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A study to determine radio-frequency interference (RFI) in low-frequency passive microwave observations of the Advanced Microwave Scanning Radiometer-2 (AMSR2) is performed. RFI detection methods, such as the spectral difference method, have already been applied on microwave satellite sensors. However, these methods may result in false RFI detection, particularly in zones with extreme environmental conditions. To overcome this problem, this paper proposes an approach that uses the additional 7.3-GHz channel of the AMSR2 sensor in a new RFI detection method. This method uses calculated standard errors of estimate to detect RFI contamination in 6.9- and 7.3-GHz observations. It was found that 6.9-GHz observations are mainly contaminated in the USA, India, Japan, and parts of Europe. The 7.3-GHz observations are contaminated in South America, Ukraine, the Middle East, Southeast Asia, and Russia. The fact that these channels are not affected by RFI in exactly the same regions is useful for studies that prefer C-band brightness temperature observations (e.g., soil moisture retrieval algorithms). Therefore, a decision tree approach was set up to determine RFI and to select reliable brightness temperature observations in the lowest frequency free of any man-made contamination. The result is a reduction of the total contaminated pixels in the 6.9-GHz observations of 66% for horizontal observations and even 85% for vertical observations when 7.3 and 10.7 GHz are used. By linking RFI maps with civilization maps, this paper further shows that RFI sources at the C-band frequency are mainly located in urbanized areas. [ABSTRACT FROM PUBLISHER]

We demonstrate the effectiveness of silicon phase masks to implement spatially resolved, multispectral imaging capabilities in the range of terahertz frequencies, using a standard setup of basic interest for time-domain spectrometry with a single-cell source and a single-cell detector. Our principle primarily aims at the development of robust and inexpensive systems. It consists of appropriate space-to-time encoding, in order to ensure single-scan triggering and then take advantage of rapid and self-consistent measurements in the two-dimensional space. The process enables very efficient discrimination giving access to a relevant spatial resolution in the analysis of small size, planar assemblies made of inhomogeneous materials. Benchmark results are provided to validate the concept, thanks to prototyping phase masks with 2 × 2 pixels, prior evidencing actual performance limitations in the case of 3 × 3 pixels. Due to the frequency bandwidth of 0.1-1.5 THz in our setup and to the available operating conditions, currently acceptable pixel resolutions lie in the range of 3-4 mm. Numerical modeling by means of finite elements helps to discuss these numbers and to investigate the relevant theoretical issues, figuring the main propagation issues in connection with a sub-picosecond seed pulse throughout various masks. This involves diffraction and trailing edge effects when crossing the mask together with residual, parasitic reflections. Finally, we give a consistent prospective for improved performance, via realistic updates regarding the architecture of the setup and complementary post-processing. Further values for the attainable spatial resolution then range from 5 × 5 to 6 × 6 pixels. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate that a wire medium (WM) slab loaded with graphene sheets enables the enhancement of the near field for subwavelength imaging at terahertz frequencies. The analysis is based on the nonlocal homogenization model for WM with the additional boundary condition at the connection of wires to graphene. The principle of the operation of the proposed lens depends on the enhancement of evanescent waves, wherein the excited surface plasmons at the lower and upper graphene interfaces are coupled by an array of metallic wires. The resolution and the operating frequency of the subwavelength imaging device are mainly determined by the tunability of graphene and the structural parameters of the WM slab. The proposed structure has a resolution better than γ/10 with the advantages of broad bandwidth, low sensitivity to losses, and tunability with respect to the chemical potential even if the distance between two graphene sheets is a significant fraction of wavelength. Severe challenges in the fabrication of the lens have been studied in-depth, and a promising approach for a practical realization has been proposed. [ABSTRACT FROM AUTHOR]

In this work, we analyze in detail the modal properties of a graphene-based planar waveguide (GPW) in the terahertz (THz) frequency range. The structure consists of a graphene sheet placed on the top of a grounded dielectric slab. As is known, the surface conductivity of the graphene sheet can easily be tuned with a bias voltage via electric-field effect; we show here how such a bias affects the propagation features of both TM and TE modes supported by the GPW. An extensive dispersion analysis is performed for complex modes in both guided and radiative (leaky) regimes, considering also dielectric and metal losses as well as nonlocal effects in graphene. In particular, we focus on the behavior of the fundamental leaky modes since they exhibit quite interesting radiation features for suitable values of the bias. These results are very promising for the development of reconfigurable leaky-wave Fabry-Perot cavity antennas based on graphene at THz frequencies. [ABSTRACT FROM AUTHOR]

