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• 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]

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]

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]

An all-solid-state electronic three-dimensional terahertz tomography system designed specifically for non-destructive inspection of polymer materials is demonstrated, which is capable of determining the positions and shapes of hidden defects accurately. The imaging radar system, based on stepped-frequency modulated continuous wave (SFMCW), with center frequency at 180 GHz, bandwidth of 60 GHz, and average power 0.5 mW, is tested against thick Teflon (polytetrafluoroethylene, PTFE) plates with internal voids as samples. The locations and shapes of the hidden holes are obtained in both electromagnetic simulation and experimental measurements with a three-dimensional image reconstruction algorithm, which features advantages of accurate reconstructed image details and fast computation speed, demonstrating that the terahertz imaging radar system combined with the algorithm developed is capable of detecting internal defects of thick polymers. Key resolution parameters are established experimentally in detail, demonstrating 2.5 mm and 1.7 mm range resolutions in free space and in Teflon separately, and lateral spot diameters ranging from 3.4 mm to 8 mm at imaging distances from 5 mm to 60 mm. • An all-solid-state electronic 3D terahertz tomography system designed for NDI of polymer materials is demonstrated. • Hidden defects are obtained by a 3D image reconstruction algorithm, which features accuracy and fast computation speed. • A judgment basis for quantitative measurements of defect's cross-range size is summarized, which is suitable for other polymers. • Key resolution parameters and appropriate detection distance are established experimentally and discussed in detail. • The new approach is far-reaching significance for non-destructive inspection of polymer materials. [ABSTRACT FROM AUTHOR]

This article proposes selected non-destructive testing methods for evaluating fibre-cement boards as to the potential occurrence of heterogeneities or defects in them. First the most common material defects in such boards are described. Then selected methods (radiography and THz imaging) exploiting high frequency electromagnetic waves are introduced. Measurements of defective and flawless boards were carried out using the methods. Selected results of the measurements are presented. The preliminary tests have proved the proposed method to be useful for testing fibre cement boards. It is demonstrated that radiographic inspection enables one to assess the homogeneity and density of fibre-cement boards and can also be used to detect cracks. In the case of delamination detection and the identification of layer thickness, the terahertz time domain system is more useful. [ABSTRACT FROM AUTHOR]

By using terahertz time-domain spectroscopy (THz-TDS), the transmission spectral images of a series of aliphatic monohydric liquid alcohols were investigated in this work. The image gray value indicated the non-uniform distribution of the liquid. After noise reduction, the depth distribution was computed from the image content, which depended on the wettability of the container surface by the liquid. Then the volume of the liquid crept up to the container surface was compared quantitatively. Our result showed that the surface wettability of the PS container surface by the liquids increased with the alcohol alkyl chain increasing, which was verified by the contact angle of the liquid and the surface. The study indicated that the THz image might offer a potential technique for detecting the wettability of liquid directly without an additional contact angle experiment. [ABSTRACT FROM AUTHOR]

[Display omitted] • Overcoming the diffraction limit in a direct terahertz imaging system in transmission mode. • An entirely new use of the Terajet effect for resolution enhancement of terahertz imaging. • Experimental demonstration of the feasibility of overcoming the diffraction limit. • A special pinhole structure was designated and characterised to validate results. We report on resolution enhancement of sub-terahertz (THz) images by using the terajet effect. A mesoscale cuboid dielectric particle, used to establish the terajet, was placed in front of an object located at the focus of the THz beam. The object under study was based on a printed circuit board (PCB) perforated with different holes with diameters ranging from 1.8 to 3.0 mm and separated from each other by a distance that varies from 0.25 to 4 mm. The sample was illuminated by a continuous wave source at a frequency of 0.3 THz and the image was obtained using a sensor based on a strained-Si Field-Effect Transistor. The image was formed pixel-by-pixel in a transmission mode configuration. A clearer image with enhanced resolution was obtained when the mesoscale cube was introduced in the optical path. The terajet effect made possible to resolve a separation between holes of around 0.5 mm (lower than the wavelength, 1 mm), that is, below the diffraction limit. The method described is easy to implement, cost effective and could be used to improve the resolution of any real imaging system. [ABSTRACT FROM AUTHOR]

