Terahertz Reflection Imaging Research Articles

Terahertz reflective imaging systems with improved angle compatibility realized by the designed quasi-scattering screen

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.

Exploring the impedance-matching effect in terahertz reflection imaging of skin tissue.

Terahertz (THz) electromagnetic waves, known for their unique response to water, offer promising opportunities for next-generation biomedical diagnostics and novel cancer therapy technologies. This study investigated the impedance-matching effect, which enhances the efficiency of THz wave delivery into tissues and compensates for the signal distortion induced by the refractive index mismatch between the target and the sample substrate. Three candidate biocompatible materials, water, glycerol, and petroleum jelly were applied to a skin phantom and compared using THz two-dimensional imaging and time-of-flight imaging methods. Finally, we successfully demonstrated impedance-matching effect on mouse skin tissues.

Spectroscopic and imaging considerations of THz-TDS and ULF-Raman techniques towards practical security applications.

Nitrogen-containing high-energy organic compounds represent a class of materials with critical implications in various fields, including military, aerospace, and chemical industries. The precise characterization and analysis of these compounds are essential for both safety and performance considerations. Spectroscopic characterization in the far-infrared region has great potential for non-destructive investigation of high energetic and related compounds. This research article presents a comprehensive study of common organic energetic materials in the far-infrared region (5-200 cm-1), aiming to enhance security measures through the utilization of cutting-edge spectroscopic techniques. Broadband terahertz time-domain spectroscopy and ultra-low frequency Raman spectroscopy are employed as powerful tools to probe the vibrational and rotational modes of various explosive materials. One of the key objectives of this present work is unveiling the characteristic spectral features and optical parameters of five common nitrogen based high energy organic compounds towards rapid and accurate identification. Further, we have explored the potential of terahertz reflection imaging for non-contact through barrier sensing, a critical requirement in security applications. Based on the spectral features obtained from the spectroscopic studies and using advanced imaging algorithms we have been able to detect these compounds under various barriers including paper, cloth, backpack, etc. Subsequently, this study highlights the capabilities of the two techniques offering a pathway to enhance their utility over a wide range of practical security applications.

Detection of water leakage defects in cable insulation sheath based on terahertz time-domain spectroscopy

Water leakage of the insulation sheath can lead to a decrease in the insulation performance of the power cable, which poses a hidden danger to the power cable. Accurate detection and positioning of the water leakage area are crucial for improving power safety and stability. In response to the water leakage defect of the cable insulation sheath, this article proposes a method based on reflective terahertz time-of-flight tomography technology to measure the water leakage area of the insulation sheath. A sample of insulation sheath water leakage defect was prepared by simulating real scenarios. By analysing the terahertz reflection time-domain optical spectrum and imaging results of the sample, the location and area of the insulation sheath water leakage defect can be accurately measured, verifying the application potential of reflective terahertz time-of-flight tomography technology in detecting water seepage defects in cable insulation sheaths.

Identification of Rice Freshness Using Terahertz Imaging and Deep Learning

Retention of rice freshness is highly dependent on storage temperature. Timely and accurate identification of rice freshness is critical to ensure food security. Here, we characterize the freshness of rice in reference to storage temperature. Terahertz reflection imaging is a non-destructive and deeply penetrating technique that can be used for detecting rice freshness. Due to the shortcomings of traditional machine learning, such as limited processing of nonlinear problems and insufficient computing power. Deep learning has the advantages of strong learning ability and high portability. Therefore, for rice freshness identification, the VGG19 network and the Inception-ResNet-v2 network were used in this paper. Moreover, we propose an improved 1D-VGG19-Inception-ResNet-A network. This network possesses the advantages of low time consumption from the 1D-VGG19 network and high classification accuracy from the 1D-Inception-ResNet-V2 network. Compared with the traditional algorithms, the accuracy of the proposed network is significantly improved, with the rice freshness recognition accuracy of 99.80%. The experimental results indicate that terahertz spectral imaging and deep learning algorithms are viable tools for monitoring rice freshness.

Multiresolution spectrally-encoded terahertz reflection imaging through a highly diffusive cloak.

