Metallic nanorod arrays: negative refraction and optical properties explained by retarded dipolar interactions

We show that the optical properties of arrays of parallel-aligned metallic nanorods can be understood by means of a retarded dipolar interaction model. Exemplarily, arrays of gold nanorods having various lengths and diameters are investigated experimentally. A strong diameter dependence of the long-axis surface plasmon resonance (LSPR) as well as a lower energy limit of this mode for varying length was found. The model also shows that, for small nanorod distances (d<λ/2), the optical properties are independent of the azimuthal angle of the incoming plane wave and of the detailed arrangement of the nanorods. Furthermore, the model was used to explain the dependence of the LSPR on the angle of incidence and to find the conditions for which negative and extraordinary positive refractions occur in these structures.