Excitons and optical nonlinearities in hybrid organic-inorganic nanostructures

We present a theoretical review of the properties of electronic excitations in nanostructures based on combinations of organic materials with inorganic semiconductors, having respectively Frenkel excitons and Wannier-Mott excitons with nearly equal energies. We show that in this case the resonant coupling between organic and inorganic quantum wells (or wires or dots) may lead to several interesting effects, such as splining of the excitonic spectrum and enhancement of the resonant optical nonlinearities. First, we discuss the properties of hybrid Frenkel-Wanner-Mott excitons, which appear when the energy splitting of the excitonic spectrum is large compared to the width of the exciton resonances (the case of strong resonant coupling). Such peculiar excitations share at the same time both the properties of the Wannier excitons (e.g., the large radius) and those of the Frenkel excitons (e.g., the large oscillator strength). We discuss mainly two-dimensional configurations (interfaces or coupled quantum wells) which are the most extensively studied. In particular, we show that hybrid excitons are expected to have resonant optical nonlinearities significantly enhanced with respect to those of traditional inorganic or organic systems. We also consider analogous phenomena in microcavities where the exciton resonances are close to the cavity photon mode resonance. Next, we consider the case of weak resonant coupling and show the relevance of the Forster mechanism of energy transfer from an inorganic quantum well to an organic overlayer. Such an effect may be especially interesting for applications: the electrical pumping of excitons in the semiconductor quantum well can be used to efficiently turn on the organic material luminescence.