Terahertz Time-domain Spectroscopy Imaging Research Articles

A Row Displacement Correction Algorithm for High-speed and Accurate Terahertz Raster Scanning Imaging

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

Nondestructive evaluation of cork phenolic-based aerospace structure using Terahertz time domain spectroscopy and imaging

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

THz Imaging to Map the Lateral Microporosity Distribution in Carbonate Rocks

Terahertz (THz) spectroscopy is a nondestructive tool used in many industries to analyze materials, including measuring the water content and the distribution of water in biological samples. THz time-domain spectroscopy (THz-TDS) measures the dielectric and structural properties of a sample by probing it with an ultrafast THz pulse and measuring the change in amplitude and phase. In this study, we demonstrate the use of THz-TDS imaging to quickly map lateral (i.e., two-dimensional) variations in microporosity (ϕμ) using the THz attenuation due to water in the pores after clearing the large pores via centrifugation. Three carbonate rock plugs with differing ϕ and pore-size distributions were subsampled for this study. Three water saturation states were produced for each sample: saturated, centrifuged, and dry. At each saturation state, the sample is weighed and imaged using THz-TDS to spatially map and measure ϕμ. The results show that for each sample the ϕμ obtained using THz-TDS imaging is in excellent agreement with that obtained from both mass balance and MICP. In addition, the THz-TDS maps show significant differences in the spatial distribution of the microporosity for samples having similar composition. This method provides a means to measure ϕ and ϕμ while mapping the spatial distribution of ϕμ toward improved petrophysical characterization of carbonate reservoir rocks.

Studying Oral Tissue via Real-Time High-Resolution Terahertz Spectroscopic Imaging

Terahertz (THz) waves are extremely sensitive to water content and biological composition (proteins, genes, and metabolites), which are the key regulator of cancer development. Hence, the application of THz spectroscopy and imaging is promising in the field of biomedicine, especially in cancer diagnosis. Currently, the diagnosis of cancer patients (including tumor borders, tumor stage, etc.) mainly rely on optical microscope observation of pathological slices. The preparation of the slices is complicated and time consuming, and the microscopic field of view is small, which cannot effectively guide the precise removal of the tumor during the operation in time. To solve this problem, a label-free real-time high-resolution near-field THz spectroscopy imaging technique was developed. In this study, we successfully apply this technique to the oral tissue. Compared with conventional point-by-point scanning terahertz time-domain spectroscopy imaging, this technique can exhibit a real-time imaging rate (as high as 20 frames/s at 512 \ifmmode\times\else\texttimes\fi{} 1024 pixels) and a field of view approximately 5 mm in diameter with a spatial resolution up to \ensuremath{\lambda}/10 at 1 THz in near-field condition. The experimental results show that the terahertz spectroscopic imaging in this paper can clearly detect and identify not only biological tissues with different components, but also normal and cancerous tissues, as well as tissues with the same composition but different slice directions. This work lays an experimental foundation for the application of THz spectroscopy imaging to rapid label-free intraoperative diagnosis, and it provides a basis for promoting the wide application of THz technology in the biomedical field.

Interface evolution behavior of Si-based adhesive structures under tensile loading based on terahertz time-domain spectroscopy

The thickness of the bonding interface changes after the structure has been subjected to tensile load, thereby affecting the strength of the bonding structure. It is necessary to monitor the evolution of the bonding interface thickness under tensile load. Through terahertz time-domain spectroscopy imaging and scale-invariant feature transform image processing, the thickness variation of three interfaces of 150 silicon-based adhesive structures under tensile load were analyzed. It was determined that variation at the edge was larger than that in the middle area. The research results also provide data support for the bonding structure health monitoring systems.

Fast Terahertz Imaging Model Based on Group Sparsity and Nonlocal Self-Similarity.

In order to solve the problems of long-term image acquisition time and massive data processing in a terahertz time domain spectroscopy imaging system, a novel fast terahertz imaging model, combined with group sparsity and nonlocal self-similarity (GSNS), is proposed in this paper. In GSNS, the structure similarity and sparsity of image patches in both two-dimensional and three-dimensional space are utilized to obtain high-quality terahertz images. It has the advantages of detail clarity and edge preservation. Furthermore, to overcome the high computational costs of matrix inversion in traditional split Bregman iteration, an acceleration scheme based on conjugate gradient method is proposed to solve the terahertz imaging model more efficiently. Experiments results demonstrate that the proposed approach can lead to better terahertz image reconstruction performance at low sampling rates.

Non-destructive Detection of Insect Foreign Bodies in Finishing Tea Product Based on Terahertz Spectrum and Image.

