Significance of This Article

An optical near-field (ONF) is a nonpropagating light localized in close proximity to, in particular, nanostructures irradiated by an external far-field light. The ONF has been discussed very frequently in the context of light concentration (i.e., electric field enhancement). Indeed, the electric field enhancement due to plasmon excitation in metals such as gold and silver clusters is a major topic in the field of plasmonics. However, a more intrinsic feature of the ONF is nonuniformity of the electric field. Since the ONF rapidly decays with increasing distance from its source, the electric field gradient plays an important role in the ONF and matter interaction. In this case, matter experiences a nonuniform electric field, the effect of which is completely neglected in a far-field propagating light. Then, the conventional theoretical treatment of a light–matter interaction based on a dipole approximation no longer works well in the ONF excitation dynamics because the treatment practically assumes that matter interacts with uniform electric fields.

In the present paper, we have demonstrated in a two-dimensional quantum dot (QD) model that a second-harmonic electric field component is concomitant with the ONF and matter interaction. This electric field caused two characteristic nonlinear optical responses: a two-photon absorption and an induction of a magnetic dipole moment. These optical phenomena were not due to usual second-order nonlinear response and were verified only when we explicitly considered the electron dynamics interacting with the ONF beyond the dipole approximation. Moreover, the effect of the second-harmonic electric field component is not limited in the specific QD model considered but is expected to intrinsically appear in any ONF excitation dynamics. Therefore, our findings provide a basis for a fundamental ONF and matter interaction in nanostructures and will pave the way for developing quantum devices with optical functions related to the ONF excitation processes.