reference-free material characterization of dielectric objects using a terahertz time-domain spectroscopy system without a priori knowledge about the object and its position. The proposed method is based on ellipsometry combined with lensless imaging for the estimation of the refractive index. The method is a multistage procedure designed for small test objects in reflection mode. The diffracted terahertz radiation is separated from the specular reflected radiation in a post-processing step to enable a valid material parameter estimation. In this way, small dielectric objects can be located, imaged with a sub-mm resolution, and their material parameters extracted.

Back projection algorithm (BPA) and range migration algorithm (RMA) are two common approaches for image reconstruction in synthetic aperture radar (SAR) imaging. Imaging is implemented in spatial domain and spatial frequency domain, respectively and these two methods are often considered to generate the same results. This is true in normal microwave/millimeter wave SAR imaging systems, since the object of interests is located far away and the applied wavelength is long. In this case, the location of the object is the most important issue and there is no impetus to image the object with very fine details. In case of terahertz (THz) imaging, we have found some differences by using the two algorithms above, including the position of the object and resolution in 2D and 3D imaging. Some insights are given and we believe there should be some guidelines regarding the pros and cons before choosing the exact algorithm, especially when high-performance imaging is pursued.

This paper presents the design and evaluation of a multi-band terahertz (THz) metamaterial-based biomedical (BioMed) sensor for blood component detection such as Deoxyribonucleic acid (DNA), Ribonucleic acid (RNA), plasma, water, Red Blood Cells (RBC), White Blood Cells (WBC) and haemoglobin. The proposed THz sensor is a multimode resonator with concentric square loops enclosing concentric octagonal loops. The proposed sensor has a metamaterial feature with a footprint of 0.164λeff × 0.164λeff. The THz sensor operates between 1 and 2 THz with four independently tunable operating bands, viz. 0.33, 0.90, 1.42 and 1.85 THz. The sensitivity of the THz sensor is estimated using the absorption characteristics, and the estimated average absorptivity is 99% for the proposed BioMed sensor. The THz sensor has a rotational symmetry offering both polarization and angular stability (up to 60°). The performance of the sensor is evaluated for different BioMed samples providing unique refractive indices. Furthermore, the impact of sample thickness on the sensor performance has been evaluated and presented. From the results, it is inferred that the proposed THz sensor is suitable for nucleic acids and other components detection in blood samples. [ABSTRACT FROM AUTHOR]

2D Bi2O2Se has shown great potential in photodetector from visible to infrared (IR) owing to its high mobility, ambient stability, and layer‐tunable bandgaps. However, for the terahertz (THz) band with longer wavelength and richer spectral information, there are few reports on the research of THz detection based on 2D materials. Herein, an antenna‐assisted Bi2O2Se photodetector is constructed to achieve broadband photodetection from IR to THz ranges driven by multi‐mechanism of electromagnetic waves to electrical conversion. The good tradeoff between the bandgap and high mobility results in a broad spectral detection. In the IR region, the nonequilibrium carriers result from photo‐induced electron‐hole pairs in the Bi2O2Se body. While in the THz region, the carriers are caused by the injected electrons from the metal electrodes by the electromagnetic‐induced well. The Bi2O2Se photodetector achieves a broadband responsivity of 58 A W‐1 at 1550 nm, 2.7 × 104 V W‐1 at 0.17 THz, and 1.9 × 108 V W‐1 at 0.029 THz, respectively. Surprisingly, an ultrafast response time of 476 ns and a quite low noise equivalent power of 0.2 pW Hz−1/2 are acquired at room temperature. Our researches exhibit promising prospects of Bi2O2Se in broadband detection, THz imaging, and ultrafast sensing. [ABSTRACT FROM AUTHOR]

Leaf water content is regarded as an important index of crop health in agriculture. It is in great need to develop a rapid, non-destructive and accurate diagnose method which can measure and assess water level of different types of crops. In this research, winter wheat leaves were selected and the reliability and applicability of THz imaging technique for destructive and non-destructive water content monitoring were studied. Based on the measurement of THz transmission, smaller water loss in distal regions of a winter wheat leaf is shown when compared with that in basal regions during the natural dehydration process. A high correlation between the gravimetric water status level and THz transmission is observed during dehydration and after rehydration. The results show that the transmission of THz waves can be used to evaluate the water content of winter wheat leaves by destructive and non-destructive geometries, which suggests that THz imaging can be a good potential tool to measure the water content to diagnosis crop health in agriculture in a rapid, non-destructive and accurate way.

