Size and dielectric-dependent plasmonic resonances in CdS@Ag core–shell quantum dots: Field enhancement, dispersion, and slow-light effects

Abstract

This study presents a comprehensive theoretical and numerical investigation of size- and host-medium dielectric-dependent plasmonic resonances in CdS@Ag core–shell quantum dots, with particular emphasis on field enhancement, optical dispersion, and slow-light effects. A hybrid framework combining the Maxwell–Garnett effective medium theory with a size-corrected electrostatic model was employed to compute the effective dielectric response and group velocity characteristics. The results reveal that local field enhancement is maximized by thicker Ag shells and low-permittivity hosts, enabling strong amplification of near-field intensities. Dual plasmon resonances, arising from the CdS/Ag and Ag/host interfaces, govern the field enhancement factor, refractive index and absorption spectra, producing tunable resonance shifts with variations in core radius, shell thickness, and host permittivity. Near these resonances, pronounced dispersion leads to a substantial increase in the group index, with group velocity reduced by more than an order of magnitude and, in certain regimes, reversed to negative values. Enhanced slow-light effects are particularly evident in high-permittivity hosts such as ZnO, where plasmon–exciton coupling further intensifies dispersion and suppresses pulse propagation. These findings provide new insights into the structural and dielectric control of plasmonic quantum dots and establish design guidelines for their application in optical delay lines, photonic modulators, sensors, and nonlinear optical devices.