Non-destructive detection of coating delamination using reflected time-domain terahertz waveforms

 Tobar, Daniel, Sri Kambhampati, Anthony J. Fitzgerald, Brett Nener, Thunyaluk Pojtanabuntoeng, and Vincent P. Wallace. "Non-destructive detection of coating delamination using reflected time-domain terahertz waveforms." Scientific Reports 15, no. 1 (2025): 42401.

Abstract
Adhesive failure leading to delamination in anti-corrosion coating systems compromises structural integrity by creating thin gaps that allow the ingress of corrosive agents like water. This study investigates the capability of terahertz pulsed imaging to detect air gaps thinner than a THz system’s minimum resolvable thickness by measuring apparent thickness changes in overlaying layers. The minimum resolvable thickness in terahertz pulsed imaging, referred to as the axial resolution, is determined by the system’s bandwidth. To test this approach, THz measurements were conducted on a 2 mm thick quartz window positioned above a metal substrate with an intervening 19 m air gap. An increase in the apparent thickness of the quartz window was observed, consistent with theoretical expectations, despite the air gap being undetectable as a distinct reflection. This analysis was extended to multi-layered paint systems consisting of top, mid, and base coatings on steel plates. Cyclic ageing induced a delamination layer between the metal substrate and base coating, estimated to be 10 m by scanning electron microscopy. Terahertz pulsed imaging analysis showed a corresponding 9.9–13.1 m increase in the apparent base coating thickness, consistent with the quartz window results. These findings demonstrate that terahertz pulsed imaging offers a non-destructive method for identifying sub-resolution air gaps, enabling early-stage detection of delamination in protective coatings.

see https://www.nature.com/articles/s41598-025-26552-6

Reference-free humidity sensing based on terahertz time-domain spectroscopy

Zhu, Ting, Shuting Fan, Jian Huang, Bin Ye, Di Zhao, Guangyou Fang, and Xuequan Chen. "Reference-free humidity sensing based on terahertz time-domain spectroscopy." Sensors and Actuators B: Chemical (2026): 139500.

Abstract

Humidity sensing is crucial across various applications, yet conventional electrical sensors rely on physical contact, limiting their accuracy, response speed, and spatial flexibility. Water vapor exhibits dense rotational transitions in the terahertz regime, making terahertz spectroscopy an attractive tool for humidity sensing. However, absorption-based spectroscopic sensing demands a frequently measured zero-concentration reference to extract intrinsic absorption spectra, hindering its practical deployment. In this work, we propose a reference-free humidity sensing method based on terahertz time-domain spectroscopy. The method employs a database-comparison strategy empowered by a two-step calibration procedure, which efficiently adapts the database to variations in optical path length, temperature and spectral response. By comparing any newly measured signal with the database, humidity levels can be directly determined from raw signals without reference and absorption models. The method achieved a root mean square error of relative humidity down to 0.62 % across various measurement conditions and systems. Furthermore, its contact-free nature enables spatially averaged and real-time response, as demonstrated in the respiration monitoring measurement. Overall, the technique outperforms conventional humidity sensing devices over multiple aspects, showing strong potential for applications in medical diagnosis, extreme industrial environments and advanced scientific instrumentation.

see https://www.sciencedirect.com/science/article/abs/pii/S092540052600078X

THz Non-Destructive Testing Method for Internal Moisture Defects in Composite Insulators

 Wang, Pei, Yanyan Bao, Yongliang Yao, Yueyi Wang, Pin Jiang, Yushuo Wu, Pengzhen Wu, and Shuaibing Li. "THz Non-Destructive Testing Method for Internal Moisture Defects in Composite Insulators." IEEE Access 14 (2026): 7204-7216.