Turbid media, made of wavelength-scale inhomogeneous particles, can give rise to many significant imaging and spectroscopy challenges. The random variation of the refractive index within such media distorts the spherical wavefronts, resulting in smeared and speckly images. The scattering-induced artifacts can obscure the characteristic spectral fingerprints of the chemicals in a sample. This in turn prevents accurate chemical imaging and characterization of the materials cloaked with a diffusive medium. In this work, we present a novel computational technique for creating spatially- and spectrally-resolved chemical maps through a diffusive cloak using terahertz time-domain spectroscopy. We use the maximal overlap discrete wavelet transform to obtain a multiresolution spectral decomposition of THz extinction coefficients. We define a new spectroscopic concept dubbed the "bimodality coefficient spectrum" using the skewness and kurtosis of the spectral images. We demonstrate that broadband wavelet-based reconstruction of the bimodality coefficient spectrum can resolve the signature resonant frequencies through the scattering layers. Additionally, we show that our approach can achieve spectral images with diffraction-limited resolution. This technique can be used for stand-off characterization of materials and spectral imaging in nondestructive testing and biological applications.

Extended Kalman filter and extended sliding innovation filter in terahertz spectral acquisition

Terahertz spectral acquisition has a fundamental limitation in implementation due to long experimental acquisition time. The long experimental acquisition time in terahertz spectral acquisition is a result of the required high integration time associated with usable dynamic range signals acquired through delay stage interferometry. This work evaluates the effectiveness of a non-linear version of the Kalman Filter, known as the extended Kalman filter (EKF), and the recently developed extended sliding innovation filter (ESIF), for increasing dynamic range in terahertz spectral acquisition. The comparison establishes that the EKF and ESIF can reduce integration time (time constant) of terahertz spectral acquisition, with EKF reducing the integration time by a factor of 23.7 for high noise signals and 1.66 for low noise signals to achieve similar dynamic ranges. The EKF developed in this work is comparable to a nominal application of wavelet denoising, conventionally used in terahertz spectral acquisitions. The implementation of this filter addresses a fundamental limitation of terahertz spectral acquisition by reducing acquisition time for usable dynamic range spectra. Incorporating this real-time post-processing technique in existing terahertz implementations to improve dynamic range will permit the application of terahertz spectral acquisition on a wide array of time sensitive systems, such as terahertz reflection imaging, and terahertz microfluidics. This is the first implementation, to our knowledge, of Kalman filtering methods on terahertz spectral acquisition.

Front cover

Continuous‐wave terahertz reflection imaging is a potential tool for biological tissues. Based on our home‐made continuouswave terahertz reflection imaging system, the effect of both polarization mode and reflection window on the imaging performance is studied theoretically and experimentally, showing good agreement. By taking obtaining sample information and image contrast into consideration, p‐polarized terahertz waves are recommended. Moreover, considering the sample boundary identification and the image contrast, selection criteria for reflection window are proposed. This work will help to improve the performance of continuous‐wave terahertz reflection imaging and accelerate the THz imaging in biological application.Further details can be found in the article by Limin Wu, Yuye Wang, Haibin Li, Zelong Wang, Meilan Ge, Degang Xu, and Jianquan Yao (e202100245). image

Research on red and green porcelain sherd of Bayi Kiln using terahertz time-domain spectroscopy and imaging

Terahertz (THz) technology is particularly suitable for non-destructive detection of porcelain sherd due to the unique properties of non-ionizing, high penetration, and broad-spectrum. In this paper, we investigated the reflection characteristics of the pigments and sediments for red and green porcelain sherd of Bayi Kiln made in Shanxi province during the Jin Dynasty by terahertz time-domain spectroscopy (THz-TDS), obtaining the spectral responses of different components. Furthermore, Raman spectroscopy was used as a complementary technique to complete the qualitative analysis of the pigments and sediments. In addition, the reflective THz imaging was performed to visualize the pigments and sediments in different regions of the porcelain sherd, realizing the visual detection of the pattern on the surface of the porcelain sherd. These results show that THz-TDS and THz-imaging can serve as effective tools for the detection and analysis of ancient Chinese porcelain.

Optimization for continuous-wave terahertz reflection imaging for biological tissues.