The paper deals with the experimental performance assessment of Compressive Sampling (CS) based Terahertz (THz) Imaging systems, an emerging approach for carrying out non-destructive tests of materials with the aim of detecting defects and flaws. Differently from traditional methods based on raster scan, CS approach allows to reconstruct the image of interest through a reduced number of measurements, with a notable reduction of the time of investigation. Although both simulated and experimental results concerning the performance assessment of THz Imaging technique are available in literature, the additional uncertainty due to the application of CS approach has never been in-depth taken into account, since the step of CS processing has been considered as ideal. Due to the success of CS-based Imaging THz technique and the promising performance of its exploitation also in industrial applications, the considered assumption is no more acceptable. Therefore, the authors focused their attention on the uncertainty sources associated with the experimental application of CS to THz Imaging systems and on their impact on the overall quality of the reconstructed image. Several numerical tests, conducted by means of an optimized design of experiments, allow to (i) assess the sensitivity to relevant uncertainty sources of the reconstructed image quality and (ii) define a suitable performance factor capable of driving experimenters towards a proper configuration of the measurement station. In particular, misalignment of the CS masks turns out to be the most impacting uncertainty source, as confirmed by experimental tests carried out through an actual THz Imaging system. Nevertheless, the performance factor estimated on the reconstructed image of a reference target is capable of highlighting the presence of an incorrect configured Imaging system, thus making it possible to remedy and provide accurate and reliable THz images. [ABSTRACT FROM AUTHOR]

Low-frequency modes of L -Asp and L -Asn were studied in the range from 0.1 to 3.0 THz using time-domain Terahertz spectroscopy and density functional theory calculation. The results show that PBE-D2 shows more success than BLYP-D2 in prediction of THz absorption spectra. To compare their low-frequency modes, we adopted “vibrational character ID strips” proposed by Schmuttenmaer and coworkers [ Journal of Physical Chemistry B , 117 , 10444(2013)]. We found that the most intense THz absorption peaks of two compounds both involve severe distortion of their hydrogen bonding networks. Due to less rigid hydrogen bonding network in L -Asp, the side chain (carboxyl group) of L -Asp exhibits larger motions than that (carboxamide group) of L -Asn in low-frequency modes. [ABSTRACT FROM AUTHOR]

We provide a comprehensive account of recent advances in biomedical image analysis and classification from two complementary imaging modalities: terahertz (THz) pulse imaging and dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI). The work aims to highlight underlining commonalities in both data structures so that a common multi-channel data fusion framework can be developed. Signal pre-processing in both datasets is discussed briefly taking into consideration advances in multi-resolution analysis and model based fractional order calculus system identification. Developments in statistical signal processing using principal component and independent component analysis are also considered. These algorithms have been developed independently by the THz-pulse imaging and DCE-MRI communities, and there is scope to place them in a common multi-channel framework to provide better software standardization at the pre-processing de-noising stage. A comprehensive discussion of feature selection strategies is also provided and the importance of preserving textural information is highlighted. Feature extraction and classification methods taking into consideration recent advances in support vector machine (SVM) and extreme learning machine (ELM) classifiers and their complex extensions are presented. An outlook on Clifford algebra classifiers and deep learning techniques suitable to both types of datasets is also provided. The work points toward the direction of developing a new unified multi-channel signal processing framework for biomedical image analysis that will explore synergies from both sensing modalities for inferring disease proliferation. [ABSTRACT FROM AUTHOR]

We investigated high-field terahertz (THz) responses and the nonlinear conductivities of n- and p-type GaAs thin films in the THz region. As the THz pulse intensity was increased to 59 μJ/cm 2 , the THz transmission of the n-type GaAs thin film was significantly increased, more than that of the p-type film. This result is correlated with the conductivity of the electronic system caused by scattering processes. Intervalley scattering was dominant in n-type GaAs, whereas hot–cold hole interactions and intervalence band transitions were the main factors reducing the conductivity in p-type GaAs. In addition, we extracted the frequency-domain conductivity of the n- and p-type GaAs thin films and used the Drude–Smith model to explain non-Drude-like behavior due to high-field excitation. [ABSTRACT FROM AUTHOR]

We propose an engineering of the optical properties of GaAs/AlGaAs quantum wells using AlAs and InAs monolayer insertions. A quantitative study of the effects of the monolayer position and the well thickness on the interband and intersubband transitions, based on the extended-basis sp 3 d 5 s ⁎ tight-binding model, is presented. The effect of insertion on the interband transitions is compared with existing experimental data. As for intersubband transitions, we show that in a GaAs/AlGaAs quantum well including two AlAs and one InAs insertions, a three level { e 1 , e 2 , e 3 } system where the transition energy e 3 − e 2 is lower and the transition energy e 2 − e 1 larger than the longitudinal optical phonon energy (36 meV) can be engineered together with a e 3 − e 2 transition energy widely tunable through the TeraHertz range. [ABSTRACT FROM AUTHOR]

