Size-Dependent Dispersion and Slow-Light Effects in CdS@Ag Core-Shell Quantum Dots: A Theoretical Study of Plasmonic Resonances and Group Velocity Modulation

This study investigates the size-dependent dispersion and slow-light phenomena in CdS@Ag core-shell quantum dot nanostructures embedded within various dielectric host materials. A comprehensive theoretical framework is developed by integrating the Maxwell-Garnett effective medium theory with the electrostatic approximation, alongside a size-corrected Drude model to characterize the metallic shell. This approach enables the accurate modeling of the effective dielectric function, polarizability, refractive index, and group velocity. Numerical simulations are conducted across a range of core radii, shell thicknesses, and host permittivities to systematically examine the influence of geometrical and environmental parameters on plasmonic resonances and pulse propagation dynamics. The results reveal two distinct, tunable surface plasmon resonances at the CdS/Ag and Ag/host interfaces, whose hybridization significantly alters the refractive index dispersion. Pronounced slow-light effects are observed in proximity to these resonances, including a reduction in group velocity by more than an order of magnitude and the emergence of negative group velocity regimes. These findings offer valuable insights into the geometry- and environment-dependent plasmon-exciton coupling mechanisms in core-shell quantum dots and provide guiding principles for the design of advanced nanophotonic devices such as optical delay lines, modulators, and plasmon-enhanced sensors.