Precision millimeter-wave dielectric measurements of birefringent crystalline sapphire and ceramic alumina

A quasi-optical technique, namely, dispersive Fourier transform spectrometry, has now been improved to provide high-precision continuous data of complex refractive index, complex dielectric permittivity, and loss tangent of materials at millimeter wavelengths. The use of a polarizing two-beam interferameter is ideally suited for the measurement of birefringent materials. A massive biréfringent effect (δ∊′ 2.193) is now observed for crystalline sapphire (Al2O3). The bifringent effect for crystalline quartz is much smaller (δ∊′ ⋍ 0.2). New millimeter-wave results of commercially available ceramic alumina (Al2O3) are compared with those of crystalline sapphire.