This study presents a theoretical and numerical investigation of local field enhancement and optical bistability in ZnSe-based core–shell nanocomposites, providing insights for photonic applications. The structures consist of a ZnSe dielectric core and a metallic shell of either silver ( \(\textrm{Ag}\) ) or gold ( \(\textrm{Au}\) ), embedded in oxide matrices ( \(\textrm{SiO}_2\) , \(\textrm{ZnO}\) , or \(\textrm{HFO}_2\) ) with varying permittivities. Using the quasi-static approximation and the Lorentz–Drude model, we analyze how variations in the core radius (keeping the outer radius fixed) influence spectral response and local field enhancement. Results show that smaller cores (thicker shells) yield stronger local field enhancement factors ( \(\textrm{LFEF}\) ) due to increased plasmonic confinement. Ag shells produce sharper, more intense resonances than Au, attributed to lower damping and superior plasmonic performance. The dielectric environment also plays a key role: low-permittivity matrices like \(\textrm{SiO}_2\) support higher field localization, while high-permittivity ones such as \(\textrm{HFO}_2\) weaken confinement and blue-shift the resonances. Nonlinear analysis reveals that thicker shells and lower matrix permittivity enhance bistability and reduce switching thresholds, particularly in Ag-based systems. These findings highlight the critical influence of geometry and material choice on both linear and nonlinear optical responses. The results offer practical guidance for engineering core–shell nanocomposites tailored for applications in photonic devices, optical sensors, and all-optical switching.
