The Performance of Active Coated Nanoparticles Based on Quantum-Dot Gain Media

Quantum-dots (QDs) provide an exciting option for the gain media incorporated in active coated nanoparticles (CNPs) because they possess large gain coefficients resulting from their extreme confinement effects. The optical properties of core/shell QDs can be tuned by changing the relative size of the core/shell, that is, by effectively changing its band gap structure. Similarly, the resonance of a CNP can be adjusted by changing the relative sizes of its layers. It is demonstrated here that by optimally locating the QDs inside a resonant CNP structure it is possible to greatly enhance the intrinsic amplifying behavior of the combined QD-CNP system. 1. Introduction The active coated nanoparticle (CNP) designs studied previously considered active silica cores coated with either Ag or Au shells (depending on the wavelength region of interest) [1–3] or the corresponding “inside-out” (IO) designs, that is, metallic cores covered with active silica coatings [2, 4]. In general, these designs used a simple gain model that did not include the dispersion behavior typical of a physical, active medium. These nanoamplifier designs were predicted to require a gain value of 104–105？cm？1. This is achievable with quantum dots (QDs); and, consequently, they have been a preferred choice for some discussions on the physical realizations of these nanoamplifiers [3]. QDs offer an alternative, more robust option to dye-based gain media, which are susceptible to bleaching. Nonetheless, the active IO-CNP has been experimentally verified with a dye impregnated silica coating [4]. Moreover, it and related studies [5, 6] suggest that the strong localized field and large cross-section effects associated with these plasmonic nanostructures reduce the model-based large gain values to more practical ones. We demonstrate here that QDs, which can be obtained commercially and could be integrated with the CNPs, do in fact represent an exciting practical option for a variety of active CNP designs. Moreover, it is established that the number of QDs needed for a successful active CNP design is considerably smaller than anticipated. 2. Quantum-Dot Gain Model The effective permittivity of a core-shell QD has been modeled by Holmstr？m et al. and is applicable for core-shell QDs in the strong confinement regime [7]. This model is motivated by Maxwell-Garnett effective medium theory and is given by the following: where is the volume of the entire core-shell structure; is the oscillator strength; and are, respectively, the conduction and valence band carrier distribution functions; is a dampening