Unified treatment for scattering, absorption, and photoluminescence of coupled metallic nanoparticles with vertical polarized excitation

Optical properties of coupled metallic nanoparticles (MNPs) have been widely reported due to their unique characteristics such as peak shift/splitting of the coupling spectra and electromagnetic enhancement at subwavelength scale, etc. In a previous work, we have investigated the coupling spectra of two coupled MNPs with parallel polarized excitation. In this study, we investigate the vertical polarization case in detail. Different from the parallel one, the vertical one has its unique properties: (a) three coupling coefficients; (b) positive coupling terms in the coupling equations; (c) blue-shifts of the peaks with the increasing coupling strength for identical MNPs spectra, including scattering, absorption, and photoluminescence (PL). The model shows that for the two resonant MNPs, the spectra reveal only one mode that blue shifts as the coupling strength increases; while for the two non-resonant MNPs, the spectra reveal two splitting modes, one of which blue shifts and the other of which red shifts as the coupling strength increases. The relative intensity of the two modes varies with the coupling strength. Furthermore, the PL efficiency (relative to the scattering) is about the order of 10 ^−2 . Comparison with published experimental results shows the validity of this model. This work provides a deeper understanding on the optical properties of coupled MNPs and is beneficial to relevant applications.