Fig. 1. Electromagnetic spectrum and terahertz gap^[11]

Fig. 2. Structure diagram

Fig. 3. The relationship between NRCS and incidence angle for clean sea surfaces and oil-covered sea surfaces at 1 THz. (a) VV polarization; (b) HH polarization^[27]

Fig. 4. Electromagnetic scattering from an air-oil-water double-interface structure (left); Equivalent single-interface structure (right) ^[28]

Fig. 5. (a) The effect of oil film on single-stable NRCS at 1 GHz; (b) The effect of oil film on single-stable NRCS at 5 GHz; (c) The effect of oil film on single-stable NRCS at 0.5 THz; (d) The effect of oil film on single-stable NRCS at 1 THz; (e) The relationship between NRCS and oil thickness^[28]

Fig. 6. (a) Refractive index distribution; (b) Absorption coefficient distribution; (c) Real part of the dielectric constant distribution; (d) Imaginary part of the dielectric constant distribution^[36]

Fig. 7. The real dielectric constant (a) and imaginary dielectric constant (b) of water, ethanol, and fuel treatment; The real dielectric constant (c) and imaginary dielectric constant (d) of gasoline 95, gasoline 98, and diesel; The real dielectric constant (e) and imaginary dielectric constant (f) of 50/50 mixed gasoline/diesel, kerosene, and general bottle cleaner ^[39]

Fig. 8. (a) Schematic diagram of the THz-TDS system; (b) Physical image of the THz-TDS system and the sample with defects for detection^[46]

Fig. 9. (a) Glass fiber sample tested using THz-TDS, with significant color change after water immersion; Absolute measurements of the dielectric constant of the glass fiber sample after water immersion at low-frequency THz: (b) Real dielectric constant; (c) Loss tangent ^[46]

Fig. 10. Composite transmission imaging of two glass fiber samples in the 0.1-0.4 THz range. (a) Sample thickness of 9.0 mm; (b) Sample thickness of 14.6 mm^[46]

Fig. 11. Terahertz image of glass fiber reinforced composite with incomplete hole defects^[47]

Fig. 12. Theoretical model of terahertz wave propagation in marine protective coatings^[52]

Fig. 13. Original terahertz signal from the coating steel substrate. (a) Two layers of 152 mm anticorrosive coating on the steel substrate; (b) Three layers of 127 mm antifouling coating and two layers of 152 mm anticorrosive coating on the steel substrate^[52]

Fig. 14. (a) Thickness distribution map of ship protective coatings; (b) Paint peeling defect detection of ship protective coatings; (c) Corrosion defect detection of ship protective coatings^[52]

Fig. 15. Reflection principle of terahertz waves^[56]

Fig. 16. (a) Physical image of PE pipe with pre-fabricated defects; (b) 2D lattice scan image^[56]

Fig. 17. (a) E_P obtained at different levels of damage; (b) THz-TDS under air (green), intact PE plate (red), intact PE plate with gaps (black), and intact PE plate with water (blue); E_P of PE plates with different water (c) and oil (d) contents at different times^[57]

Fig. 18. (a) Astaxanthin absorption peak; (b) \begin{document}$ \beta $\end{document}-carotene absorption peak; (c) Starch absorption peak; (d) Terahertz absorption spectra of Haematococcus pluvialis after baseline removal at different stress durations^[65]

Fig. 19. Terahertz absorption spectroscopy-based linear prediction models for (a) astaxanthin; (b) \begin{document}$ \beta $\end{document}-carotene and (c) starch^[65]

Fig. 20. Terahertz absorbance of riboflavin and bacillus subtilis at different culture times. (a) The absorbance of riboflavin; (b) The absorbance of bacillus subtilis strain with very low riboflavin yield; (c) The absorbance of Bacillus subtilis strain with high riboflavin yield^[66]

Fig. 21. (a) Schematic diagram of cesium atoms placed in a cage, which will vibrate slowly and resonate with low-frequency terahertz light; (b) Schematic diagram of a cyanide-bridged manganese-iron metal framework adsorbing Cs ions from a solution^[67]

Fig. 22. (a) THz-TDS absorption spectrum of the cyanide-bridged manganese-iron metal framework; (b) Terahertz spectra of samples collected from Cs solutions of different concentrations; (c) Terahertz spectral components generated due to the cesium vibration mode at 1.5 THz; (d) Relationship between the cesium ion composition in the sample and the absorption peak area of the samples recovered from various cesium solutions^[67]