Terahertz imaging is applied to characterize a hybrid fiber-reinforced composite laminate subject to low-velocity impact in this study. The hybrid fiber-reinforced composite laminate comprises unidirectional glass/epoxy and carbon/epoxy plies with a cross-ply stack pattern. Both impact-induced intra- and inter-laminar damages are successfully detected, and the damage evolution throughout the thickness is also evaluated. The interaction between the terahertz polarization and carbon-fiber orientation is investigated in detail. Inter-laminar damage at the interface and the intra-laminar damage close to the same interface can be differentiated via polarization-resolved imaging. With a parameter fitting method based on multiple regression analysis, delamination is characterized quantitatively. Terahertz C- and B-scan images clearly exhibit the propagation of the damage from the top to the bottom surface in three dimensions. [ABSTRACT FROM AUTHOR]

It is particularly important to make quality evaluation for paint spraying process. This work presented an efficient method based on wavelet packet energy for terahertz non-destructive evaluation of organic protective paint. The wavelet packet energy analysis algorithm was summarized. Various coating structures were experimentally investigated using terahertz pulsed imaging system and the relative wavelet energy and the Shannon entropy of the detected signal were calculated as wavelet energy indexes. The proposed algorithm was performed on the evaluation of coating unevenness and the identification of hidden corrosion defect. The results demonstrated that the wavelet energy indexes are sensitive to coating structure change when compared to the terahertz peak-finding method, and it allowed more accurate estimate of micro corrosion defect. Therefore, the proposed approach based on wavelet packet energy could be recommended for quick quality evaluation of organic coating with terahertz non-destructive technology. • An efficient method based on wavelet packet energy for terahertz non-destructive evaluation of organic paint was proposed. • Experimental measurement of various coating samples was carried on and the wavelet energy indexes were calculated. • Evaluation of coating unevenness and the identification of hidden corrosion defect were carried out. [ABSTRACT FROM AUTHOR]

Terahertz (THz) imaging is an innovative technology of imaging which can supply a large amount of data unavailable through other sensors. However, the higher dimension of THz images can be a hurdle to their display, their analysis and their interpretation. In this study, we propose a weighted feature space and a simple random sampling in k -means clustering for THz image segmentation. Our approach consists in estimating the expected centers, selecting the relevant features and their scores, and classifying the observed pixels of THz images. Automatic estimation of the random sample size and the selected feature number are also proposed in this paper. Our approach is more appropriate for achieving the best compactness inside clusters, the best discrimination of features, and the best tradeoff between the clustering accuracy and the low computational cost. Our approach of segmentation is evaluated by measuring performances and appraised by a comparison with some related works. [ABSTRACT FROM AUTHOR]

We introduce a near-field scanning probe terahertz (THz) microscopy technique for probing surface plasmon waves on graphene. Based on THz time-domain spectroscopy method, this near-field imaging approach is well suited for studying the excitation and evolution of THz plasmon waves on graphene as well as for mapping of graphene properties at THz frequencies on the sub-wavelength scale. [ABSTRACT FROM AUTHOR]

We investigate the shortening of the gate-length in submicrometric and nanometric field-effect transistors as a powerful tool to improve their self-oscillations performances in the terahertz frequency region due to the appearance of the Dyakonov–Shur instability. The theoretical model is based on the numerical solution of hydrodynamic equations for the electron transport in FETs/HEMTs channels. We show that a decrease of the gate length allows, on the one hand, to increase the intrinsic resonant frequencies near 1 THz and, on the other hand, to improve the conditions for the onset of the Dyakonov–Shur instability and related phenomena. The small-signal characteristics calculated under constant drain-voltage operation are compared with the drain-voltage self-oscillations calculated under constant drain-current operation. [ABSTRACT FROM AUTHOR]

Terahertz digital holography is a combination of terahertz imaging and digital holography. During reconstruction, the key point is to find accurately the propagation distance from which the distribution of focused samples can be reconstructed. In this paper, we use a continuous-wave terahertz in-line digital holographic imaging system to record holograms. Moreover, the autofocusing algorithms through which the reconstructed distance can be calculated are applied to the reconstruction. The in-line schematic is beneficial to the terahertz wave imaging, which, however, inevitably produces the object's twin image. In the refocusing process, both the reconstructed image with low signal-to-noise ratio and contrast and the twin image induce the formation of false peaks corresponding to improper distances on the autofocusing curves. To restrain the disturbance factors and improve the accuracy of the judgment, a phase retrieval method is implemented in the reconstruction. The results demonstrate the feasibility of the autofocusing method with phase retrieval in terahertz in-line digital holographic imaging system. The proposed method provides an automated and efficient evaluation which helps to obtain the optimized propagation distance. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new compressed sensing reconstruction method is proposed for fast reconstruction of terahertz spectral images. • Compressed sensing algorithms using deep learning can significantly reduce algorithm complexity and increase reconfiguration speed. • High-quality reconstruction of low-sampling-rate terahertz spectral images. • This work will help expand the cause of terahertz imaging in the field of non-destructive testing. Terahertz time-domain spectroscopy (THz-TDS) is widely applied in the field of rapid nondestructive detection of grain owing to its low photon energy and high penetrating power. Nevertheless, terahertz imaging systems suffer from the problems of long image acquisition time and massive data processing. To mitigate these issues, this work presents an adaptive compressed sensing reconstruction algorithm for terahertz spectral images based on residual learning (ATResCS). The algorithm compresses the number of data samples, reducing the amount of data required for imaging and improving the imaging speed. Further, ATResCS reduces the time complexity by employing a convolutional neural network. The algorithm is validated by acquiring terahertz spectral image data via a THz-TDS system. ATResCS outperforms conventional algorithms regarding peak signal-to-noise ratio (PSNR) and structural similarity, significantly reducing the reconstruction time and, thus, enabling real-time reconstruction. Specifically, at low sampling rates (0.1), ATResCS retains key spectral image information. The average PSNR is 0.96 – 1.015 dB higher than that of DR2-Net, reducing the average reconstruction time by 0.1 – 0.2 s. Experiments demonstrate that ATResCS has better reconfiguration capability and lower algorithm complexity, enabling high-quality and fast reconstruction of terahertz spectral images. [ABSTRACT FROM AUTHOR]

