Dielectric properties of ferroelectric lithium tantalate crystals studied by terahertz time-domain spectroscopy

The terahertz (THz) dynamics of ferroelectric lithium tantalate crystals has been investigated using the latest THz time-domain spectroscopy. The intensity and phase shift of transmitted THz radiation have been accurately measured for the propagation of the THz radiation along the y-axis down to 3 cm(-1) (0.1 THz). The phonon-polariton dispersion relations have been determined from the phase shift for A(1)(z) and E(x, y) symmetry polaritons. The dispersion relations of both symmetries show no "avoided crossing" below 60cm(-1) (1.8THz). The complex dielectric constants epsilon(z)(omega) and epsilon(x)(omega) in the sub-THz to THz regions have also been determined as functions of frequency. Both the real and imaginary parts of epsilon(x)(omega) are well reproduced using a single damped harmonic oscillator model. However, those of epsilon(z)(omega) indicate the excess part, which may be attributed to a relaxation process related to the order-disorder nature of ferroelectricity.