Enhanced light emission in nanostructures

Abstract

The enhancement of light emissive processes in different quantum nanometric systems is presented in this review. The plasmonic enhanced photoluminescence (fluorescence) of metals and metal nanoparticles, molecules and semiconductor nanostructures, as well as surface-enhanced Raman scattering is initially considered. The enhancement of excitonic photoluminescence intensity in semiconductor confined systems such as quantum wells, quantum wires and quantum dots, and microcavities are then discussed. Finally, the experimental results of the enhanced exciton photoluminescence from GaAs homojunctions, δ-doped GaAs structures, GaAs/AlGaAs selectively doped and AlInN/GaN heterostuctures is presented. These results can be applied to enhance the emission of light-emitting diodes, as well as to increase the efficiency of solar cells. 1. Bulk and surface plasmons in metals A few metals and their nanoparticles offer reso­ nances of their carrier transitions that fall near or in the visible region of the electromagnetic spectrum. The best material candidates are alkali and noble metals. Noble metals have the additional advantage of being chemically inert, i. e. not very reactive, and hence do not require vacuum in order to perform an experiment. The two most common metals are silver (Ag) and gold (Au), which are widely used in various nanophotonics applications [1]. Noble metal nanoparticles have been used to change the colours of reflected or transmitted light in both architecture and art over several hundreds of years. The staining of glass windows or orna­ mental cups are the best known examples [2]. More practical optical applications have been limited to reflectors due to the large free electron densities of metals, in which the electrons are highly delocal­ ized over a large surface space. However, we can decrease the size of the metal surface and confine its electronic motion. The decrease in size of metal nanoparticles gives rise to an intense absorption of light, which is called surface plasmon absorption. This intense absorption induces a strong coupling of the nanoparticles to the electromagnetic radia­ tion of light. Plasmons consist of coherent oscillations of con­ duction­band­electrons and are primarily respon­ sible for the optical properties of metals. The Drude plasma frequency for the free electron gas equals , where n is the number of elec­ trons per unit volume, m e is electron mass, e is the electron charge, and ε 0 is the electric constant [3, 4]. The dielectric constant of metals is a complex func­ tion of frequency, and the real part changes from