Investigation of THz birefringence measurement and calculation in Al₂O₃ and LiNbO₃.

Based on the polarization-sensitive terahertz time-domain spectroscopy, we measured the birefringence for Al₂O₃ and LiNbO₃ single crystals, which correspond to trigonal structures that have an uniaxial birefringence, in the THz frequency range of 0.25 to 1.4 THz. For more comprehensive understanding of the THz birefringence, the measured birefringence is compared with the results of ab initio calculations. The measured birefringence shows good agreement with the calculated value.