Optical non-destructive testing (NDT) has gained more and more attention in recent years, mainly because of its non-destructive imaging characteristics with high precision and sensitivity. This paper provides a review of the main optical NDT technologies, including fibre optics, electronic speckle, infrared thermography, endoscopic and terahertz technology. Among them, fibre optics features easy integration and embedding, electronic speckle focuses on whole-field high precision detection, infrared thermography has unique advantages for tests of combined materials, endoscopic technology provides images of the internal surface of the object directly, and terahertz technology opens a new direction of internal NDT because of its excellent penetration capability to most of non-metallic materials. Typical engineering applications of these technologies are illustrated, with a brief introduction of the history and discussion of recent progress.

Terahertz metamaterial absorbers are important functional devices for liquid analyte detection. In contrast to general metamaterial absorbers with single-layer metasurfaces that possess only one resonant mode, a triple-band terahertz metamaterial absorber formed by a single layer of symmetrically arrayed snowflake-shaped resonators was proposed in this study. The simulation results showed that the absorption of the metamaterial absorber reached 97.43% at 0.550 THz, 79.22% at 1.249 THz, and 99.02% at 1.867 THz with narrow resonant peaks. The resonant frequencies were sensitive to the refractive index of the surrounding medium at a fixed analyte thickness, which would play an important role in the performance of the sensor for detecting changes in the surrounding refractive index. The maximum value of the refractive index sensitivity was 137.70 GHz/RIU, 306.25 GHz/RIU, and 473.86 GHz/RIU, with a figure of merit (FoM) of 3.14, 2.33, and 6.46, respectively, for refractive index values ranging from 1.0 to 2.2 under three resonant modes. It is worth noting that most of the liquid samples showed a refractive index ranging from 1.0 to 2.0. Furthermore, the identification of peanut oil, carbon disulfide, and turpentine was considered to verify that the proposed terahertz sensor could be used for high-sensitivity liquid detection and has broad development prospects in the field of detecting and sensing.

A theoretical model for the electrical conductivity size effect of square nanowires is proposed in this manuscript, which features combining the three main carrier scattering mechanisms in polycrystalline nanowires together, namely, background scattering, external surface scattering, as well as grain boundary scattering. Comparisons to traditional models and experiment data show that this model achieves a higher correlation with the experiment data.