Theories behind Non Linear Optics

Nonlinear optics is the study of phenomena that occur as a consequence of the modification of the optical properties of a material system by the presence of light. Nonlinear optical phenomena are “nonlinear” in the sense that they occur when the response of a material system to an applied field depends in a nonlinear manner on the strength of the optical field. Typically, only laser light is sufficiently intense to modify the optical properties of a material system. In nonlinear optics, superposition principle no longer holds.