optique

Sum-Frequency Generation (SFG) is an optical phenomenon, wherein two photons impinge on a sample surface and generate a third photon at the sum of their frequency.

Optical crystallography is the branch of science that focuses on the optical properties of crystals. Also known as crystal optics, it describes the behavior of light in anisotropic media, such as crystals, in which the light behaves differently depending on which direction the light is traveling.