Imbalanced water management in a proton exchange membrane (PEM) fuel cell significantly reduces the cell performance and durability. Visualization of water distribution and transport can provide greater comprehension toward optimization of the PEM fuel cell. In this work, we are interested in water flooding issues that occurred in flow channels on cathode side of the PEM fuel cell. The sample cell was fabricated with addition of a transparent acrylic window allowing light access and observed the process of flooding formation (in situ) via a CCD camera. We then explore potential use of terahertz (THz) imaging, consisting of femtoelectron-based THz source and off-angle reflective-mode imaging, to identify water presence in the sample cell. We present simulations of two hydration states (water and nonwater area), which are in agreement with the THz image results. A line-scan plot is utilized for quantitative analysis and for defining spatial resolution of the image. Implementing metal mesh filtering can improve spatial resolution of our THz imaging system. [ABSTRACT FROM AUTHOR]

A terahertz (THz) imaging scheme based on the effect of self-mixing in a 2.9 THz quantum cascade (QC) amplifier has been demonstrated. By coupling an antireflective-coated silicon lens to the facet of a QC laser, with no external optical feedback, the laser mirror losses are enhanced to fully suppress lasing action, creating a THz QC amplifier. The addition of reflection from an external target to the amplifier creates enough optical feedback to initiate lasing action and the resulting emission enhances photon-assisted transport, which in turn reduces the voltage across the device. At the peak gain point, the maximum photon density coupled back leads to a prominent self-mixing effect in the QC amplifier, leading to a high sensitivity, with a signal to noise ratio up to 55 dB, along with a fast data acquisition speed of 20 000 points per second. [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]

We propose a scalable, low-noise imager architecture for terahertz recordings that helps to build large-scale integrated arrays from any field-effect transistor (FET)- or HEMT-based terahertz detector. It enhances the signal-to-noise ratio (SNR) by inherently enabling complex sampling schemes. The distinguishing feature of the architecture is the serially connected detectors with electronically controllable photoresponse. We show that this architecture facilitate room temperature imaging by decreasing the low-noise amplifier (LNA) noise to one-sixteenth of a non-serial sensor while also reducing the number of multiplexed signals in the same proportion. The serially coupled architecture can be combined with the existing read-out circuit organizations to create high-resolution, coarse-grain sensor arrays. Besides, it adds the capability to suppress overall noise with increasing array size. The theoretical considerations are proven on a 4 by 4 detector array manufactured on 180 nm feature sized standard CMOS technology. The detector array is integrated with a low-noise AC-coupled amplifier of 40 dB gain and has a resonant peak at 460 GHz with 200 kV/W overall sensitivity. [ABSTRACT FROM AUTHOR]

This paper describes the development of new instrumentation at the Linac Coherent Light Source for conducting THz excitation experiments in an ultra high vacuum environment probed by soft X-ray diffraction. This consists of a cantilevered, fully motorized mirror system which can provide 600 kV cm−1 electric field strengths across the sample and an X-ray detector that can span the full Ewald sphere with in-vacuum motion. The scientific applications motivated by this development, the details of the instrument, and spectra demonstrating the field strengths achieved using this newly developed system are discussed. [ABSTRACT FROM AUTHOR]

This paper presents imaging and analysis of heterogeneous breast cancer tissue using pulsed terahertz (THz) imaging technology. The goal of this research is to validate and standardize a methodology for THz imaging capable of differentiating between heterogeneous regions of breast tumors. The specimens utilized here were obtained from breast tumors diagnosed as triple negative infiltrating ductal carcinoma (IDC). These tissues were fixed in formalin, embedded in paraffin, and cut into sections of three thicknesses: 10, 20, and \mathbf30\;\boldsymbol\upmu\mathbfm. All tissues were prepared on standard glass slides used in regular histopathology of hematoxylin and eosin (H&E) stained sections. The THz pulsed system is used to scan the two dimensional tissue sections with step size of 400, 200, and \mathbf50\;\boldsymbol\upmu\mathbfm. The experimentally measured THz fields reflected from single pixels identified in each region of the tumor are validated with the Fresnel reflection coefficient formulation. A variety of signal normalization and processing methods are investigated. The images are also validated with the standard histopathology images. The obtained results of three different tumors demonstrate strong capability of THz reflection imaging mode to distinguish between the heterogeneous regions in the tumor. [ABSTRACT FROM PUBLISHER]

