Raman Inactive Phonon–Polariton Dispersion of Quantum Paraelectric KTaO3 Proved by Broadband Terahertz Time-Domain Spectroscopy and FTIR

KTaO3 (KTO) is a quantum paraelectric perovskite oxide which belongs to the cubic space group Pm3¯m in a large temperature range. Polar optical modes with a T1u symmetry of KTO are infrared-active and Raman-inactive according to the centrosymmetric exclusion principle of the selection rule. In general, the soft modes responsible for ferroelectric instability are infrared-active and Raman-inactive in the paraelectric phase. Therefore, there are still not enough studies on Raman-inactive soft modes and related phonon polaritons. In the present study, Raman-inactive polar modes and related polaritons of KTO crystals are studied by Terahertz Time-Domain spectroscopy (THz-TDS) and FTIR. The real and imaginary parts of a dielectric constant along the [100] axis are uniquely determined by transmission and reflection THz-TDS without any fitting in the low-frequency range between 6 and 225 cm−1, which covers the two lowest-frequency polar modes. The reflectivity is determined by reflection FTIR in the range between 50 and 1200 cm−1, and the complex dielectric constant is also estimated by the fitting in the range between 6 and 1200 cm−1. The phonon–polariton dispersion relations of the real and imaginary parts of the polariton wavevector are also studied in the range between 6 and 1200 cm−1. The crossover from photon-like to phonon-like polaritons and related polariton decay are observed, while no anomaly related to polariton scattering and coupling to other elementary excitations is observed in the polariton dispersion.