Mie calculations and FDTD simulations

Mie calculations of spherical Au nanoparticles and Au@semiconductor core@shell nanoparticles in a homogeneous environment are performed using a MATLAB script⁴⁶. The calculations are performed using the formalism given by Bohren and Huffman¹⁰.

Lumerical FDTD simulations are performed to study the optical properties of Au and Au@semiconductor core@shell spheres/rods on a quartz substrate. In our simulations, the Au nanosphere/rod is placed on top of a quartz substrate, and the entire system is suspended in a background refractive index of 1.333 corresponding to water. For Au, we use the dielectric function measured by Johnson and Christy⁴⁷. The refractive index of quartz is obtained from the supplier website, while a constant refractive index of 2.3 is used for the ceria shell. A total-field scattered-field source is used to illuminate the Au particles from beneath the quartz-nanoparticle interface at a distance of 600 nm from the nanoparticle. Absorption and scattering monitors kept at distances of 300 and 900 nm respectively, are used to record the nanoparticle cross-sections. The mesh region covers the entire nanoparticle and the mesh size of all simulations is kept at 0.5 nm. Perfectly matched layer (PML) absorbing boundaries are used to border the simulation region.