Optical Absorption in Single-Domain Ferroelectric Barium Titanate

Abstract

The optical-absorption coefficients for polarized light have been measured for single-domain crystals of $n$-type BaTi${\mathrm{O}}_3$. The measurements were made over a wavelength range from approximately 0.4 to 12 μ, and over a temperature range from 26 to 130°C. This temperature range includes both the cubic and the ferroelectric tetragonal states. The most significant feature of the measurements is a broad peak in the optical-absorption coefficient $\alpha$ centered near a photon energy of 0.6 eV; the absorption exhibits a threshold at a photon energy of approximately 0.2 eV. In addition, a large anisotropy, comparable to that observed in the transport behavior, is noted in the infrared values of $\alpha$ in the tetragonal state. It is shown that a consistent explanation of the optical-absorption properties is that most of the absorption in the visible and near infrared is the result of optical excitation of electrons in impurity states approximately 0.2 to 0.3 eV below the conduction-band minimum, superimposed on a free-carrier and lattice-absorption background. The deep impurity states are probably associated with oxygen vacancies.