Non-destructive testing of low-density and organic foreign bodies is the main challenge for food safety control. Terahertz time-domain spectroscopy (THz-TDS) and imaging technologies were applied to explore the feasibility of detection for insect foreign bodies in the finishing tea products. THz-TDS of tea leaves and foreign bodies of insects demonstrated significant differences in terms of time domain and frequency signals in the range of 0.3–1.0 THz. These signals were corrected by the use of adaptive iteratively reweighted penalized least squares (AirPLS), asymmetric least squares (AsLS), and baseline estimation and de-noising using sparsity (BEADS) for reducing baseline drift and enhancing effective spectral information. The K-nearest neighbor (KNN) and partial least squares discrimination analysis (PLS-DA) models showed the best performance after AirPLS correction with the prediction accuracy of 98 and 100%, respectively. In addition, the locations and outlines of insect bodies could be clearly presented via the THz-TDS image. These results suggested that THz-TDS spectroscopy and imaging provide an alternative tool for the detection of insect foreign bodies in finishing tea products.

Hybrid Sparsity Model for Fast Terahertz Imaging.

In order to shorten the long-term image acquisition time of the terahertz time domain spectroscopy imaging system while ensuring the imaging quality, a hybrid sparsity model (HSM) is proposed for fast terahertz imaging in this paper, which incorporates both intrinsic sparsity prior and nonlocal self-similarity constraints in a unified statistical model. In HSM, a weighted exponentiation shift-invariant wavelet transform is introduced to enhance the sparsity of the terahertz image. Simultaneously, the nonlocal self-similarity by means of the three-dimensional sparsity in the transform domain is exploited to ensure high-quality terahertz image reconstruction. Finally, a new split Bregman-based iteration algorithm is developed to solve the terahertz imaging model more efficiently. Experiments are presented to verify the effectiveness of the proposed approach.

Spatial Domain Terahertz Image Reconstruction Based on Dual Sparsity Constraints.

Terahertz time domain spectroscopy imaging systems suffer from the problems of long image acquisition time and massive data processing. Reducing the sampling rate will lead to the degradation of the imaging reconstruction quality. To solve this issue, a novel terahertz imaging model, named the dual sparsity constraints terahertz image reconstruction model (DSC-THz), is proposed in this paper. DSC-THz fuses the sparsity constraints of the terahertz image in wavelet and gradient domains into the terahertz image reconstruction model. Differing from the conventional wavelet transform, we introduce a non-linear exponentiation transform into the shift invariant wavelet coefficients, which can amplify the significant coefficients and suppress the small ones. Simultaneously, the sparsity of the terahertz image in gradient domain is used to enhance the sparsity of the image, which has the advantage of edge preserving property. The split Bregman iteration scheme is utilized to tackle the optimization problem. By using the idea of separation of variables, the optimization problem is decomposed into subproblems to solve. Compared with the conventional single sparsity constraint terahertz image reconstruction model, the experiments verified that the proposed approach can achieve higher terahertz image reconstruction quality at low sampling rates.

Application of THz-TDS Technology in on-line detection of vacuum degree of arc extinguishing chamber

Terahertz time-domain spectroscopy (THz-TDS) is a non-destructive detection technology for carrying information inside a sample through a wide-band terahertz pulse carrying medium information. This paper briefly describes the imaging principle, characteristics, development and application direction of THz-TDS technology. The theoretical basis of THz-TDS technology applied in vacuum degree on-line monitoring is analyzed and verified by experiments. The experimental results show that with the change of vacuum degree, the signal amplitude attenuation and time delay of THz spectrum will change regularly. It shows that the spectrum and absorption spectrum obtained by THz-TDS imaging technology can sensitively distinguish the change of pressure in vacuum environment, that is, the change of vacuum degree. At the same time, the related air environmental pollution information also has strong absorption characteristics in the terahertz band, which makes the use of terahertz spectroscopy technology to detect pollutants in the atmosphere have a good development prospect.

A Deep Transfer Learning Model for Packaged Integrated Circuit Failure Detection by Terahertz Imaging

Terahertz time-domain spectroscopy imaging system (THz-TDS) is becoming a promising tool for packaged integrated circuit (IC) failure detection due to its nonmetal penetrability and low radiation. However, two major obstacles are hindering the industrial application of the THz-TDS based IC detection method: 1) the low resolution of THz images may affect the detection accuracy; 2) the failure detection tasks are always carried out manually, which is inefficient and inaccurate. Thus, in this paper, we firstly enhanced the quality of IC THz images with a deconvolution algorithm and a mathematically simulated point spread function (PSF), and then we proposed a deep convolutional neural network (CNN) based failure detection framework to achieve end-to-end IC inspection automatically. Besides, we introduced transfer learning to overcome the limitation of the IC dataset size. The result demonstrated that our proposed method achieved excellent performance concerning both accuracy and efficiency.

