The terahertz (THz)-based thickness measurement of thermal barrier coatings (TBCs) demands smart determination of THz peaks instead of human intervention for efficiency and accuracy, especially in the case of uneven microstructure due to imperfect spraying conditions. To this end, in this article, a long short-term memory (LSTM) framework driven by a theoretical model of THz signals is proposed to locate peaks automatically and location errors are eliminated by finding the maximum around the originally located results. Then, the local extrema are used to extract peaks for calculating refractive index online to remove the influence of the change in refractive index on thickness measurement. Finally, the simulations and experiments were carried out to verify the proposed method. The results show that our method generated identical results with those from the manual intervention, while the efficiency is significantly improved.

In this paper, a polarization-independent graphene metasurface structure is designed to realize the excellent functions of slow-light and sensing generated based on plasma-induced transparency (PIT) in terahertz region. The structure includes a cross-shaped and four square graphene resonators that are identical. The unique PIT transparent window generated by bright-bright mode coupling is studied by using finite difference time domain (FDTD) and coupled mode theory (CMT), which are highly consistent. By changing the Fermi level and carrier mobility of graphene, the transmission spectrum of PIT can be tuned effectively. Interestingly, due to the field enhancement and strong dispersion properties of surface plasma, the graphene metasurface proposed in this paper has excellent effects in slow-light, the maximum group index can reach 658. At the same time, because the transmission spectrum of PIT is sensitive to the external refractive index, the sensitivity and figure of merit (FOM) used to evaluate the sensing performance of the structure are 1.7134 THz/RIU and 144.45, respectively. Finally, because the graphene metasurface is a center symmetric structure, it has polarization-insensitive performance. The proposed structure can be used as a slow-light device or applied to biomolecular recognition, environmental monitoring and other terahertz sensing fields.

This paper proposes a methane sensor based on localized surface plasmon resonance (LSPR) of a hexagonal periodic gold nanoring array. The effects of structural parameters on the extinction spectrum and refractive index (RI) sensitivity are analyzed to obtain optimal parameters. In particular, the RI sensitivity can reach 550.08 nm/RIU through improvement of the sensor structure, which is an increase of 17.4% over the original value. After coating a methane-sensitive membrane on the inner and outer surfaces of the gold rings, the methane concentration can be accurately measured with a gas sensitivity of −1.02 nm/%. The proposed method is also applicable to quantitative analyses of components concentration and qualitative analyses of gas composition.