Terahertz spectroscopy and its applications to dielectric and electrical insulating materials

Calcium carbonate (CaCO₃) is a white painting material and shows high absorption at approximately 3 THz. In this paper, we investigate the relationship between absorption and the condition of CaCO₃ crystals . By employing terahertz time-domain spectroscopy (THz-TDS), we verify that terahertz absorbance differs from visible light absorbance and depends on the crystal size and direction. It is observed that when the direction of the terahertz electric field is nearly parallel to the optic axis, the absorbance became large. This shows that the CO₃ group in the crystal, which has a dipole moment, affects absorption at 3 THz. From the above observation, we demonstrate that we can easily discriminate between CaCO₃ and other white painting materials by employing THz-TDS; moreover, the CaCO₃ crystal condition can also be determined.