Precise millimeter-wave dielectric measurements of single crystal ferroelectric materials

Transmittance measurements on various single crystal ferroelectric materials over a broad millimeter-wave frequency range have been performed. The frequency dependence of the complex dielectric permittivity has been determined in the millimeter wave region for the first time. The measurements have been employed using a free-space quasi-optical millimeter-wave spectrometer equipped with a set of high power backward wave oscillators (BWOs) as sources of coherent radiation, tunable in the range from 30 – 120 GHz. The uncertainties and possible sources of instrumentation and measurement errors related to the free-space millimeter-wave technique are discussed. This work has demonstrated that precise MMW permittivities can be obtained even on small thin crystals using the BWO quasi-optical approach. The real and imaginary parts of dielectric permittivity have been extracted from the accurate transmittance spectra. The effect of crystallographic orientation on complex permittivity, which is shown to be more than a factor of 1.5 between different orientations, has been carefully examined for Lithium Niobate.