Identification of Multipolar Surface Plasmon Resonances in Triangular Silver Nanoprisms with Very High Aspect Ratios Using the DDA Method

The extinction spectra of 5-nm thick, triangular silver nanoprisms are calculated using the discrete dipole approximation (DDA) method. The calculations are proved to accurately converged by satisfying the usual criteria related to the applicability of the DDA. The ultrathin thickness of the nanoprisms considered here has the advantage of making it possible to largely tune their aspect ratio (AR) from 5 to 40, and simultaneously limit all their dimensions below the wavelength of the incident light. For nanoprisms with AR >= 15, several intense bands are observed. These bands correspond to the well-known, in-plane, dipolar surface plasmon resonance (SPR) and several multipolar modes emerging at higher energies. Because of the high AR of the nanoprisms considered in this work, the multipolar SPR are particularly well observed, thus making it possible to examine them in detail. The calculated extinction spectrum shows a clear dependence on the edge length, the thickness, the aspect ratio and the volume of the nanoprism. The evolution of the extinction spectrum when simulating the presence of a substrate is also investigated as well as that induced by changing the size of the truncation in snipped nanoprisms. The qualitative agreement of the presented simulations with previous experimental observations made by other groups confirms the ability of the DDA method to predict the optical properties of such ultrathin triangular nanoprisms.