Significance of This Article
The main contribution of our paper is a discussion on the optical selection rules and hyperpolarizabilities of a second harmonic generation (SHG) induced by a nonuniform optical near field (ONF) on the basis of first-principles calculations. We demonstrated that the ONF causes the SHG even in symmetric materials and the first hyperpolarizability of an asymmetric molecule is one to two orders of magnitude larger for the ONF excitation than for the far-field excitation. The origin of these unusual optical properties caused by the ONF excitation is the nonuniformity of the ONF in space. Our theoretical approach describes light-matter interactions of materials in a more rigorous manner beyond the conventional electric dipole approximation. Furthermore, our findings paves the way for creating new optical functions in materials based on the nonuniform ONF-matter interaction.