Optical–dielectric characterization and contactless thickness measurement of ceramics based on terahertz spectroscopy

In this study, the optical dielectric properties of 95# and 99# ceramics in the terahertz (THz) band are investigated using THz time-domain spectroscopy. The refractive indices of the 95# and 99# ceramic samples remain relatively stable in the 0.3-1.6 THz frequency range, at 2.95 and 3.1, respectively. The absorption coefficient increases with frequency, with the 95# ceramic exhibiting a higher value due to its higher impurity content. Ceramic 99# exhibits higher polarization capability and energy storage capacity, while ceramic 95# has a larger imaginary part, which may be associated with its higher dielectric loss, thereby leading to stronger terahertz wave attenuation. The imaginary part of the conductivity of both ceramics is negative, indicating that the polarization effect dominates the energy response. Furthermore, three thickness measurement models based on time-domain spectra-peak-time fitting, reflection, and transmission models-are evaluated. Experimental results show that all models effectively detect sample thickness, with a minimum detectable thickness of less than 34 µm. These methods enable non-contact thickness measurement, wear assessment, and quality control.