Over the past two decades, terahertz (THz) technology has gained in importance and visibility as an emerging technique to be applied in a variety of fields. Since 2006, noninvasive THz time domain spectroscopy (TDS) imaging technology has been proposed for the study of artworks. When applied to panel paintings, THz-TDS imaging makes it possible to fill in the information gap on these artworks between two diagnostic imaging techniques, namely IR reflectography and X-ray radiography, because it provides useful data on the internal physical structure of non-metallic objects without touching the artworks investigated. The resulting noninvasive cross-section image of the object is acquired by extracting the reflected pulse from a particular interface of two media that have different refractive indexes. In this paper, we used a portable THz-TDS imaging system to acquire THz images on a laboratory panel painting and on two artworks, namely the Badia Polyptych (c. 1300-1) by Giotto di Bondone in the Uffizi Gallery in Florence and the San Giovenale Triptych (1422) by Masaccio in the Masaccio Museum at the Pieve (Parish Church) of San Pietro at Cascia di Reggello, near Florence. This was the first application of THz-TDS on masterpieces that is part of conservation – restoration projects focussed on acquiring data on the materials and techniques used by the artists and the state of conservation of the paintings, as well as on testing the potentials of an application of the new emerging THz-TDS imaging technique for the art conservation field. In particular, the data acquired noninvasively on the panel paintings provided useful information on the internal structure of the paintings and on their conditions, as well as a tentative chemical composition of the works or some of their parts. Lastly, this paper offers some basic information on the application of the THz-TDS imaging technique to panel paintings. [ABSTRACT FROM AUTHOR]

Abstract: Recently, significant attention has been paid to the food safety issue of foreign-body detection in the food industry. Commercialized X-ray imaging is extensively used for monitoring any foreign bodies that may be embedded in food products. However, foreign bodies such as low-density organic materials cannot be easily observed by conventional detection methods. The sub-terahertz and terahertz wave methods can provide not only the identification of a low-density foreign body but also a safe inspection due to the non-ionizing radiation. In this study, foreign bodies concealed in dry food were detected with a high-resolution raster scan imaging system at 210 GHz in the sub-terahertz wave range. Our raster-scan imaging system using Gaussian beam focusing demonstrated the diffraction-limited spatial resolution in the sub-terahertz wave range. Transmission images of foreign bodies showed better contrast than reflection images. Nondense materials, such as insects and plastics, were observed with the continuous sub-terahertz wave, in contrast to the difficulty in observing them in X-ray transmission imaging. [Copyright &y& Elsevier]

Highlights: [•] TPI could provide spatial distribution of surface density/roughness of core tablet. [•] Tensile strength, friability provided no information on visible surface defect risk. [•] TPI helped in estimating tablet abrasion risk prior to film coating. [•] TPI revealed the relationship between surface strength and blending condition. [Copyright &y& Elsevier]

Abstract: An original design of generic interest is proposed for fast imaging, in the field of sub-terahertz frequencies, by means of resonant coupling between an ultra-thin photo-thermal converter and a metallic grid upstream an infrared camera. The material is a sheet of polyimide material with a high content of absorptive carbon inclusions. We make use of the large difference between the IR and THz wavelengths in a quarter-wave planar geometry, to ensure a highly efficient and stable conversion process. A complete setup has been implemented for demonstration purposes, using the beam from a Gunn diode at 110GHz. Experimental results are in good agreement with the predictions from the numerical model, which helps to validate the concept and the requirements for geometrical adjustment. [Copyright &y& Elsevier]

