High-precision dielectric measurements on liquids and solids at millimeter and submillimeter wavelengths

A brief outline is given of the improvements which have been made in the measurement techniques available for the determination of the complex relative permittivity of liquids and low-loss solids in the frequency range v = 100 GHz to 18 THz. These improvements involve very stable interferometric systems with either dielectric beam splitters for the upper frequency end or else free standing wire grid beam splitters for the lower frequency end, novel cell designs, the use of sensitive detectors, and of new computational procedures. The results can be checked by independent spot frequency measurements using HCN and H
2
O lasers coupled to an improved Mach-Zehnder interferometer. The optical properties of the polymeric (polyethylene, TPX, polypropylene, PTFE) and single crystal (quartz, silicon germanium) windows now assume an increased importance and we have determined refractive index n (v) and power absorption coefficient α(v) to much improved precision. With our present technique we can now determine the complex permittivity of very heavily absorbing liquids such as primary alcohols and water as far as 6 THz and beyond in favorable cases as functions of frequency over a wide temperature range.