Anisotropy of W-band complex permittivity in Al₂O₃

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.