Abstract: Recent development of analytical techniques to monitor the coating process of pharmaceutical solid dosage forms such as pellets and tablets are described. The progress from off- or at-line measurements to on- or in-line applications is shown for the spectroscopic methods near infrared (NIR) and Raman spectroscopy as well as for terahertz pulsed imaging (TPI) and image analysis. The common goal of all these methods is to control or at least to monitor the coating process and/or to estimate the coating end point through timely measurements. [Copyright &y& Elsevier]

Abstract: Terahertz pulsed imaging (TPI) was employed to explore its suitability for detecting differences in the film coating thickness and drug layer uniformity of multilayered, sustained-release coated, standard size pellets (approximately 1mm in diameter). Pellets consisting of a sugar starter core and a metoprolol succinate layer were coated with a Kollicoat® SR:Kollicoat® IR polymer blend for different times giving three groups of pellets (batches I, II and III), each with a different coating thickness according to weight gain. Ten pellets from each batch were mapped individually to evaluate the coating thickness and drug layer thickness between batches, between pellets within each batch, and across individual pellets (uniformity). From the terahertz waveform the terahertz electric field peak strength (TEFPS) was used to define a circular area (approximately 0.13mm2) in the TPI maps, where no signal distortion was found due to pellet curvature in the measurement set-up used. The average coating thicknesses were 46μm, 71μm and 114μm, for batches I, II and III respectively, whilst no drug layer thickness difference between batches was observed. No statistically significant differences in the average coating thickness and drug layer thickness within batches (between pellets) but high thickness variability across individual pellets was observed. These results were confirmed by scanning electron microscopy (SEM). The coating thickness results correlated with the subsequent drug release behaviour. The fastest drug release was obtained from batch I with the lowest coating thickness and the slowest from batch III with the highest coating thickness. In conclusion, TPI is suitable for detailed, non-destructive evaluation of film coating and drug layer thicknesses in multilayered standard size pellets. [Copyright &y& Elsevier]

Abstract: Solid dosage forms are the pharmaceutical drug delivery systems of choice for oral drug delivery. These solid dosage forms are often coated to modify the physico-chemical properties of the active pharmaceutical ingredients (APIs), in particular to alter release kinetics. Since the product performance of coated dosage forms is a function of their critical coating attributes, including coating thickness, uniformity, and density, more advanced quality control techniques than weight gain are required. A recently introduced non-destructive method to quantitatively characterise coating quality is terahertz pulsed imaging (TPI). The ability of terahertz radiation to penetrate many pharmaceutical materials enables structural features of coated solid dosage forms to be probed at depth, which is not readily achievable with other established imaging techniques, e.g. near-infrared (NIR) and Raman spectroscopy. In this review TPI is introduced and various applications of the technique in pharmaceutical coating analysis are discussed. These include evaluation of coating thickness, uniformity, surface morphology, density, defects and buried structures as well as correlation between TPI measurements and drug release performance, coating process monitoring and scale up. Furthermore, challenges and limitations of the technique are discussed. [Copyright &y& Elsevier]

Abstract: The purpose of this study was to evaluate the intra-tablet coating uniformity and the identification of critical process parameters in an active pan coating process using terahertz pulsed imaging (TPI). A design of experiments (DoE) was performed with drum load, drum speed, spray rate, run duration and spray pressure as factors. Different measures of intra-tablet uniformity were investigated: the average thickness on the individual tablet faces, spatial variation in layer thickness over the tablet surface, and the coefficient of variation (CVintra). Data analysis revealed that the process parameters in the investigated parameter space had hardly any influence on the difference in layer thickness of the tablet faces and centre band. No increase or decrease in layer thickness – as described in the literature – was found towards the edges of the tablet face. In overwetted process conditions a higher layer thickness at the centre band edges could be observed. Still, the highest variability in coating thickness was found along the circumference of the centre band rather than the height. In general, higher CVintra of layer thickness were found on the centre bands in comparison with the tablet faces. The analysis of the DoE model revealed that the run duration had the highest influence on the CVintra on the tablet faces. TPI showed high potential in the assessment of intra-tablet uniformity and layer thickness distributions over the whole tablet surface. It was successfully used to identify critical process parameters regarding intra-tablet coating uniformity. [Copyright &y& Elsevier]

