What is Ellipsometry?
Ellipsometry is a sensitive optical technique for determining properties of surfaces and thin films. If linearly polarized light of a known orientation is reflected at oblique incidence from a surface then the reflected light is elliptically polarized. The shape and orientation of the ellipse depend on the angle of incidence, the direction of the polarization of the incident light, and the reflection properties of the surface. We can measure the polarization of the reflected light with a quarter-wave plate followed by an analyzer; the orientations of the quarter-wave plate and the analyzer are varied until no light passes though the analyzer. From these orientations and the direction of polarization of the incident light we can calculate the relative phase change,
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and the relative amplitude change, 
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introduced by reflection from the surface.
An ellipsometer measures the changes in the polarization state of light when it is reflected from a sample. If the sample undergoes a change, for example a thin film on the surface changes its thickness, then its reflection properties will also change. Measuring these changes in the reflection properties can allow us to deduce the actual change in the film's thickness.
Since the instrument measures changes in polarization it probably should have been given the name `polarimeter'; however, at the time when the ellipsometer was named the term polarimeter was already in use as the name of an instrument for measuring the specific rotation of optically active materials, which is something different from the reflection properties of a film-covered surface. Since the general polarization state of polarized light reflected from a surface is elliptical, the term ellipsometer was chosen.
The most important application of ellipsometry is to study thin films. In the context of ellipsometry a thin film is one that ranges from essentially zero thickness to several thousand Angstroms, although this range can be extended in some cases. If a film is thin enough that it shows an interference color then it will probably be a good ellipsometric sample. The sensitivity of an ellipsometer is such that a change if film thickness of a few Angstroms is usually easy to detect.
Reflectance spectroscopy, in general, measures the change in intensity of reflected light, ignoring other properties of the light ray. Ellipsometry measures the change in polarisation, as well as the intensity, upon reflection from an electrode. Linearly polarised light, when reflected from an electrode surface, will change its state to Elliptically polarised. Viewed end on the electric vector of the light, E, appears to describe an ellipse.
Analysis of this reflected light gives Epsilon (=tan-1(b/a)) and Theta (the azimuthal angle), used to calculate the refractive index and thickness of any film present on the electrode.
If the film is electrochemically active, applying a potential difference across the film may change the film parameters, and hence the polarisation of the reflected light. Monitoring the polarisation of the reflected light while changing the applied voltage is in-situ Electrochemical Ellipsometry.
