  Ellipsometry is a technique whereby the measurement of the two orthogonal
polarization components of light reflected at glancing incidence allows a
characterization of the optical properties of a material at a particular
frequency. Importantly, it obviates the need for measurement against a standard
reference sample, and so can provide reliable spectroscopic information even
when surface morphology is unknown, of marginal quality and/or a reference is
unavailable. Although a standard technique in the visible range, it has not
been widely applied in the Terahertz (THz) spectral range despite its potential
utility. This is largely because of the technical difficulties that these
frequencies present. This review details recent progress in the implementation
of THz range ellipsometry. We discuss a variety of configurations including
various kinds of laboratory and facility based sources using both continuous
wave and pulsed spectroscopic methods. We discuss the general problems
encountered when trying to import the methodologies of visible range
ellipsometry to the THz range and give examples of where the technique has been
successful thus far.