Abstract: In this study, terahertz pulsed imaging (TPI) was employed to investigate the effect of the coating equipment (fluid bed and drum coater) on the structure of the applied film coating and subsequent dissolution behaviour. Six tablets from every batch coated with the same delayed release coating formulation under recommended process conditions (provided by the coating polymer supplier) were mapped individually to evaluate the effect of coating device on critical coating characteristics (coating thickness, surface morphology and density). Although the traditional coating quality parameter (weight gain) indicated no differences between both batches, TPI analysis revealed a lower mean coating thickness (CT) for tablets coated in the drum coater compared to fluid bed coated tablets (p <0.05). Moreover, drum coated tablets showed a more pronounced CT variation between the two sides and the centre band of the biconvex tablets, with the CT around the centre band being 22.5% thinner than the top and bottom sides for the drum coated tablets and 12.5% thinner for fluid bed coated tablets. The TPI analysis suggested a denser coating for the drum coated tablets. Dissolution testing confirmed that the film coating density was the drug release governing factor, with faster drug release for tablets coated in the fluid bed coater (98±4% after 6h) compared to drum coated tablets (72±6% after 6h). Overall, TPI investigation revealed substantial differences in the applied film coating quality between tablets coated in the two coaters, which in turn correlated with the subsequent dissolution performance. [Copyright &y& Elsevier]

We present terahertz pulsed imaging (TPI) as a novel tool to quantify the hardness and surface density distribution of pharmaceutical tablets. Good agreement between the surface refractive index (SRI) measured by TPI and the crushing force measured from diametral compression tests was found using a set of tablets that were compacted at various compression forces. We also found a strong correlation between TPI results and tablet bulk density, and how these relate to tablet hardness. Numerical simulations of tablet surface density distribution by finite element analysis exhibit excellent agreement with the TPI measured SRI maps. These results show that TPI has an advantage over traditional diametral compression and is more suitable for nondestructive hardness and density distribution monitoring and control of pharmaceutical manufacturing processes. © 2013 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 102:2179-2186, 2013 [ABSTRACT FROM AUTHOR]

The present work gives an insight into some key measurement and signal processing considerations in terahertz pulsed imaging (TPI). TPI is increasingly used for the measurement of the spatial variation of coating thickness on coated solid dosage forms. The potential of TPI for the assessment of coating thickness distributions and the use in process development is described in recent literature. However, some critical factors need to be taken into account when working with this technique. These are (1) the signal processing of the raw data, (2) the influence of the composition of the sample matrix on the TPI signals and subsequent coating analysis, (3) signal distortions that can occur at tablet edges or areas with defects, and (4) the refractive index as a key parameter in the quantification of layer thickness. In this paper, we will highlight to what extent these factors impact on the qualitative and quantitative analysis of TPI data and how artifacts and misinterpretation of data can be avoided to ensure fully quantitative and robust measurements. © 2013 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 102:1813-1824, 2013 [ABSTRACT FROM AUTHOR]

Abstract: Terahertz reflection imaging (at frequencies ∼0.1–10THz/1012 Hz) is non-ionizing and has potential as a medical imaging technique; however, there is currently no consensus on the optimum imaging parameters to use and the procedure for data analysis. This may be holding back the progress of the technique. This article describes the use of various intelligent analysis methods to choose relevant imaging parameters and optimize the processing of terahertz data in the diagnosis of ex vivo colon cancer samples. Decision trees were used to find important parameters, and neural networks and support vector machines were used to classify the terahertz data as indicating normal or abnormal samples. This work reanalyzes the data described in Reid et al. (2011) (Physics in Medicine and Biology, 56, 4333–4353), and improves on their reported diagnostic accuracy, finding sensitivities of 90–100% and specificities of 86–90%. This optimization of the analysis of terahertz data allows certain recommendations to be suggested concerning terahertz reflection imaging of colon cancer samples. [Copyright &y& Elsevier]

Abstract: Specific biorecognition is essential for many biological processes, for which highly sensitive and label-free biosensors are strongly demanded. The recently developed metamaterials are a potential choice for biosensing due to their exotic properties. In the current work, a label-free and specific sensor for streptavidin–agarose (SA) was fabricated based on terahertz metamaterial functionlized by octadecanthiols and biotins. Both low and high frequency resonant modes from the metamaterials are found applicable for the detection of SA, and a redshift up to 6.76GHz for the high frequency mode was measured in the undiluted commercial solution. The low frequency mode is attributed to inductor–capacitor (LC) oscillation, while the high frequency mode originates from the plasmonic dipole oscillator, both of which are highly sensitive to the micro-environment change. Adsorption of SA of different concentrations causes different redshifts, and the replacement of high refractive-index substrate with low refractive-index substrate can efficiently promote the sensitivity, well agreeing with the numerical simulation. Moreover, for a particular biomolecule, the sensitivity can be further improved by optimizing the metamaterial design. This method might be very helpful for desirable biorecognition in biology, medicine, and drug industry. [Copyright &y& Elsevier]

