Anisotropy of W-band complex permittivity in Al₂O₃

Abstract

The dielectric anisotropy of Al 2 O 3 is studied here by characterizing Wband (75-110 GHz) complex permittivity of four different orientations of sapphire (Al 2 O 3 single crystals). This was done using freespace, focused beam methods. Dielectric polarizability (α D ) 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. SnO 2 , LiGaO 2 ) to predict permittivity for different orientations, enabling rapid design of highfrequency systems (e.g. radomes, electromagnetic windows). These permittivity measurements were also used to determine the dominant polarization mechanisms leading to dielectric anisotropy of Al 2 O 3 in the Wband; electronic and ionic polarization orthogonal to the direction of the focused beam.