In order to optimise the design of millimetre wave quasi-optical components it is necessary to know accurately the dielectric permittivity (and sometimes the magnetic permeability) of a candidate material as a function of frequency. The refractive index and loss of a material are related to its dielectric permittivity and magnetic permeability and so understanding these properties will influence the selection of the most appropriate material to use for a given component as well as affecting the geometry of the final design for this device.
Materials with low loss and relatively low refractive index are typically used to make lenses, optical windows, radomes and as anti-reflection matching layers, whereas materials with high loss and usually larger refractive indices are used for the construction of absorbers and loads (beam dumps).
Over the past 20 years, our group has used both waveguide and free-space (quasi-optical) methods in order to characterise the complex optical properties of materials in the millimetre wave region. These methods are based upon the measurement of the transmission through and reflection from samples of a known length, l, as a function of frequency.
This results in transmission and reflection interferograms with periodic features that depend upon the length of the sample and its optical properties.
A model is fitted to these data to determine the optical parameters that are required to produce these interferograms from a sample of this known length.
By measuring over as wide a frequency range as possible, any dispersion (variation of these material properties as a function of frequency) can also be determined. Samples of several different lengths are measured for each material in order to reduce any ambiguity or uncertainty in the fitted values.