Surface Plasmon Resonance in Metal Nanoshells with Air-Core: Computational Study Using DDA

Metallic nanoshells with air cores exhibit tunable plasmonic properties across the visible to near-infrared spectrum, making them highly relevant for applications in sensing, photothermal therapy, and optoelectronics. The surface plasmon resonance (SPR) peak of a 15 nm Au nanosphere in a medium with a refractive index of 1.54 appears at 545 nm, whereas for an Au nanoshell of the same size, it shifts to 805 nm (near-IR). This study systematically investigates the effects of shell material, shell thickness (S.T.), surrounding medium, and size parameters on the extinction spectra of nanoshells using both a quasi-static dipole model and a numerical approach based on the Discrete Dipole Approximation (DDA). Our results demonstrate that reducing S.T. leads to a redshift in the plasmon resonance, while increasing the refractive index of the surrounding medium significantly enhances the localized electric field intensity. The calculated figure of merit (FoM) values of 2.0 for the nanoshell and 1.33 for the nanosphere highlight the superior sensitivity of nanoshells, making them more effective for sensing applications. The numerical results are validated through analytical calculations, reinforcing the robustness of the DDA method in modeling nanoscale plasmonic behavior. This work provides deeper insights into the tunability of plasmonic nanoshells, contributing to the optimization of nanostructured materials for advanced optical and sensing applications.