Refractive Index and Absorption Coefficient of Undoped and Mg-Doped Lithium Tantalate in the Terahertz Range

Abstract

Dielectric material parameters of lithium tantalate (LT) in the terahertz region have been investigated using terahertz time-domain spectroscopy (THz-TDS). Undoped congruent, undoped stoichiometric, and Mg-doped stoichiometric LT crystals were measured. The Mg content was 0.5 and 1.0 mol% for the stoichiometric composition. Index of refraction and absorption coefficient spectra were determined in the 0.3–2.0-THz frequency range for beam polarization both parallel (extraordinary polarization) and perpendicular (ordinary polarization) to the optical axis [001] of the crystal at room temperature. For the calculation of the refractive index and absorption coefficient spectra from the measured data, we used TeraMat software (Menlo System) belonging to the spectrometer. The refractive index and the absorption coefficient for stoichiometric crystals were lower than for the congruent one. In the case of stoichiometric crystals, the Mg dopant caused a slight reduction of both ordinary and extraordinary refractive index compared to the undoped crystal. However, the presence of Mg did not reduce the absorption coefficient either for the ordinary or for the extraordinary polarization. In order to fit the measurement data, a Lorentz oscillator model was used. Good agreement was obtained between the measured data and the fitting curves by using the Lorentz oscillator model containing three terms.