From slow to superluminal propagation: Dispersive properties of surface plasmon polaritons in linear chains of metallic nanospheroids

We consider propagation of surface plasmon polaritons (SPPs) in linear periodic chains (LPCs) of prolate and oblate metallic spheroids. We show that the SPP group velocity can be efficiently controlled by varying the aspect ratio of the spheroids. For sufficiently small aspect ratios, a gap appears in the first Brillouin zone of the chain lattice in which propagating modes do not exist. Depending on the SPP polarization, the gap extends to certain intervals of the Bloch wave number q. Thus, for transverse polarization, no propagating SPPs exist with wave numbers q such that q(c)(perpendicular to)