Material characterization over broad band millimeter-wave frequency range using free space quasi-optical spectrometer

With the increasing interests of wireless device design in millimeter-wave frequency range, it is necessary to investigate the physical properties of different materials. Comparison among several state-of-the-art techniques is performed. Theoretical analysis and derivation of transmittance and reflectance spectra in terms of refractive and absorption indices using free space quasi-optical technique is studied based on the theory of multiple interferences in lossless and lossy materials. Transmittance and reflectance measurements of a variety of substances including powdered materials, solid materials, single crystals, as well as scattering and absorbing materials over a broad millimeter-wave frequency range are performed. The difference between non-periodic transmittance and periodic reflectance spectra are discussed based on the theory of scattering. Refractive and absorption indices are obtained by analyzing the transmittance spectra or reflectance spectra or both. Thus complex dielectric permittivity can be calculated according to the known refractive and absorption indices. The measurements are performed in both transmittance and reflectance modes using a free space quasi-optical spectrometer equipped with a set of high-power backward-wave oscillators as sources of coherent radiation, which are tunable in the range from 30 to 120 GHz. Unique physical properties of some materials are discovered in millimeter-wave frequency range for the first time.