Effect of symmetry breaking and substrate on the optical properties of silver sphere-like nanoparticles in different surrounding media

The effect of the substrate to the absorption spectra of silver nanoparticles of sphere-like shapes is investigated. Silver nanoparticles of broken spherical symmetry are placed on substrates of different dielectric functions at various contact angles (alpha). The absorption efficiency of the supported nanoparticles is calculated by using the Discrete Dipole Approximation (DDA) method. Increasing the value of alpha and, hence the contact area of the supported nanoparticles, results in an inhomogeneous distribution of the polarization charges over the nanoparticle-substrate system. This leads to the excitation of plasmonic bands of different characters (dipolar and quadrupole modes). The admixture of both dipolar and quadrupole modes is found to be more pronounced when a nanoparticle with highest contact area (alpha = 90 degrees, hemispherical shape) is considered. The band position of the Longitudinal Mode (LM) is red-shifted with alpha, while the resonance wavelength of the Transverse Mode (TM) is blue-shifted.