Continuous-wave terahertz reflection imaging is a potential tool for biological tissues. Based on our home-made continuous-wave terahertz reflection imaging system, the effect of both polarization mode and reflection window on the imaging performance is studied theoretically and experimentally, showing good agreement. By taking obtaining sample information and image contrast into consideration, p-polarized terahertz waves are recommended. Moreover, considering the sample boundary identification and the image contrast, selection criteria for reflection window are proposed. This work will help to improve the performance of continuous-wave terahertz reflection imaging and accelerate the THz imaging in biological application.

Diagnosis of injection-molded weld lines in ABS thermoplastic by polarized terahertz reflective imaging

We carry out a nondestructive study of injection-mold weld lines in an acrylonitrile butadiene styrene (ABS) thermoplastic electrical receptacle plate. Terahertz techniques show morphological features at the weld lines and reveal terahertz birefringence properties localized around the welds, indicating frozen-in molecular orientation and stress proximate to the welds. Surface acoustic microscopy corroborates the morphological features at the welds shown by terahertz imaging and also reveals anomalous mechanical properties at the weld lines. These results indicate that THz-based techniques provide nondestructive reliability assessment and quality control for injection-molded thermoplastic products.

Continuous-wave terahertz reflective ptychography by oblique illumination.

Massive usage scenarios prompt the prosperity of terahertz (THz) reflective imaging methods. In this Letter, we apply ptychography to continuous-wave THz reflective imaging. Our scheme has a compact lensless layout and uses a full-field oblique-illumination recording mode. Diffraction patterns are corrected through tilted plane correction. This method can be used to retrieve the complex-valued object function and to suppress the negative effect of non-uniform illumination. The feasibility is investigated using two metal samples.

Interference elimination based on the inversion method for continuous-wave terahertz reflection imaging.

We propose a novel approach based on the inversion method to eliminate interference in the continuous-wave (CW) terahertz (THz) reflection imaging. Through the study on the imaging window of the CW-THz reflection imaging with the interference mechanism, inverse processing is introduced to realize the interference elimination. Based on the theoretical calculation, high resistivity float-zone silicon (HRFZ-Si) with high refractive index is selected as the imaging window to improve the dynamic range of the THz image. The interference elimination method is verified experimentally by a CW-THz reflection imaging system based on a THz quantum cascade laser (QCL) lasing at 4.3THz. The reflectivities of liquid samples of water and ethanol are restored by the interference elimination method, which corresponds well with the theoretical calculation. Moreover, the interference elimination method is performed on THz images of fresh biological tissues. The image contrast of tissue can be greatly enhanced with the accurate reflective information.

High-Speed and Cost-Effective Reflective Terahertz Imaging System Using a Novel 2D Beam Scanner

Nondestructive THz imaging technology is a promising technology in several industrial applications such as process monitoring and quality control. To further popularize the application of this technique in the industry, we developed photonics-based devices such as a dual-mode laser, uni-travelling carrier photodiode, and Schottky-barrier diode that enable the realization of a compact, low-cost, multifunctional continuous wave (CW) THz system. To develop CW THz reflective imaging systems with these devices, a telecentric f-θ lens and novel two-dimensional (2D) beam scanner were developed. A high-speed, low-cost 2D reflector with a 4-sided polygon mirror, a brushless DC (BLDC) motor rotating at approximately 2,990 revolutions per minute (RPM) and a stepping motor was designed to ensure that a THz image sized 500 × 400 pixels could be scanned in approximately 2 s. However, it is difficult to maintain a constant speed in high-speed rotating BLDC motors. The measured maximum deviation is approximately 0.7%, which results in distorted images. To compensate for this distortion, a sync signal reflector was designed to monitor the real-time RPM of the BLDC motor. Using the novel 2D beam scanner and sync signal reflector, the distorted images could be corrected simply, enabling the development of a 2D THz reflective imaging system using a single emitter and detector; this system could exhibit a higher speed and was more cost effective than the existing system. Although further improvements to photonics-based THz technologies are necessary, we believe that these efforts will begin an era of THz technologies as a widely used industrial technique.

Scanning laser terahertz near-field reflection imaging system

We developed a scanning laser terahertz (THz) near-field reflection imaging system using a fiber-coupled femtosecond laser source for high spatial resolution measurements without any sub-wavelength probes. In this system, THz wave pulses generated at the laser focusing spot in a non-linear optical crystal interact with a sample set on the crystal. Using this system, we obtained the THz reflection images of metallic structures with a resolution of ∼λTHz/34 (λTHz = 680 μm) and also observed paraffin-embedded human breast tissue of several 10 μm in size. These results promise that this system can be suitable for industrial applications which require micron-scale resolution.