Terahertz, sandwiched between conventional microwave and optical frequencies, has inspired increasing interest due to its uniqueness and high potential applications, such as imaging, sensing, and communications. This article, on the other hand, focuses on one emerging application of the terahertz spectrum: THz interconnect. Intra-/inter-chip communication has doubled every two years over recent decades, and the trend is projected to continue in the future. However, the bandwidth supportable by chip I/O pins cannot keep up with the requirement, which forms the increasing gap between the bandwidth requirement and support capability, or the interconnect gap. To ultimately solve the problem and close the gap, both bandwidth density and energy efficiency should be boosted. THz interconnect holds high potential to boost key performance by leveraging the advantages of both high-speed electronics devices and low-loss quasi-optical channels. This article discusses THz interconnect from different aspects: system architecture, circuit specifications, design challenges, and non-ideality effects. Particularly, this article exemplifies both active and passive circuit design techniques for THz interconnect, a 140 GHz transceiver and a terahertz generator in 65 nm CMOS technology, and a low-loss and process- compatible silicon waveguide channel. THz interconnect opens high potential new revenue to solve the long-standing interconnect issue. [ABSTRACT FROM PUBLISHER]

[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]

Measurement of chest wall displacement is an important approach for measuring mechanics of chest wall, which has considerable significance for assessing respiratory system and diagnosing pulmonary diseases. However, existing optical methods for measuring chest wall displacement are inconvenient for some specific patients such as the female patients and the patients with bandaged chest. In this letter, we proposed a method for measuring chest wall displacement based on terahertz wave and established corresponding mathematic model and set up a terahertz measurement system. The main advantages of this method are that it can measure the chest wall displacement of the subjects without taking off clothes or arranging any markers. To validate this method and assess the performance of the terahertz system, in vitro, the displacement of a water module driven by a linear guide rail was measured by the terahertz system and compared with the actual displacement of the water module. The results showed that the waveforms measured with two methods have a good agreement, and the relative error is less than 5% and sufficiently good for measurement demands. In vivo, the synchronous experiment was performed on five human volunteers with the terahertz system and a respiratory belt transducer. The results demonstrate that this method has good performance and promising prospects for measuring chest wall displacement. [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]

A Polymethylmethacrylate (PMMA) rectangular tapered probe with metal coating on the sides is analyzed as a near-field imaging probe at 100 GHz in Ansoft High Frequency Structure Simulator (HFSS). Normally, highly resistive silicon and sapphire, which are costly, are used as a near-field probe due to their low loss and high permittivity. PMMA near-field probe is usually used in Scanning Near-field Optical Microscopy (SNOM), which is made from PMMA optical fibers. We propose for the first time to use PMMA as a near-field probe in millimeter and Terahertz wave scanning near-field imaging applications. The geometrical optimization of the tapered probe is carried out on the basis of different coupling methods. The beam shape merging from the end of the tapered tip is analyzed. The operation efficiency of two-side tapered and four-side tapered probes has been compared in view of the fabrication technique. A knife edge is simulated in HFSS to define the lateral resolution. Longitudinal resolution is discussed through setting a stair step shaped sample. A high lateral resolution around the end of the probe size can be achieved and even higher longitudinal resolution. The impact of the tip-sample distance and the lateral resolution are clearly illustrated via simulations. Experiments are carried out using a two-side tapered probe provided with an aluminum coating. The resolution is defined by scanning a PMMA board which was half coated with aluminum. [ABSTRACT FROM AUTHOR]

We show that the transmission of a terahertz (THz) pulse through single-layer graphene is strongly nonlinear. As the peak electric field of the THz pulse exceeds 50kV/cm, the graphene becomes increasingly transparent to the THz radiation. When field strength reaches 800 kV/cm, the increased transparency corresponds to a two-fold decrease in the time-average sheet conductivity of the graphene (time averaged over the duration of the pulse). Time-resolved measurements reveal that the leading portion of the pulse creates transparency for the trailing portion, with a 10-fold suppression in sheet conductivity at the tail of the strongest THz pulse. Comparing the THz-induced transparency phenomena in different sample geometries shows that substrate-free graphene is the best geometry for maximizing the nonlinear transparency effect. [ABSTRACT FROM AUTHOR]

