Progress of ultrafast terahertz time-domain spectroscopy: Raman inactive soft mode in quantum paraelectric SrTiO3

Abstract

We have measured the complex dielectric constants of quantum paraelectric SrTiO3 (STO) bulk single crystals by using the ultrafast and broadband terahertz time-domain spectroscopy (THz-TDS) with the reflection configuration. The observed complex dielectric dispersion consists of the Raman inactive TO1 soft mode and is well reproduced by damped harmonic oscillator model where the resonant frequency and the damping constant are 2.75 THz and 0.63 THz, respectively, in good agreement with the previous reports. The ultrafast THz-TDS used in this study employs the technique of high-speed asynchronous optical sampling and Cherenkov type THz generator. The sampling time of one scan is about 10 milliseconds and available frequency of the THz source is from 0.5 to 7 THz. The reflection configuration combined with the ultrafast system enabled to detect the dispersion of a STO bulk sample which has high permittivity and opaque in the THz region.