Nowadays, terahertz (THz) imaging is quite promising in scientific and biomedical applications. However, the much longer wavelength degrades the resolution of obtained images compared with classical imaging at optical frequencies. Imaging is considered to be a convolution process between the object function and point spread function (PSF) of the system. Thus, how to describe the object accurately from a blurred image is an inverse problem. In this paper, we demonstrate how well such a method can function. We would illustrate how to make current facilities overcome the physical limitations in imaging, which can image at the sub-spot level. Periodical stripes with a period of 0.8 mm are resolved at 0.3 THz. The resolution is 0.32 times the physical size of the focused beam. Moreover, this method is extended to the mathematical operations in a complex domain, which is distinct from previous image studies via visible/IR light. Physically, it means that we essentially deal with phase by making use of the wave nature of THz light. For the first time, it is demonstrated that the reconstructed phase can also describe the shape of the object by applying such an approach. In addition, we have successfully measured the diameter of human hair with a relative error of less than 10% at 3 THz. As an example, we did an experiment on tissues of colorectal cancer and a better image is obtained, which would be helpful for medical diagnosis. We believe that such an approach is able to improve imaging performance at THz frequencies and many applications will gain from it.

To meet increasingly demanding technological needs in modern security and industrial applications involving rapid close-range screening, we have developed a 100-GHz linear scanner. Having incorporated a novel approach in terahertz sensing and an advanced IMPATT-diode signal generating technique, the proposed system offers an efficient non-destructive testing (NDT) solution that is absolutely safe, fast, highly portable, and cost-effective. The test results demonstrate outstanding capability of the scanner to provide continuous, high-throughput security screening of mail. The system can perform real-time imaging with effective resolution approaching 5 mm at conveyor speeds of up to 15 m/s. [ABSTRACT FROM AUTHOR]

This paper presents our comprehensive study on a THz imaging system using linear sparse periodic array (SPA) at 220 GHz. A 8 Tx and 8 Rx SPA is designed and simulated, considering the issues of theoretical resolution, practical field of view (FOV) and ghost images and suggesting the guidelines on designing such a THz SPA imaging system. In order to verify the proposed THz SPA imaging system experimentally, we have investigated a scheme that the Tx and Rx elements are located on two scanning tracks separated by a distance and devised a simplified experimental set-up using only 1 Tx and 1 Rx channels. The further simulation that takes account of effects resulting from track separation and beam properties correlates to the final measurement quite well. This experimental approach provides a low-cost solution to a practical implementation of THz SPA imaging system. [ABSTRACT FROM AUTHOR]

A 500 GHz heterodyne imager adopting a source‐driven fourth subharmonic single balanced resistive mixer is implemented in 65‐nm CMOS technology. The adopted passive mixer improves the port‐to‐port isolation and minimizes the input matching loss. At an LO power of 5 dBm, the imager achieves a conversion loss and double sideband noise figure of 30.2 dB and 51 dB, respectively, which corresponds to −93 dBm and 15.8 fW/Hz0.5 of sensitivity and the effective noise equivalent power, respectively, assuming a noise bandwidth of 1 kHz. To the best of authors' knowledge, the proposed work is the highest frequency implementation as a CMOS‐based heterodyne imagers adopting resistive mixers. [ABSTRACT FROM AUTHOR]

THz radiation is reflected by conducting materials and has the capability to penetrate a wide variety of non-conducting materials. With such abilities, THz radiation has great potential for imaging applications, e.g., screening and inspection for packaged electronics chips and electronics circuit boards. High intensity THz radiation have been produced by the accelerator-based radiation source at the Plasma and Beam Physics Research Facility, Chiang Mai University, Thailand. The coherent THz transition radiation was generated from short electron bunches of around 8 MeV and an Al-foil tilted by 45°. Characterization of the backward transition radiation properties was conducted, including spatial distribution, polarization and radiation spectrum. The THz radiation was then used to create THz images of a print circuit board by reflective THz imaging setup. THz imaging with polarized transition radiation is demonstrated. With selected radiation polarization, reflective THz imaging can be improved. [ABSTRACT FROM AUTHOR]