Terahertz reflectance probing of buried surface roughness in solid dielectrics

Surface roughness plays a critical role in interfacial breakdown and surface flashover of solid dielectrics used in high-voltage power equipment. However, conventional profilometric techniques are limited to surface-accessible regions and cannot characterize buried surface morphologies within dielectric materials. Here, we report a terahertz (THz) time-domain spectroscopy approach that exploits the interaction between THz waves and rough surfaces to quantify surface roughness. A reflection model based on the scattering loss factor (ρS) is established, indicating that micro-scale surface roughness can induce significant scattering effects on THz wave reflection, thereby supporting the feasibility of THz-based roughness characterization in solid dielectrics. Experimental results show that the THz reflectance (R) decreases nonlinearly with increasing root mean square height (Rq), exhibiting a concave trend within the range of 0.3–9 μm. The relationship between R and Rq can be empirically described using a cubic fitting function with high agreement (sum of squared error < 0.1). Furthermore, an effective reconstruction of buried surface roughness is achieved using a frequency-domain filtering-based approach. The reconstructed roughness values show good agreement with profilometer measurements, with deviations within 0.2 μm. This work provides a non-destructive and quantitative approach for evaluating both accessible and buried surface roughness in solid insulation, offering a potential pathway for interface characterization in dielectrics.