Abstract: An analysis has been carried out to estimate the spectral characteristics of an integrated photomixer/antenna in a homodyne photomixing system. The analysis adopts the impedance mismatch factor and Friis power transmission formula used in communication links based on the conventional analysis theory of a terahertz photomixer. The analysis and experimental results have proved that an impedance matching condition between the impedance of a photomixer and the input impedance of an antenna is directly related with photomixing terahertz wave generation. The Friis formula is introduced to calculate the propagation loss of the wave from a transmitter to a receiver in a homodyne photomixing system. A log-periodic antenna was used to ensure a high dynamic range in a broad frequency region. The dynamic range of the homodyne terahertz photomixing system was about 60dB near 100GHz and decreased with an increasing frequency from 10GHz to 1000GHz. The measured results agree well with the theoretically analyzed results and prove that the terahertz photomixing power is closely related to impedance mismatch factor and it could be estimated in the homodyne terahertz photomixing system without a terahertz power detector. [Copyright &y& Elsevier]

Abstract: A method to monitor the oil pollution with reflective pulsed terahertz system is demonstrated. A cup of water covered by a layer of sesame oil is analyzed by the reflection terahertz imaging technology for simulating oil pollution monitoring. The optical thickness of sesame oil is accurately calculated using the reflective terahertz pulses and the diffuse areas of sesame oil with different densities are clearly determined by two-dimensional transverse reflection terahertz images. The application of reflective pulsed terahertz imaging is extended to the oil detection in water pollution monitoring. [Copyright &y& Elsevier]

Abstract: Terahertz imaging and conventional X-ray have been used to investigate a sealed Ancient Egyptian jar preserved at the Museum of Aquitaine (France). Terahertz radiation revealed an unknown content that could not have been visualized by X-ray. By comparison with a model object, we concluded that this content was composed of organic materials explaining their relative radiolucency. [Copyright &y& Elsevier]

Abstract: Absorption and reflection spectra have successfully been used for substance identification, which is also applicable to the THz spectral range. For optical spectroscopy, the Kramers–Kronig transformation (KKT) is a powerful tool to determine the complex refractive index (with k, n absorption and refractive indices, respectively) from either an absorption or reflection measurement. By terahertz time-domain spectroscopy (THz-TDS) materials are probed with short pulses of radiation. The detection is sensitive to the sample on both the amplitude and the phase of the electrical field. Thus, THz-TDS spectroscopy can provide more information than conventional Fourier-transform spectroscopy, by which a power spectrum is measured. In the case of transmission measurements, for instance, formulae exist by which the frequency-dependent complex refractive index is directly calculated from the time-dependent electrical field (waveform) without the necessity of a KKT. In the case of reflection experiments, a comparable computation is possible for the frequency-dependent phase angle; here a KKT can help to recover essential parameters. We present a combination of KKT and TDS methods for the calculation of optical constants in the THz regime. [Copyright &y& Elsevier]

In this paper, the performance of a quad-band rectangular microstrip patch antenna comprising sandwiched metasurface (MS) between two dielectric substrates has been reported in the terahertz spectrum. The inset feed technique has been used in the proposed patch antenna. The simulation of the proposed antenna shows respective fractional bandwidths of 7.28%, 10%, 11.15% and 3.5% at the frequencies of 0.92 THz, 1.28 THz, 1.68 THz and 2.08 THz. The maximum realized gain of 8.5 dBi has been achieved at 2.08 THz. The radiation efficiency has been found to be more than 50% at all the operating frequencies. This type of antenna can be used in medical, security purposes and video imaging systems in the terahertz domain. • Quad-band terahertz antenna comprises with microstrip patch and metasurface. • The maximum Fractional bandwidth is 11.15% at 1.68 THz. • The maximum realized gain is 8.5 dBi at 2.08 THz. • The radiation efficiency is more than 50% at all the operating bands. [ABSTRACT FROM AUTHOR]

Abstract: The optical properties of zinc dialkyldithiophosphate (ZDDP) were investigated using a transmission-mode terahertz time-domain spectroscopy (THz-TDS) measurement. Comparing with the middle-infrared spectra of ZDDP, THz spectra of ZDDP show the weak broad absorption bands, and the absorption coefficients increased monotonously as the frequency increases within the effective frequency range. The selected frequency, where absorption constants show the lowest variance by using optimum thickness formula, was used for quantitative analysis of ZDDP with different alkyl substituents. [Copyright &y& Elsevier]

Continuous wave terahertz (CW THz) imaging, is a variant of terahertz imaging that has been gaining scientific and technological relevance in multiple areas. In this paper the fundamental phenomena of CW THz were studied and a mathematical model was developed that successfully describes the Fabry–Perot interference for such a system, opening the possibility for measurement of thicknesses and surface curvatures. The capabilities of the system were tested using different types of defects, such as voids, water infiltrations and thin metallic wires. The interactions between different materials, features and the radiation beam were numerically studied using finite element method and the results agreed with the experiments. By comparing the results with other Non-Destructive Testing methods, it was found that CW THz imaging is particularly interesting to image water infiltrations and composite materials that incorporate conductive wires. • Fabry–Perot effect is crucial to understand Continuous Wave Terahertz Imaging. • An analytical model comprising Fabry–Perot and attenuation phenomena was validated. • Numerical simulation provided valuable insight on experimental THz NDT results. • 35 μ m metallic wires and water infiltrations were detected in dielectric parts. • THz radiation presented high reliability compared to some other NDT techniques. [ABSTRACT FROM AUTHOR]

