Sample size limitations for frequency-swept free-space material characterization

THz devices depend on precise knowledge of material parameters, which, as the frequency coverage of VNA's pushes into the THz region, can be obtained from free space measurements. However the interpretation of the results generally assumes an infinite plane wave propagating through the sample, whereas in practice a Gaussian beam-waist is typically located there. We discuss the error introduced by this, and the resulting limits on the size of the beam-waist and therefore the size of the material sample.