Phonon-polariton is the coupled excitation between optical phonon and photon. The remarkable frequency vs. wavevector dispersion relation of phonon-polariton contributes to important technological applications such as tunable terahertz radiation sources and basic materials science to clarify the terahertz dynamics of condensed matter such as lattice instability in ferroelectrics. This paper studies the broadband dispersion relation of phonon-polariton between 10 cm-1 and 1200 cm-1 in uniaxial ferroeletric crystals, LiNbO3 (LN) and LiTaO3 (LT) with polar trigonal system on the basis of the observed results using THz-Raman spectroscopy, THz time domain spectroscopy, and far-infrared spectroscopy. The dispersion on the lowest-frequency TO mode with A1(z) symmetry of LN and LT crystals, which are assigned as ferroelectric soft modes, is discussed.
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