Terahertz (THz) technology has firmly established itself as an effective sensing and nondestructive testing technique for the detection of substances and physico-chemical evaluation of materials and structural systems since its first emergence almost three decades ago. To date, both the effectiveness and accuracy of this technology have been extensively demonstrated in a myriad of applications across the spectrum of research and development all the way to process analytical technology, quality control, nondestructive testing, and structural health monitoring. These applications are generally enabled by the production and availability of advanced, versatile, robust, highly accurate, and industrially rugged THz spectroscopy and imaging systems, the unique properties of THz waves compared with other electromagnetic waves, as well as the advancements in electronics, photonics, and THz metamaterial systems development. This article presents a comprehensive state-of-the-art and state-of-the-practice review of sensing and nondestructive testing applications of THz technology and analyzes the role of THz metamaterials in enhancing the resolution and sensitivity of THz systems. The study also provides a general overview of the fundamentals of THz spectroscopy and imaging systems and discusses the suitability of THz sensing and nondestructive testing in a variety of real-world application scenarios ( e.g . composites’ defect detection and evaluation, paints and coatings thickness measurement and characterization, biomolecule detection, etc .). Aspects such as the noise caused by the presence of barriers, challenges with experimental implementations and operability of THz systems, long times required to acquire THz images, as well as limited customizability and portability of currently available THz systems are also discussed.

While thermal barrier coatings (TBCs) are being sprayed onto aero-engine turbine blades, or while the engine blade is working, high temperatures and strong impact forces will damage TBCs under thermal cycles, resulting in the coating peeling off from the blades. The current method of using ECT, IRT, or another method alone cannot achieve the real-time detection of coating defects with both high precision and high penetration power. Two detection methods, namely, terahertz pulsed imaging (TPI) and optical coherence tomography (OCT), were combined to evaluate typical defects observed in TBCs (including internal debonding cracks, surface high-temperature cracks, and surface etched cracks). The results showed that the OCT system successfully obtained the micron-level axial resolution, but the detection depth of the OCT system was limited. The TPI system achieved a higher penetration depth than OCT—hence, it can be used for the nondestructive detection and evaluation of the internal debonding defects in the sample—but its resolution needs to be improved. Following this conclusion, a method is proposed using TPI and OCT concurrently for the nondestructive testing and quantitative evaluation of TBCs on etched cracks, thus achieving progress both in terms of depth and resolution. In our experiment, defects with a depth of 519 μm and a width of 100 μm were measured. The proposed method is suitable for situations where multiple defects in TBC samples of blades need to be detected simultaneously during the working process. When there are defects deep inside the sample, more small cracks on the surface can be evaluated to achieve a combination of depth and accuracy.

The prism-coupled absorber with electrically-driven MoS ₂ excites perturbed plasmon modes to induce susceptible dielectric response and unique dual phase singularities, which achieve active multidomain manipulations in an ultra-broadband range.