This paper presents a high-power 0.53 THz source module with programmable diversity to adjust the brightness and the direction of light to obtain the desired diffuse lighting conditions in THz imaging applications. The source module consists of a single SiGe BiCMOS chip which operates an array of 16 source-pixel incoherently. Each source pixel consists of a primary on-chip ring-antenna and two triple-push oscillators locked 180° out-of-phase. The module provides a total radiated power of up to 1 mW (0 dBm) with 62.5 µW (–12 dBm) per source pixel on average and an EIRP per pixel of 25 dBm. The circuit layout is scalable in size and output power. The chip consumes up to 2.5 W from a 2.4 V supply and 3.2 mW from a digital 1.2 V supply respectively. The module includes a secondary silicon lens, is programmable through a CPLD, and supplied from a USB port. The THz radiation can be recorded with a CMOS 1 k-pixel THz video camera and represent an all silicon solution for real-time active THz imaging. [ABSTRACT FROM AUTHOR]

We propose a porous-core octagonal photonic crystal fiber for low-loss terahertz (THz) waveguiding. Great attention is given to the geometries of the fiber inside the core to increase the fraction of power transmitted through the air holes. At an operating frequency f = 1 THz, this design exhibits a low effective material loss which is approximately 0.05 cm-1 or 0.2 dB/cm. In addition to the confinement loss, some other properties like the power fraction of the core air holes, responses of the effective material loss, and power fraction with respect to frequency have been also reported. This design is useful for efficient transmission of broadband terahertz radiation. [ABSTRACT FROM AUTHOR]

A hybrid shape reconstruction algorithm based on the near-field microwave imaging approach is presented. A combination of the linear sampling and level set methods is used to calculate the shape of three-dimensional scatterers in a background region with compact support. In this regard, the dyadic Green's function of the background environment is calculated numerically. Since the discretization of the near-field integral equation in the matrix form is ill conditioned, regularization is used for solving this problem. The Chan–Vese and level set method are used to retrieve the accurate shape. Different imaging examples are presented to show the stability of the method once exposed to the noise. Furthermore, application of the method in through the wall and immersed object imaging is demonstrated. [ABSTRACT FROM PUBLISHER]

The main characteristics of millimeter-wave (MM-wave) image detector were simulated by means of accurate numerical modeling of thermophysical processes in a metamaterial MM-to-IR converter. The converter represents a multilayer structure consisting of an ultra thin resonant metamaterial absorber and a perfect emissive layer. The absorber consists of a dielectric self-supporting film that is metallized from both sides. A micro-pattern is fabricated from one side. Resonant absorption of the MM waves induces the converter heating that yields enhancement of IR emission from the emissive layer. IR emission is detected by IR camera. In this contribution an accurate numerical model for simulation of the thermal processes in the converter structure was created by using COMSOL Multiphysics software. The simulation results are in a good agreement with experimental results that validates the model. The simulation shows that the real-time operation is provided for the converter thickness less than 3 μm and time response can be improved by decreasing of the converter thickness. The energy conversion efficiency of MM waves into IR radiation is over 80%. The converter temperature increase is a linear function of a MM-wave radiation power within three orders of the dynamic range. The blooming effect and ways of its reducing are also discussed. The model allows us to choose the ways of converter structure optimization and improvement of image detector parameters. [ABSTRACT FROM AUTHOR]

One attractive option to achieve real-time terahertz (THz) imaging is a microelectromechanical systems (MEMS) bimaterial sensor with embedded metamaterial absorbers. We have demonstrated that metamaterial films can be designed using standard MEMS materials such as silicon oxide (SiOx), silicon oxinitrate (SiOxNy), and aluminum (Al) to achieve nearly 100% resonant absorption matched to the illumination source, providing structural support, desired thermomechanical properties and access to external optical readout. The metamaterial structure absorbs the incident THz radiation and transfers the heat to bimaterial microcantilevers that are connected to the substrate, which acts as a heat sink via thermal insulating legs, allowing the overall structure to deform proportionally to the absorbed power. The amount of deformation can be probed by measuring the displacement of a laser beam reflected from the sensor's metallic ground plane. Several sensor configurations have been designed, fabricated, and characterized to optimize responsivity and speed of operation and to minimize structural residual stress. Measured responsivity values as high as 1.2 deg /µW and time constants as low as 20ms with detectable power on the order of 10 nW were obtained, indicating that the THz MEMS sensors have a great potential for real-time imaging. [ABSTRACT FROM AUTHOR]

