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 A1(z) and E(x,y) symmetry polaritons. The dispersion relations of both symmetries show no avoided crossing below 60 cm-1 (1.8 THz). The complex dielectric constants Εz(ω) and Εx(ω) in the sub-THz to THz regions have also been determined as functions of frequency. Both the real and imaginary parts of Εx(ω) are well reproduced using a single damped harmonic oscillator model. However, those of Εz(ω) indicate the excess part, which may be attributed to a relaxation process related to the order-disorder nature of ferroelectricity.