Plasmon hybridization and magnetic-permeability-dependent optical absorption in \(\mathrm{Fe_3O_4}\)-Cs core–shell nanostructures

Abstract

Magneto-plasmonic core–shell nanostructures provide a versatile platform for tailoring optical absorption through coupled magnetic and plasmonic interactions. In this work, the optical absorption properties of \(\mathrm{Fe_3O_4}\)–Cs core–shell nanostructures embedded in alkali halide matrices (CsCl, CsBr, and CsI) are theoretically investigated using Maxwell–Garnett effective medium theory combined with frequency-dependent dielectric and magnetic responses. Two configurations are analyzed: (i) \(\mathrm{Fe_3O_4}\) core–Cs shell (Model A) and (ii) Cs core–\(\mathrm{Fe_3O_4}\) shell (Model B). The absorption spectra are investigated as functions of core–shell volume parameter, host dielectric constant, and nanoparticle filling factor. Model A exhibits two hybridized plasmon resonances arising from coupling between the inner \(\mathrm{Fe_3O_4}\)–Cs and outer Cs–host interfaces, while Model B shows a single broadened resonance dominated by the Cs core. Increasing host dielectric constant produces pronounced red shifts, whereas increasing filling factor primarily enhances absorption intensity with minimal change in resonance wavelength. Simulations further show that increasing the magnetic permeability of the \(\mathrm{Fe_3O_4}\) shell enhances magnetic damping, resulting in resonance broadening and reduced plasmon coherence. The results demonstrate that \(\mathrm{Fe_3O_4}\)–Cs nanostructures exhibit tunable magneto-plasmonic absorption characteristics that may be useful for dielectric-environment-sensitive photonic and sensing applications. The calculated optical response suggests that \(\mathrm{Fe_3O_4}\)/Cs core-shell structures may be of interest for future magneto-plasmonic and optical sensing studies.

Magneto-plasmonic core–shell nanostructures enable tunable optical absorption via coupled magnetic and plasmonic interactions, which is relevant for the design of dielectric-environment-sensitive photonic devices and refractive-index-based sensors. The effects of magnetic damping on the optical response were systematically investigated, revealing resonance broadening, reduced plasmonic coherence, and the emergence of weak Fano-type spectral asymmetry.

The optical properties were modeled within the quasistatic approximation using Maxwell–Garnett effective medium theory combined with frequency-dependent dielectric functions (Drude model for Cs) and constant magnetic permeability for \(\mathrm{Fe_3O_4}\). Effective permittivity and permeability of the composite were calculated for varying core–shell volume parameter (\(\beta \)), host dielectric constant (\(\epsilon _{h}\)), filling factor (F), and magnetic permeability (\(\mu \)). Absorption spectra were derived from the imaginary part of the effective refractive index. Python 3.13 was used to implement the numerical simulations and optical modeling of the core–shell nanostructures. The code was developed to calculate effective dielectric and magnetic properties, simulate absorption spectra, and analyze the influence of structural and material parameters on plasmonic behavior (code provided in Supplementary Information).