Plasmonic photoconductive antennas are generally used for terahertz wave generation and detection in the time-domain terahertz imaging and spectroscopy systems. The proper performance of a photoconductive antenna is the high optical-to-terahertz conversion efficiency. Here, we present a novel plasmonic nano-antenna array with stepped strips at the edge of contact electrodes that offers better performance compared with rectangular strips. It is shown by simulation that this photoconductive antenna under an optical pump power of 50 mW can transmit light to the low-temperature grown (LT) GaAs substrate more than 95% and does not any reflect. In addition, it is found that photoconductive antenna with stepped strips can produce an optical current of 215 nA and broadband terahertz detection over the 0.1-8 THz frequency range, which shows good improvement over previous structures. [ABSTRACT FROM AUTHOR]

In this paper, a high-performance modulator based on FSS and double layers graphene has been designed and numerically investigated. The proposed structure is composed of graphene, dielectric, FSS, dielectric and graphene. The amplitude of terahertz signal which transmit from the modulator can be dynamically controlled by the graphene, and the FSS is used to enhance the light-matter interaction in graphene. The advantages of the proposed modulator are high modulation depth and a broad bandwidth. The maximum modulation depth can reach 100%, and the modulation depth can reach more than 90% even if the wavelength ranges from 500 um to 1300 um (0.23 THz to 0.6 THz). Moreover, the performance is demonstrated by the equivalent circuit theoretically. The numerically results are in well agreement with equivalent circuit results. The proposed modulator shows enormous potential in terahertz imaging and communications systems. • Metamaterial → FSS. • f d ε = 1 1 + e (ε − | u c |) ∕ k B T ; k B is the Boltzmann's constant and u c is the chemical potential. • Figure 6. Simulated and theoretical transmission of the modulator (dotted line stands for theoretical result; the solid line represents simulated result). (a) is this modulator with double-layers graphene, and (b) is with single layer graphene. • Figure 8. The calculated normalized amplitude modulation when the chemical potential of graphene is 0.9 eV. [ABSTRACT FROM AUTHOR]

In this Letter, a quad-band linear frequency conversion prototype is proposed and investigated in terahertz region based on a time-varying metasurface. The unit cell of the metasurface consists of four pairs of concentric split-ring resonators loaded with photoactive silicon. By changing the conductivity of photoactive silicon via optical pump, both static and time-varying metasurfaces are achieved and investigated. Numerical results show that the transmission spectrum of the static metasurface has four merged resonant frequencies at 0.49 THz, 0.58 THz, 0.71 THz, 0.81 THz respectively, and linear frequency conversion appears at these four frequencies under the modulation of time-varying metasurface. The conversion linearity is also verified by applying dual-band bandpass filters with different center frequency. The coefficients of determination are 0.98, 0.94, 0.87, 0.96 correspondingly. This prototype provides a new approach for realizing quad-band frequency conversion for ambient terahertz wave and has wide applications in terahertz imaging and communication systems. [ABSTRACT FROM AUTHOR]

Nafion membranes are considered as the industry standard electrolyte material for proton exchange membrane fuel cells. These membranes require adequate hydration in order to reach a high proton conductivity. The relatively high sensitivity of terahertz radiation to liquid water enables contrast to be observed for inspecting water presence in Nafion electrolyte membranes. Utilising a commercially available terahertz source and camera, this paper investigates the feasibility of a compact terahertz imaging system for visualising and quantifying liquid water during an ambient air desorption process for Nafion membranes of a wide range of thicknesses – NRE-212 (50 μm), N-115 (127 μm), N-117 (180 μm) and N-1110 (254 μm). We demonstrate that the terahertz imaging system is able to quantify liquid water in the 25–500 μm thickness range, estimate membrane weight change related to liquid water desorption, which correlated well against simultaneous gravimetric analysis and visualise the room temperature liquid water desorption process of a partially hydrated Nafion N-117 membrane. • Simple terahertz imaging system for inspecting liquid water in Nafion membranes. • Demonstrated sensitivity to liquid water in the 25–500 μm thickness range. • Estimated water weight correlates to gravimetric analysis. • Imaged water distribution across partially hydrated Nafion during desorption. [ABSTRACT FROM AUTHOR]