Numerical Simulation of Electromagnetic Wave Interaction with Spheroidal Core-Shell Nanoparticle: Dependence of Surface Plasmon Resonance on Core-Shell Composition

Abstract

In the present work, we report our observations drawn from the numerical simulation of absorption and scattering efficiencies of spheroid shape nanostructures using discrete dipole approximation technique. The absorption and scattering spectra are discussed in terms of shell thickness, composition of core and shell material, aspect ratio and size of the spheroidal nanoparticle (NP). The normalized electric field intensity around the sphere and spheroidal core@shell geometry has been studied in the present work. The characteristics of surface plasmon resonance (SPR) depend upon the symmetry of NP. For core@shell spherical nanogeometry, only one plasmon mode is found while for asymmetric nanogeometry (such as oblate and prolate), more distinct plasmonic modes are found. It was accounted that for oblate shape TiO2@Au spheroid of effective core radius 80 nm with 0.1 aspect ratio, two SPR peaks have been found, one is in visible range (725 nm) corresponding to transverse mode (TM) and the other is in an infrared region (1099 nm) corresponding to longitudinal mode (LM). In this paper, two cases of core-shell oblate shape nanogeometry are considered to study the optical properties; one is nanospheroid of metal (Au, Ag) shell with an oxide (TiO2) core, and the other is metal core with an oxide shell. Numerical results illustrate that the behaviour of SPR of these two core@shell structures are quite different from each other. On increasing the shell thickness of metal@dielectric nanogeometry, SPR peak is red shifted while blue shifted for dielectric@metal nanogeometry. The comparison has been done between analytical (electrostatic approach) and numerical (DDA) approaches for ellipsoidal nanogeometries. It was observed that numerical results are in good agreement with analytical predictions for very small sized NP (d < < λ).