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 60 cm -1 (1.8 THz). The complex dielectric constants ε z (ω) and ε x (ω) in the sub-THz to THz regions have also been determined as functions of frequency. Both the real and imaginary parts of ε x (ω) are well reproduced using a single damped harmonic oscillator model. However, those of ε z (ω) indicate the excess part, which may be attributed to a relaxation process related to the order-disorder nature of ferroelectricity.