Terahertz (THz) spectroscopy and imaging have been heralded for some time as potentially revolutionary techniques for biomedical applications. Label-free detection of molecules and recognition of molecular events are often mentioned as the most exciting possibilities. A crucial practical goal, however, is the ability to perform such measurements on tiny amounts of biological fluids or even on individual organic structures. Living cells, for instance, have diameters at most of some tens of micrometers, i.e. at least λ/10 even for few-THz radiation. Furthermore, all analyses relevant for a biological perspective must be performed in a water environment, which presents a strong absorption across the whole THz spectral range, severely limiting the penetration of the electromagnetic field. Here, it is shown how both issues can be overcome with a lab-on-a-chip approach based on a microfluidic platform coupled to a plasmonic antenna. Using a quantum cascade laser as THz illumination source, liquid volumes down to the picoliter range are probed, and direct operation on individual 10-µm diameter microparticles flowing in water is shown. The present demonstration opens the way to the development of THz biosensing of individual living cells and small probe volumes. [ABSTRACT FROM AUTHOR]

The dispersion of penetration depth of terahertz radiation in various human pathological tissues were obtained and analyzed for development of subcutaneous diseases diagnostics in terahertz frequency range. [ABSTRACT FROM AUTHOR]

An active terahertz wave imaging method is investigated for improved system simplicity, inexpensive implementation, and distance approximation. The proposed technique is composed of a single-pixel setup that allows acquiring the two-dimensional (2-D) Fourier transform intensity map of the imaged object and additional depth data using time of flight method. A raster scan is performed to achieve the object's 2-D Fourier transform intensity and depth information. Iterative phase retrieval methods are employed to accomplish good image reconstruction using only the measured object's Fourier transform magnitude. The proposed method uses a glow discharge detector (GDD) as its single millimeter wave pixel and offers a simple noncalibrating scheme for 2-D imaging and distance approximation. This work shows experimental results of using the GDD as a distance approximation detector and specifies the advantages, disadvantages, and constraints when using such a sensor. Basic aperture imaging (transmission imaging) experimental results are also shown, and complex aperture imaging simulations and their corresponding reconstructions are presented. Finally, both 2-D imaging and acquired depth data are fused into a single three-dimensional reconstructed image to reveal the potential of the proposed method. [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]

Purpose: Enteric coatings are used to reduce gastrointestinal side effects and control the release properties of oral medications. Although widely used, the effect of formulation and process conditions on physicochemical and functional properties of enteric coatings remains unclear. Methods: Terahertz pulsed imaging (TPI) was employed to evaluate the coat properties of enteric coated tablets (ECTs) with various acid resistance. Other analytic methods, such as loss on drying, scanning electron microscopy and X-ray computed tomography were then used to validate the relationships established among 4 TPI-derived parameters and the physicochemical properties of enteric coatings. Results: Weight gain measurement did not provide any information to assess acid resistance of enteric coating, whereas four TPI-derived parameters non-destructively reflected the coating properties such as thickness, coat uniformity, density, and water distribution, allowing the identification of the causes of poor acid resistance in certain ECT batches using a single measurement. These parameters also revealed the effect of coating conditions; in particular, coating under dry conditions led to less dense and nonuniform coatings with poor acid resistance. Conclusion: We demonstrated the utility of TPI to identify structural defects within ECTs with poor acid resistance. TPI-derived parameters can aid in formulation development and quality control of ECTs. [ABSTRACT FROM AUTHOR]

Imaging at long wavelengths, for example at terahertz and millimetre-wave frequencies, is a highly sought-after goal of researchers because of the great potential for applications ranging from security screening and skin cancer detection to all-weather navigation and biodetection. Here, we design, fabricate and demonstrate active metamaterials that function as real-time tunable, spectrally sensitive spatial masks for terahertz imaging with only a single-pixel detector. A modulation technique permits imaging with negative mask values, which is typically difficult to achieve with intensity-based components. We demonstrate compressive techniques allowing the acquisition of high-frame-rate, high-fidelity images. Our system is all solid-state with no moving parts, yields improved signal-to-noise ratios over standard raster-scanning techniques, and uses a source orders of magnitude lower in power than conventional set-ups. The demonstrated imaging system establishes a new path for terahertz imaging that is distinct from existing focal-plane-array-based cameras. [ABSTRACT FROM AUTHOR]

