Sensing and Nondestructive Testing Applications of Terahertz Spectroscopy and Imaging Systems: State-of-the-Art and State-of-the-Practice

Terahertz (THz) technology has firmly established itself as an effective sensing and nondestructive testing (NDT) technique for the detection of substances and physicochemical 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 (PAT), quality control, NDT, 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 (THz-SI) 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 NDT 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-SI systems and discusses the suitability of THz sensing and NDT in a variety of real-world application scenarios (e.g., composites' defect detection and evaluation, paints and coatings thickness measurement and characterization, and biomolecule detection). 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.