T-matrix simulations of the optical response of gold nanorods: Impact of dielectric function of nanorods on the simulated optical properties and their sensitivity to the dielectric environment

The T-matrix approach is used to elucidate the influence of the dielectric function of gold nanorods on their optical response. On the basis of extensive and systematic numerical simulations, we follow the spectral variations of extinction efficiency and field enhancement as a function of the nanorod’s dielectric properties, medium dielectric constant, and aspect ratio. Simulations with six available tabulated gold dielectric functions show different locations of the longitudinal localized surface plasmon resonance (LSPR). A shift of up to 50 nm is found and compared with experimental data available in the literature. With the increasing refractive index of the ambient medium, the peaks red shift to higher wavelengths, which can be understood in terms of the reduced plasmon energy due to the reduced restoring forces of the plasmon oscillations. A linear correlation exists between the relative shift of the resonance wavelength and the relative increment of the medium refractive index. The correlation coefficient represents the sensitivity of the resonance to the dielectric environment and is more dependent on the aspect ratio than the dielectric properties of nanorods. However, a non-linear correlation is observed between the relative changes in the resonance extinction peak and the relative refractive index increment of the medium, which is considerably affected by the dielectric properties of nanorods. Results reveal that the field enhancement is influenced primarily by the dielectric environment and the aspect ratio. Simulations with the size-corrected dielectric functions show a broadening of the LSPR with no significant shift, in addition to a reduced extinction efficiency and field enhancement.