Nondestructive evaluation of thermal barrier coating thickness degradation using pulsed IR thermography and THz-TDS measurements: A comparative study

Thermal barrier coatings are extensively used in aircraft engines. During service, the TBC coatings degrade because of erosion and sintering by hot gas flow and also by localized wear due to rubbing of flaps with spacers. It is necessary to assess the condition of the coatings as a function of service life through suitable non-destructive means. Pulse Thermography (PT) and Terahertz-Time Domain Spectroscopy (THz-TDS) techniques are used to evaluate the degree of degradation of the thin Air Plasma Sprayed (APS) TBCs top coat thickness. Infrared thermography has the advantage of fast inspection of a large area. In this work, we used a simplified analytical model aided calibration and development of a regression model to quantitatively analyze the thickness degradation in real-world TBC samples that have endured varying service life. These measurements were later verified using THz-TDS imaging, an emerging technique for accurate thickness measurements. Assuming the refractive index of the topcoat material, Yttria-Stabilized Zirconia (YSZ) as 4.8, the topcoat thickness of the entire specimen has been estimated using THz-TDS reflection mode setup. Results show that, the thickness values are varying between 94.94 μm - 114.96 μm for 500 h serviced samples and 32.5 μm - 91.96 μm in the case of 1000 h serviced samples demonstrating loss of TBC with increased service life. Comparison of the pulse thermography results with THz-TDS reveals a mean relative error of less than 10.3% in TBC thickness estimation. Further the results of both the techniques are cross-validated with Eddy current testing and optical microscopy. The proposed non-destructive techniques for TBC estimation will aid in the accurate creation of engine digital twin and will help in scheduling preventive maintenance measures thus increasing the life and safety of key aircraft engine components.

Detecting melanoma with a terahertz spectroscopy imaging technique.

Transmission mode terahertz time-domain spectroscopy system was employed to image BALB/c mouse skin tissue slices containing melanoma. The melanoma was unambiguously identified in the frequency region of 0.6-1.8 THz because melanoma has a higher refractive index as well as a higher absorption coefficient than the normal region of the skin tissue. Based on the results of hematoxylin-eosin staining and mass weighing, it was further suggested that the higher density of nucleic acids, higher water content, and lower fat content in the melanoma compared to the normal region are major factors responsible for melanoma's higher refractive index and absorption coefficient than normal tissue. The present work validates that terahertz time-domain spectroscopy imaging technique is possible to be used for the diagnosis of melanoma.

Three-dimensional water mapping of succulent Agave victoriae-reginae leaves by terahertz imaging

While terahertz imaging has been used before for the determination of water content in vegetative tissue, most studies have either presented measurements of the temporal evolution of water content at a single-point of the plant or have presented two-dimensional images of leaves, demonstrating the potential of the technique, but relatively little of such information has been used to support biologically relevant conclusions. In this article we introduce terahertz time-domain spectroscopic imaging as a technique for the determination of the three-dimensional distribution of water in succulent plant tissues. We present the first three-dimensional water mapping of an agave leaf, which demonstrates an unprecedented capability to study the water retention mechanisms within succulent plants. We found that agave leaves are composed of a low-hydration outer tissue layer, defined by the outermost layer of vascular tissue that surrounds a high-hydration tissue, the carbohydrate rich hydrenchyma. The findings are supported by histological images and the correlation between the water content and carbohydrate presence is consistent with recently published findings of a remarkably large hydration shell associated with agave fructans.

Visualization of moisturizer effects in stratum corneum in vitro using THz spectroscopic imaging.

We use terahertz time-domain spectroscopic (THz-TDS) imaging for the evaluation of moisturizing-substances effects over stratum corneum (SC) samples. Excised SC of porcine skin is used as an in vitro skin model. We evaluate the interaction of SC samples with glycerine and lanolin, two substances commonly used in moisturizers. In order to do this, THz images of SC samples after deposition of the substances are scanned. The response of the SC samples to a commercial moisturizer is also analyzed. Our results show that THz imaging is capable of sensing the distinct interaction mechanisms of the substances with the SC samples. The occlusive nature of lanolin, the hyperosmotic behavior of glycerine, and the moisturizing effect of the commercial moisturizer can be observed using THz-TDS imaging.

