Bulk second-order optical nonlinearities in centrosymmetric materials

Abstract

The semiclassical theory of electrical susceptibility predicts null even-order susceptibilities for all centrosymmetric materials. However, second harmonic generation and other even-order nonlinear optical phenomena are often seen in them. Contributions due to magnetic dipoles, bulk quadrupoles, or symmetry ruptures describe many of these effects, but not all of them, as can be seen in some experiments recalled here. This work presents an extension of the theory that renders more general expressions for susceptibilities. According to these expressions, bulk even-order susceptibilities may eventually transform as pseudo-tensors and, therefore, may have nonzero values in centrosymmetric structures. We show examples of how this fact provides an alternative or simpler explanation of the conflicting experiments. Our result may have important implications in lasing, communications, imaging, quantum optics, quantum computing, and other fields, as it widens the family of materials in which to look for nonlinear effects such as second harmonic generation, the Pockels effect, spontaneous parametric downconversion, and other phenomena.