Spectroscopic ellipsometry is a powerful technique for characterizing the optical properties of thin films and surface layers. It is based on the measurement of the change in polarization of light as it interacts with a sample. When light is reflected from a surface, the reflected light has a polarization state that depends on the optical properties of the material. By measuring the change in polarization, spectroscopic ellipsometry can provide information about the thickness, refractive index, and extinction coefficient of the sample.