Study of in vivo brain glioma in a mouse model using continuous-wave terahertz reflection imaging.

We demonstrated that in vivo brain glioma in a mouse model using a continuous-wave terahertz reflection imaging system, as well as the ex vivo fresh brain tissues in mouse model. The tumor regions of in vivo and ex vivo brain tissues can be well distinguished by THz intensity imaging at the frequency of 2.52THz. The THz images with high sensitivity correlated well with magnetic resonance, visual and hematoxylin and eosin stained images. Furthermore, the THz spectral difference between brain gliomas and normal brain tissues were obtained in the 0.6THz to 2.8THz range, where brain gliomas have the higher refractive indices and absorption coefficients, and their differences increase particularly in the high frequency range. These results suggest that THz imaging has great potential as an alternative method for the intraoperative label-free diagnosis of brain glioma in vivo.

Fast terahertz reflective confocal scanning imaging with a quantum cascade laser and a photodetector

For the terahertz imaging systems, the fast detection of terahertz light is deemed as a key technology. We present a fast terahertz reflective confocal scanning imaging system employing a fast terahertz quantum-well photodetector (QWP) and a fast rotating translational platform. The excellent performance of the terahertz QWP indicates that the device can rapidly detect the 4.3 THz radiation generated from a pulsed electrically pumped terahertz quantum cascade laser, which is used as the terahertz source of the imaging system. Two 200 μm pinholes are placed in the two focuses to filter out the scattered light and improve the resolution of the imaging system, we achieve a lateral resolution better than 110 μm and an axial resolution of about 320 μm. The terahertz two-dimension images of some objects are obtained within 5 s with a high contrast. The three-dimensional sections of the object demonstrate the great axial selectivity of the terahertz imaging system.

Diffractive Elements for Zero-Order Bessel Beam Generation With Application in the Terahertz Reflection Imaging

Novel diffractive elements are presented to produce zero-order Bessel beams in the terahertz (THz) band. First, diffractive elements are designed as phase plates to change the direction of the incident Gaussian beam. Then, they are fabricated by the emerging three-dimensional printing technology. Last, zero-order Bessel beams are generated by the diffractive elements at 0.3 THz. To verify the feasibility of this approach, the generated zero-order Bessel beams are monitored and compared with the ones generated by the axicons. The comparison confirms that the diffractive elements can produce Bessel beam effectively and efficiently. In addition, the diffractive elements are applied to introduce zero-order Bessel beam to a THz reflection imaging system. A resolution test phantom is imaged by the THz reflection imaging system with zero-order Bessel beam and a conventional THz reflection imaging system. The results indicate that the THz reflection imaging system with Bessel beam offers a significantly extended depth of field and high anti-interference capability.

Investigation of Keratinizing Squamous Cell Carcinoma of the Tongue Using Terahertz Reflection Imaging

We investigated the feasibility of using terahertz (THz) reflection imaging to detect keratinizing squamous cell carcinoma (SCC) of the tongue. Four fresh keratinizing SCC tissues were studied, which had been surgically resected. All of the keratinizing SCCs were well distinguished from normal healthy tissues. We showed that the tumor regions exhibited low THz reflection despite having higher water content than normal regions. The refractive indices and absorption coefficients were low in the tumor tissues despite the relatively high water content. Our results showed that there were dominant factors such as keratin pearls, other than the water content affecting the THz reflection signal.

Detection of foreign bodies in grain with terahertz reflection imaging

Foreign bodies embedded in grain are difficult to detect with conventional measurement methods. In this paper, terahertz time-domain spectroscopy (THz-TDS) reflection imaging is proposed as a non-destructive and deeply penetrating technique for detecting the foreign bodies found in wheat grain with different depths below the surface. Image preprocessing and threshold segment algorithm are employed to improve the THz images. In contrast to the behavior of wheat grains, foreign bodies embedded in flour are also detected and analyzed. The results indicate that the THz reflection imaging technology promises to be a useful tool for observing the presence of foreign bodies in grain.