Editing Ellipsometry (section)

===Single-wavelength vs. spectroscopic ellipsometry===
Single-wavelength ellipsometry employs a [[monochromatic]] light source. This is usually a [[laser]] in the [[visible spectrum|visible]] spectral region, for instance, a [[HeNe laser]] with a [[wavelength]] of 632.8&nbsp;nm. Therefore, single-wavelength ellipsometry is also called laser ellipsometry. The advantage of laser ellipsometry is that laser beams can be focused on a small spot size. Furthermore, lasers have a higher power than broad band light sources. Therefore, laser ellipsometry can be used for imaging (see below). However, the experimental output is restricted to one set of <math>\Psi</math> and <math>\Delta</math> values per measurement. Spectroscopic ellipsometry (SE) employs broad band light sources, which cover a certain spectral range in the [[infrared]], visible or [[ultraviolet]] spectral region. By that the complex [[refractive index]] or the [[dielectric function]] tensor in the corresponding spectral region can be obtained, which gives access to a large number of fundamental physical properties. Infrared spectroscopic ellipsometry (IRSE) can probe lattice vibrational ([[phonon]]) and free [[charge carrier]] ([[plasmon]]) properties. Spectroscopic ellipsometry in the near infrared, visible up to ultraviolet spectral region studies the [[refractive index]] in the transparency or below-[[Band gap|band-gap]] region and electronic properties, for instance, band-to-band transitions or [[exciton]]s.