Terahertz nondestructive imaging for foreign object detection in glass fibre-reinforced polymer composite panels

Terahertz time-domain spectroscopy (THz-TDS) technology, a new powerful nondestructive testing (NDT) method, can provide noninvasive, noncontact, nonionizing radiation to penetrate common nonmetallic materials. In this study, we have employed THz-TDS technology to detect hidden inclusions with various shapes and materials in glass fiber reinforced polymer (GFRP) composite solid panels. Traditional transmission and reflection THz-TDS imaging are performed for detecting inclusions in GFRP solid panels. Next, the spectral difference between defect area and non-defect area has been analyzed, and different parameters for imaging are selected according to defect characteristics, which greatly improves defect detection. The THz-TDS imaging results are compared with those employing X-ray computed tomography (CT) and ultrasonic C-scan, particularly in terms of spatial resolution and overall image quality. It is found that THz-TDS technology can successfully detect hidden inclusions with relatively high image quality, and has an acceptable precision for measuring inclusion size. As such, THz-TDS technology has certain advantages in evaluating the reliability of GFRP composites.

Terahertz time-domain spectroscopy of weld line defects formed during an injection moulding process

The terahertz time-domain spectroscopy (THz-TDS) imaging technique was used to detect the weld line defect of an injection-moulded short glass fiber-reinforced composite. To determine the effect of the weld lines on tensile strength, tensile tests were conducted with the specimens fabricated with NORYL (amorphous blends of polyphenylene ether resin and polystyrene) having a short glass fiber content ranging from 0% to 30%. The time delay phenomenon of the THz waves was observed with respect to the weld line and short glass fiber contents. The interaction between the THz waves and structural characteristics of the weld line, such as density and fiber orientation, were studied and different time delay phenomena of THz signal were observed owing to the different refractive index of the weld line defect. The refractive index of THz wave was also obtained by an analytical calculation and compared with the experimental results in order to determine the correlation between degradations of tensile strength and changes in the reflected THz wave caused by the weld line. Finally, the weld line defect of the specimens was successfully detected and identified using a THz-TDS imaging technique, showing that this technique is a nondestructive inspection method that can be utilized to evaluate the reliability of injection moulded short glass fiber-reinforced composite structures.

THz Spectroscopic Identification of Red Mineral Pigments in Ancient Chinese Artworks

Nondestructive analysis of historical objects is of significance for cultural heritage conservation. In this paper, terahertz time-domain spectroscopy (THz-TDS) was used to distinguish seven red mineral pigments used in ancient Chinese artworks. Two absorption features of natural minerals HgS and four highly resolved spectral features of mineral pigment Pb3O4 were observed and identified as their fingerprints in the range 0.2 to 3.0 THz, based on which the spatial distribution of individual chemical substances including cinnabar, vermilion, and red lead were clearly revealed at various frequencies using terahertz spectroscopy imaging. Moreover, a noncontact evaluation of thickness changing and dehydration of a wet painting was monitored by inferring time delay as well as signal amplitude of THz pulses transmitted through the painting. In order to demonstrate the feasibility of THz-TDS and THz imaging for authentic artworks detection, a complete set of THz analysis of two nineteenth century wall paintings discovered in the Fuchen Temple of the Forbidden City, Beijing, was performed and the results indicate that THz measurement techniques provide a noninvasive and nondestructive solution for the care, preservation, and restoration of cultural relics.

Nondestructive evaluation of glass fiber honeycomb sandwich panels using reflective terahertz imaging

The glass fiber honeycomb sandwich panel is an efficient structural configuration which has been widely used in several designs related to air, sea, and land transport systems. Terahertz time domain spectroscopy technology is an effective supplementary method for nondestructive evaluation of composites. Terahertz time domain spectroscopy imaging technology in the reflection configuration was employed to detect the defects of glass fiber honeycomb sandwich panels, such as a skin-core debonding and a foreign inclusion. It was shown in systematic experimental investigation that different defects were detected through terahertz imaging based on different parameters, which were obtained by comparing and analyzing the waveforms in time and/or frequency domain between normal and defective regions of the sample.

Non-contact measurement of the electrical conductivity and coverage density of silver nanowires for transparent electrodes using Terahertz spectroscopy

In this work, a terahertz time-domain spectroscopy (THz-TDS) imaging technique was used for non-contact measurement of the conductivity and coverage density (DC) of silver nanowires (SNWs) as transparent electrodes. The reflection mode of THz-TDS with an incident angle of 30° was used, and the sheet resistance (Rsh) of SNW films was measured using the four-point probe method. The correlations between the THz reflection ratio and Rsh were studied by comparing the results of the four-point probe method and the measured THz reflection ratios. Also, the DC of SNWs was evaluated using THz waveforms with a general refractivity formula. This result matched well with a conventional approximation method using a scanning electron microscope image. Furthermore, defects in the SNWs could be easily detected using the THz-TDS imaging technique. The non-contact THz-TDS measurement method that we developed is expected to be a promising technique for non-contact measurement of the Rsh and DC for transparent conductive electrodes.