Terahertz time-domain spectroscopy of infrared active soft mode and phonon-polariton dispersion

ABSTRACT

The recent progress of Terahertz Time-Domain Spectroscopy (THz-TDS) using a coherent terahertz radiation enables the accurate and reliable measurements of dielectric properties in comparison with the traditional far-infrared spectroscopy using an incoherent light source. The applications of the broadband THz-TDS to infrared active soft modes and phonon-polaritons in ferroelectrics were reviewed. The broadband THz-TDS is a powerful tool to determine the real and imaginary parts of a complex dielectric constant by the transmission and reflection measurements in the frequency range between 0.2 and 6.5 THz. The infrared active soft modes were clearly observed in quantum paraelectric SrTiO₃, incipient ferroelectric BaZrO₃, and improper ferroelectric β-Gd₂(MoO4)₃. THz-TDS enables to determine also the complex dispersion relations of phonon-polariton including the spatial decay of polariton. The polariton dispersions of Raman inactive soft mode of SrTiO₃, the lowest-frequency modes of ferroelectric LiTaO₃ and LiNbO₃ were discussed.

KEYWORDS:

• Ferroelectrics
• terahertz
• spectroscopy
• soft mode
• phonon-polariton

Acknowledgments

This work was supported in part by the JSPS KAKENHI Grant Numbers 90134204 and 24740194, and the Sasakawa Scientific Research Grant. Authors are grateful for the technical support on THz-TDS measurements to Sendai Office, Advantest Corporation.