ABSTRACT 

The presence of internal moisture defects at the interface between the core rod and skirt has been identified as a primary factor contributing to the failure of composite insulators. However, effective detection methods for such defects remain to be fully developed. In order to address this issue, the present paper puts forward a non-destructive testing method based on terahertz technology. Firstly, composite insulator samples containing moisture defects were fabricated in order to simulate real-world defects. Subsequent experimental testing yielded the terahertz time-domain/frequency-domain characteristics and imaging features of different samples. The optical parameters of the defective samples were extracted using an extended fourth-order Dedye model. The particle swarm optimization (PSO) method was then employed to overcome the limitations of traditional nonlinear fitting methods in dispersion models. Finally, based on the fitted parameters, time-domain finite-difference time-domain (FDTD) simulations were performed to invert the internal moisture defects within composite insulators in the terahertz frequency band, thus validating the accuracy of the experimental results. The findings demonstrate that the simulation inversion results align with actual test data, thus confirming that terahertz time/frequency domain parameters can effectively characterize internal moisture defects in composite insulators. The time-frequency domain characteristics facilitate qualitative analysis of defect occurrence and quantitative calculation of defect thickness. When combined with terahertz imaging results, this approach facilitates rapid determination of the location and geometric dimensions of moisture defects. Consequently, it achieves non-destructive testing of internal moisture defects in composite insulators, ensuring operational reliability and providing technical support for their safe and stable operation.

see https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=11343885

Monitoring Tensile-Induced Subsurface Damages of Woven Glass Fiber Reinforced Polymer Using Terahertz Time-of-Flight Tomography

 Zhai, Min, Haoyue Pan, Bin Xiao, Haolian Shi, Zhang Qu, Wenlong He, Cong Zhai, and Yi Tang. "Monitoring tensile-induced subsurface damages of Woven Glass Fiber Reinforced Polymer using terahertz time-of-flight tomography." NDT & E International (2026): 103645.

Abstract

Woven Glass Fiber Reinforced Polymer (GFRP) composites were studied using terahertz time-of-flight tomography to characterize failure modes in GFRP composite in a nondestructive and contactless fashion during in-situ tensile testing. The fracture morphologies of GFRP composite under different applied stresses were discussed by comparing terahertz C-and B-scan images to evaluate the dynamic evolution of tensile-induced microstructure. Our results show that significant THz-detectable damage initiation was observed at stress levels exceeding 60 MPa. In addition, tensile-induced damage can be observed not only on the surface, but also within the inner piles of GFRP composites. Finally, our work verifies the effectiveness of THz-based approach on three-dimensional dynamic monitoring the quality of GFRP composite in service and evaluating the influence of different loading conditions on structural properties and failure pattern of composite materials.

see https://www.sciencedirect.com/science/article/abs/pii/S0963869526000162

Real-time observation of coherent spin wave handedness

 Ha, Taewoo, Kyung Ik Sim, Howon Lee, Hyun Jun Shin, Sanghoon Kim, Se Kwon Kim, Jae Hoon Kim, Dong-Soo Han, Young Jai Choi, and Byung Cheol Park. "Real-time observation of coherent spin wave handedness." npj Spintronics 2, no. 1 (2024): 37.

Abstract

Magnonics, a crucial domain in information science and technology, utilizes spin waves in magnets as efficient information carriers. While antiferromagnets have been suggested for versatile magnonic platform because of the coexistence of right- and left-handed spin waves, their energetic degeneracy poses challenges for observation through spectral measurements, limiting their applicability. Recent observations of distinct spin wave handedness within the gigahertz regime have reported but, are yet to be demonstrated in terahertz (THz) frequencies of antiferromagnetic spin waves. Most of all, the coherence of spin waves is a key aspect of quantum information. Here, employing THz time-domain spectroscopy—a direct, precise, and easy probe for monitoring coherent spin wave dynamics—we discern chiral antiferromagnetic spin waves of opposite phase windings in the time domain, noting their handedness reversal across the angular momentum compensation temperature in ferrimagnets. We establish a principle for directly measuring the handedness of coherent antiferromagnetic spin waves in ferrimagnets with net magnetic moment M ≠ 0 but angular momentum L = 0. Our multidimensional access in the time and spectral domain enables the accurate determination of critical temperature and the dynamic observation of coherent chiral spin waves simultaneously in a single experiment, with potential applications in exploring other quantum chiral entities.

see https://www.nature.com/articles/s44306-024-00040-5