Anisotropy of W-band complex permittivity in Al2O3

Abstract

The dielectric anisotropy of Al2O3 is studied here by characterizing W-band (75–110 GHz) complex permittivity of four different orientations of sapphire (Al2O3 single crystals). This was done using free-space, focused beam methods. Dielectric polarizability () of these orientations is then calculated and these values are related to their complex permittivity. Based on this relationship, a framework is developed for rapid and straightforward estimation of dielectric anisotropy using a known crystal structure and a dielectric permittivity measurement performed on one orientation of the material. This framework can be applied to other materials with dielectric anisotropy (e.g. SnO2, LiGaO2) to predict permittivity for different orientations, enabling rapid design of high-frequency systems (e.g. radomes, electromagnetic windows). These permittivity measurements were also used to determine the dominant polarization mechanisms leading to dielectric anisotropy of Al2O3 in the W-band; electronic and ionic polarization orthogonal to the direction of the focused beam.