Localized surface plasmon resonance (LSPR) of coupled metal nanospheres in longitudinal, transverse and three-dimensional coupling configurations

Abstract

localized surface plasmon resonance of metal nanoparticles is highly regarded by researchers due to its special applications. In the paper, the spectral response of the coupled metal nanospheres located in a silica fiber is explored using the boundary element method as a fast and appropriate numerical method for phenomena related to scattered waves. Three different coupling configurations including longitudinal, transverse, and three-dimensional coupling is simulated and investigated. The results declare that the coupling distance and the angle between nanoparticles relative position vector and direction of light propagation, in addition to the radii of the metal nanoparticles and the refractive index of the environment, can affect the shape of scattering cross-sectional spectrum and also the position and linewidth of the resonance peaks. Furthermore, the effects of coupling in the LSPR spectrum of longitudinal coupled structures are observed even up to long coupling distances (about 300 nm for gold nanospheres), while these effects are negligible in the transverse coupled structures in the studied range. Also, a threshold can be defined for effective coupling distance, which determines whether the coupling effects are significant or not. The results show that this threshold value is independent of the radius of metal spherical nanoparticles. Finally, the results prove that the dominant coupling in the three-dimensional structures is the longitudinal coupling that controls the threshold coupling value of the structure and it is 300 nm for structures based on gold spherical nanoparticles. This threshold value is an important quantity for better decisions about LSPR-based applications.