Terahertz time-domain spectroscopy of congruent LiNbO_3 and LiTaO_3 crystals

摘要

The complex dielectric constants of congruent LiNbO_3 and LiTaO_3 single crystals in a THz frequency region have been determined directly by using broadband terahertz time-domain spectroscopy (THz-TDS) with reflection configuration. We have successfully extracted the accurate complex dielectric constants by using corrected phase information derived from the transmission measurement results. The lowest E(TO1) phonon modes of both crystals have been observed clearly. On the other hand, additional bands at approximately 2.5 THz below the lowest A_1(TO1) mode frequency have been observed. These bands have also been reported by Raman and hyper-Raman spectroscopy and attributed to intrinsic defects in congruent crystals. According to the experimental results that these additional bands were observed at almost the same frequency on Raman, hyper-Raman, and THz-TDS, these bands are suggested to be assigned as a first-order scattering process.