With many potential applications in mind, great effort is being applied to develop a terahertz-wave technology platform on which waves can be manipulated with sufficient confinement and efficient interaction for the development of smart components. Here, we utilize the in-plane resonance of a thin, planar photonic-crystal slab with negligible absorption loss to successfully demonstrate and visualize terahertz-wave trapping. We artificially introduce free carriers, which interact with the trapped waves, and capture them in the slab by absorption. Our system exhibits an experimental absorptivity (interaction efficiency) of ∼99% and a broad bandwidth (absorptivity of ≥90%) that covers 17% of the centre frequency. We also demonstrate its application to the stabilization of terahertz wireless communication systems. Our study shows the capability of photonic crystals as a terahertz-wave platform, the application of which may be extended to other components including filters, couplers, antennas, detectors, modulators, switches and emitters. [ABSTRACT FROM AUTHOR]

This work presents experimental investigations of three-dimensional (3-D) far-field holographic microwave imaging (HMI) method for diagnosing inclusions within dielectric objects, and in particular, it relates to electromagnetic imaging to reconstruct dielectric properties of inhomogeneous, lossy bodies with arbitrary shape. The apparatus is designed for operation at a single frequency of 12.6GHz. 16 antennas are located on a 2-D array plane which is placed under the object in the far-field region, with air in the space between the antenna array and the object. Experimental results indicate that the 3-D HMI system has the ability to produce a 3-D image of multimedia dielectric object and detect small inclusions embedded within an object. The invention has potential application to tissue imaging. [ABSTRACT FROM AUTHOR]

Terahertz (THz) technology is becoming a spotlight of scientific interest due to its promising myriad applications including imaging, spectroscopy, industry control and communication. However, one of the major bottlenecks for advancing this field is due to lack of well-developed solid-state sources and detectors operating at THz gap which serves to mark the boundary between electronics and photonics. Here, we demonstrate exceptionally wide tunable terahertz plasma-wave excitation can be realized in the channel of micrometer-level graphene field effect transistors (FET). Owing to the intrinsic high propagation velocity of plasma waves (>~108 cm/s) and Dirac band structure, the plasma-wave graphene-FETs yield promising prospects for fast sensing, THz detection, etc. The results indicate that the multiple guide-wave resonances in the graphene sheets can lead to the deep sub-wavelength confinement of terahertz wave and withQ-factor orders of magnitude higher than that of conventional 2DEG system at room temperature. Rooted in this understanding, the performance trade-off among signal attenuation, broadband operation, on-chip integrability can be avoided in future THz smart photonic network system by merging photonics and electronics. The unique properties presented can open up the exciting routes to compact solid state tunable THz detectors, filters, and wide band subwavelength imaging based on the graphene-FETs. [ABSTRACT FROM AUTHOR]

We present a technique for measuring radio-frequency (RF) electric field strengths with sub-wavelength resolution. We use Rydberg states of rubidium atoms to probe the RF field. The RF field causes an energy splitting of the Rydberg states via the Autler-Townes effect, and we detect the splitting via electromagnetically induced transparency (EIT). We use this technique to measure the electric field distribution inside a glass cylinder with applied RF fields at 17.04 GHz and 104.77 GHz. We achieve a spatial resolution of ≈100 μm, limited by the widths of the laser beams utilized for the EIT spectroscopy. We numerically simulate the fields in the glass cylinder and find good agreement with the measured fields. Our results suggest that this technique could be applied to image fields on a small spatial scale over a large range of frequencies, up into the sub-terahertz regime. [ABSTRACT FROM AUTHOR]

ABSTRACT A terahertz (THz) transmission imaging system using a 0.2 and 0.4 THz gyrotron as a high power source was evaluated. In the gyrotron, 0.2 THz is the first main mode and 0.4 THz is the second harmonic mode. The output power of gyrotron is about 100 W at 0.2 THz and 1 W at 0.4 THz. For fast imaging, the source output was shaped into a line beam and an array detector was used. Two reflecting mirrors were utilized for a reflective imaging system. A 0.8 mm resolution was obtained using a 0.4 THz transmission image. The feasibility of fast THz imaging for the inspection of foreign substances in food is demonstrated. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1211-1214, 2014 [ABSTRACT FROM AUTHOR]

