Nonlinear optics and organic materials

In recent years, we have witnessed a growing relationship between the organics and optics communities.Liquid crystals, for example, have become the preferred materials for optical display devices.Organics are being seriously considered as the materials of the future for optoelectronic applications.It is known that organic molecules with extensively delocalized electrons are highly nonlinear and could be used in the design of new nonlinear optical materials.Indeed, organic crystals and poled polymers with exceptionally strong second-order nonlinearities and good optical quality have been discovered.Numerous organic materials with high third-order nonlinearities have also been found.The recent discovery of organic photo-refractive materials has stimulated even more interest in this area Many conferences have been devoted to this topic in the past ten years.The recent IKET ANI Conference again had it as the main theme.Advances in the field have been exciting and encouraging, but I will refrain from further discussion and simply refer the readers to the many papers in this Special Issue.Instead, we shall consider another interesting topic relating nonlinear optics and organic materials: how nonlinear optics can be used to study organic materials.One of the main differences between linear and nonlinear responses of a medium to incoming radiation is in their symmetries. 1 It leads to the possibility that some properties of the medium could be more sensitively probed by nonlinear, rather than linear, optical means, or vise versa.A well-known example is that some vibrational modes of a medium could be Raman-active but infrared-inactive, and would be more readily observed by Raman