Nonlinear optics, high density effects and stimulated emission

Abstract

Nonlinear optics is a wide field, comprising in principle all phenomena, where one observes a nonlinear response (e.g. of the polarization of a medium) on the stimulus such as the incident electromagnetic field. It starts with the typical effects such as second or third harmonic generation, four wave mixing or the rectification of the electromagnetic field (the so-called dc-effect) and continues over two and multi photon absorption to the phenomena of extreme nonlinear optics. Another branch deals with the phenomena in dense electron–hole pair systems (so-called high exciton, high density or many particle effects), which can be created, for example by optical excitation, but also by biasing a p–n junction in forward direction or by excitation with an electron beam. The third aspect concerns the stimulated emission resulting (e.g.) from the high excitation effects and the laser emission, which is influenced by the shape of the samples and resonators, (e.g.) of bulk materials, nano rods or powders. All three aspects overlap. We present in the following selected results for ZnO following roughly the above ordering.