Exciton-Plasmon Interactions in Metal-Semiconductor Nanostructures

The complementary optical properties of metal and semiconductor nanostructures make them attractive components for many applications that require controlled flow of electromagnetic energy on the nanometer length scale. When combined into heterostructures, the nanometer-scale vicinity of the two material systems leads to interactions between quantum-confined electronic states in semiconductor nanostructures and dielectric-confined electromagnetic modes in the metal counterparts. Such exciton-plasmon interactions allow design of absorption and emission properties, control of nanoscale energy-transfer processes, creation of new excitations in the strong coupling regime, and increase of optical nonlinearities. With the advancement of novel fabrication techniques, the functionalities of metal-semiconductor nanostructures will be further increased for better control of optical properties and energy flows on nanometer length and femtosecond time scales. T he popularity of nanoscience has exploded within the last two decades, mainly because new nanofabrication processes have led to a wealth of novel, engineered nanostructures with tunable size and shape parameters, leaving researchers with unprecedented control over their electronic, optical, and mechanical properties. Some of the most prominent optical nanostructures include metal nanoparticles with controllable absorption and scattering resonances resulting from surface plasmon (SP) excitations and low-dimensional semiconductor nanostructures with engineered electronic levels that can give rise to tunable, highly efficient emission and absorption. The advantages of nanomaterials are not limited to controllable optical properties of single components but extend to the unique possibilities to combine different nanomaterials into composite structures. Such hybrid materials feature properties of two or more components and potentially synergistic properties caused by interactions between the nanoscale constituents. Interactions can be very strong as both the building blocks and the separation between the components have nanoscale dimensions. The number of possible hybrid materials that can be built from existing nanostructures is enormous; therefore, the potential for creating highly functional hybrid materials that enable,modify, and control energy processes and pathways is very promising. Here, I will discuss the perspectives that are presented by interactions of optical excitations in metal-semiconductor nanostructures. Both material systems exhibit excitations at optical frequencies that can be designed to be resonant with each other. This opens up the possibility for coupled excitations that can act differently than the optical excitations of the individual components. Optical excitations in semiconductor nanostructures are defined by the electronic levels in the conduction and valence bands. As a result of quantum confinement, the electronic levels are discrete in one or more dimensionsand canbe tunedby size and shape (Figure 1a). The fundamental optical excitations are transitions between these discrete levels in the conduction and valence bands that lead to the formation of bound electron-hole pairs or excitons. The equivalents in metal nanostructures are so-called surface plasmons that are collective oscillations of conduction band electrons. These SPs arise from the dielectric contrast between the metal nanostructure and the nonconductive environment, and the SP resonance frequency can be controlled by shape and size as well (Figure 1b). Interactions between excitons and SPs occur when metal and semiconductor nanostructures are in close proximity. One often discerns two opposite cases of weak and strong coupling. In the weak coupling regime, wave functions and electromagnetic modes of excitons and plasmons are considered unperturbed, and exciton-plasmon interactions are often described by the coupling of the exciton dipole with the electromagnetic field of the SP. This model has been used to explain the original experiments by Drexhagen, who studied the change of the excitation decay rate of an emission dipole in the proximity of a plane metal surface. In general, wellknown phenomena including enhanced absorption cross sections, increased radiative rates, and exciton-plasmon energy transfer are described in the weak coupling regime. Thechallengeremains toproperlycalculate theelectromagnetic fields in the proximity of metal nanoparticles of nontrivial Received Date: August 5, 2010 Accepted Date: September 7, 2010