Dielectric tunability of ferroelectric barium titanate at millimeter-wave frequencies

Ferroelectrics have been rarely considered as suitable candidates for the design of agile devices in the booming field of millimeter wavelengths and terahertz applications. Here a specifically designed free-space quasioptical system to determine the dielectric tunability and losses of a barium titanate single crystal at different temperatures is described. The values obtained are promising to establish ferroelectrics in the inventory of functional materials relevant at millimeter wavelengths. The potential for application of Landau's theory and a discussion of the role of phonon modes on the tunability is presented.