The n-doped and undoped silicon nanowires grown by chemical vapor deposition on quartz substrate were characterized using optical-pump terahertz-probe transmission experiments. Temporal decays of the differential transmission measurements are reproduced using a biexponential function with an initial photocarrier lifetime of ~2 ps and a longer decay time of 10 ps to a few tens of picoseconds. Based on the influence of the laser fluence, a carrier capture and recombination scenario is proposed to explain these temporal decay curves. For both samples, the capture of photocarriers by the traps present on the surface of the nanowires plays an important role in the observed photoconductivity dynamics. Frequency-dependent complex photoconductivity data curves are extracted from the terahertz (THz) traces taken at different optical-pump THz-probe delays. These data curves are reproduced using a plasmon resonance model. The fitting procedure allows us to determine carrier scattering times of about 28±6 and 14±4 fs for the undoped and doped samples, respectively. Our results show that defects and ionized impurities introduced by n doping the silicon nanowires reduce the photocarrier mobility and lifetime. [ABSTRACT FROM AUTHOR]

Real-time terahertz (THz) wave imaging has wide applications in areas such as security, industry, biology, medicine, pharmacy, and arts. In this letter, we report on real-time room-temperature THz imaging by nonlinear optical frequency up-conversion in organic 4-dimethylamino-N'-methyl-4'- stilbazolium tosylate crystal. The active projection-imaging system consisted of (1) THz wave generation, (2) THz-near-infrared hybrid optics, (3) THz wave up-conversion, and (4) an InGaAs camera working at 60 frames per second. The pumping laser system consisted of two optical parametric oscillators pumped by a nano-second frequency-doubled Nd:YAG laser. THz-wave images of handmade samples at 19.3 THz were taken, and videos of a sample moving and a ruler stuck with a black polyethylene film moving were supplied online to show real-time ability. Thanks to the high speed and high responsivity of this technology, real-time THz imaging with a higher signal-to-noise ratio than a commercially available THz micro-bolometer camera was proven to be feasible. By changing the phase-matching condition, i.e., by changing the wavelength of the pumping laser, we suggest THz imaging with a narrow THz frequency band of interest in a wide range from approximately 2 to 30 THz is possible. [ABSTRACT FROM AUTHOR]

In this contribution we present the upgrade of an existing reflector antenna system to provide a THz imaging Radar with refocusing capabilities. The method used to perform this refocusing is based on a mechanical translation of the transceiver, and it allows a 50% change of the focusing distance with respect to the normal position (25 m) maintaining well focused beams and high efficiency. The refocusing system has been validated with measurements and a resolution under 1.5 cm is obtained for the whole focusing range with spillover losses lower than 0.75 dB. [ABSTRACT FROM PUBLISHER]

Uncertainties in the retrievals of microwave land-surface emissivities are quantified over two types of land surfaces: desert and tropical rainforest. Retrievals from satellite-based microwave imagers, including the Special Sensor Microwave Imager, the Tropical Rainfall Measuring Mission Microwave Imager, and the Advanced Microwave Scanning Radiometer for Earth Observing System, are studied. Our results show that there are considerable differences between the retrievals from different sensors and from different groups over these two land-surface types. In addition, the mean emissivity values show different spectral behavior across the frequencies. With the true emissivity assumed largely constant over both of the two sites throughout the study period, the differences are largely attributed to the systematic and random errors in the retrievals. Generally, these retrievals tend to agree better at lower frequencies than at higher ones, with systematic differences ranging 1%-4% (3-12 K) over desert and 1%-7% (3-20 K) over rainforest. The random errors within each retrieval dataset are in the range of 0.5%-2% (2-6 K). In particular, at 85.5/89.0 GHz, there are very large differences between the different retrieval datasets, and within each retrieval dataset itself. Further investigation reveals that these differences are most likely caused by rain/cloud contamination, which can lead to random errors up to 10-17 K under the most severe conditions. [ABSTRACT FROM AUTHOR]

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]

Terahertz (THz) imaging provides cutting edge technique in biology, medical sciences and non-destructive evaluation. However, due to the long wavelength of the THz wave, the obtained resolution of THz imaging is normally a few hundred microns and is much lower than that of the traditional optical imaging. We introduce a sub-wavelength resolution THz imaging technique which uses the THz radiation generated by a femtosecond laser filament in air as the probe. This method is based on the fact that the femtosecond laser filament forms a waveguide for the THz wave in air. The diameter of the THz beam, which propagates inside the filament, varies from 20 μm to 50 μm, which is significantly smaller than the wavelength of the THz wave. Using this highly spatially confined THz beam as the probe, THz imaging with resolution as high as 20 μm (∼ μ/38 at 0.4 THz) can be realized [ABSTRACT FROM AUTHOR]

