Metallic nanoantennas

In the last years, the interaction between electromagnetic waves and metallic nanostructures has become particularly important in nanophotonics. Metallic nanostructures support SPRs, which are collective excitations of the electronic charge density localized at the metallic boundaries. The particles sustaining SPRs – plasmonic nanoparticles – exhibit unique optical properties derived from the huge enhancement and localization of the electromagnetic fi elds associated with the excitation of SPRs. The optical properties of these metallic nanostructures di ffer from the optical response of the same metallic material in bulk. The div ergences are not originated by quantum effects of the confi nement of the materials, but they appear due to electrodynamical e ffects and also to the modi cation of the dielectric environment.

Although plasmonics is a novel area of research, the understanding of the optical properties of small particles and the existence of electromagnetic surface waves have been long known. About one century ago, Mie explained the strong absorption of green light by subwavelength gold spheres when illuminated by plane waves by means of classical electrodynamics. SPRs have been studied for decades since the theoretical prediction of plasma losses in metallic slabs by Ritchie in 1957, and the consequent experimental evidence by Powell and Swan. As a fundamental excitation in condensed matter, during the second half of the XXth century, SPRs have been extensively analyzed mainly from the perspective of surface science, giving rise to a good understanding of the SPR properties when excited on metallic surfaces and small particles.

When, instead of extended surfaces, metallic nanoparticles are considered, electromagnetic waves are localized on the surface of the particle, adopting the terminology of Localized Surface Plasmon Resonances (LSPRs). A key property of metallic nanoparticles is the dependence of the energies of their LSPRs on the geometry of the structure, as well as their sensitivity to the dielectric environment. Thus, depending
on the particular property of interest and, by engineering the shape, the materials or the geometric confi guration, a huge variety of nanostructures has been synthesized, such as rings, nanoparticle dimers, nanoshells, nanoeggs, rods, nanorices or nanostars.

Doctoral thesis of Olalla Perez