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]

A reflection type of imaging system is shown at sub-terahertz frequencies generated from high-Tcsuperconducting intrinsic Josephson junction mesa structures fabricated by single crystalline Bi2Sr2CaCu2O8+δ to demonstrate how the sub-terahertz imaging technique using monochromatic radiation is powerful and unique for the variety of practical applications. Several examples are discussed in detail and are compared to other terahertz imaging systems. [ABSTRACT FROM AUTHOR]

We summarize three lines of development and investigation of foundry-processed patch-antenna-coupled Si MOSFETs as detectors of THz radiation: (i) Exploiting the pinciple of plasma-waved-based mixing in the two-dimensional electron gas of the transistors' channels, we demonstrate efficient detection at frequencies as high as 9 THz, much above the transit-time-limited cut-off frequencies of the devices (tens of GHz). Real-time imaging at 600 GHz with a 12 × 12 detector array is explored. (ii) Given the limited THz power usually available for applications, we explore imaging with enhanced sensitivity in heterodyne mode. We show that real-time operation of a 100 × 100-pixel heterodyne camera should be possible at 600 GHz with a better dynamic range (30 dB) than for direct power detection (20 dB), even if only a quarter-milliwatt of local-oscillator power, distributed radiatively over all detector pixels, is available. (iii) Finally, we present an all-electronic raster-scan imaging system for 220 GHz entirely based on CMOS devices, combining the CMOS detectors with an emitter circuit implemented in a 90-nm CMOS process and delivering radiation with a power on the 100- μW scale. Considering progress in the field, we anticipate that the emitter concept of oscillator-based power generation with on-chip frequency multiplication will carry well into the sub-millimeter-wave regime. [ABSTRACT FROM AUTHOR]

A terahertz (THz) imaging system and high efficient terahertz sources and detectors for medical applications were developed. A fiber laser based compact time domain terahertz tomography system was developed with a high depth resolution of less than 20 μm. Three-dimensional images of porcine skin were obtained including some physical properties such as applied skin creams. The discrimination between healthy human tissue and tumor tissue has been achieved using reflection spectra. To improve the THz imaging system, a ridge waveguide LiNbO based nonlinear terahertz generator was studied to achieve high output power. A ridge waveguide with 5-7 μm width was designed for high efficiency emission from the LiNbO crystal by the electro-optic Cherenkov effect. Terahertz electronic sources and detectors were also realized for future imaging systems. As electronic source devices, resonant tunneling diode (RTD) oscillators with a patch antenna were fabricated using an InGaAs/InAlAs/AlAs triple barrier structure. On the other side, Schottky barrier diode (SBD) detectors with a log-periodic antenna were fabricated by thin-film technology on a Si substrate. Both devices operate above 1 THz at room temperature. This electronic THz device set could provide a future high performance imaging system. [ABSTRACT FROM AUTHOR]

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]

Dissolution stability over the entire shelf life duration is of critical importance to ensure the quality of solid dosage forms. Changes in the drug release profile during storage may affect the bioavailability of drug products. This study investigated the stability of a commercial tablet (Lescol® XL) when stored under accelerated conditions (40 °C/75% r.h.). Terahertz pulsed imaging (TPI) was used to investigate the structure of the tablet coating before and after the accelerated aging process. The results indicate that the coating was reduced in thickness and exhibited a higher density after being stored under accelerated conditions for four weeks. In situ magnetic resonance imaging (MRI) of the water penetration processes during tablet dissolution in a USP-IV dissolution cell equipped with an in-line UV-vis analyzer was carried out to study local differences in water uptake into the tablet matrix between the stressed and unstressed state. The drug release profiles of the Lescol® XL tablet before and after the accelerated storage stability testing were compared using a "difference" factor f1 and a "similarity" factor f2. The results reveal that even though the physical properties of the coating layers changed significantly during the stress testing, the coating protected the tablet matrix and the densification of the coating polymer had no adverse effect on the drug release performance [ABSTRACT FROM AUTHOR]