Localized Surface Plasmons

Abstract

Here we introduce the second fundamental excitation of plasmonics-localized surface plasmons. We have seen in the preceding chapters that SPPs are propagating, dispersive electromagnetic waves coupled to the electron plasma of a conductor at a dielectric interface. Localized surface plasmons on the other hand are non-propagating excitations of the conduction electrons of metallic nanostructures coupled to the electromagnetic field. We will see that these modes arise naturally from the scattering problem of a small, sub-wavelength conductive nanoparticle in an oscillating electromagnetic field. The curved surface of the particle excerts an effective restoring force on the driven electrons, so that a resonance can arise, leading to field amplification both inside and in the near-field zone outside the particle. This resonance is called the localized surface plasmon or short localized plasmon resonance. Another consequence of the curved surface is that plasmon resonances can be excited by direct light illumination, in contrast to propagating SPPs, where the phase-matching techniques described in chapter 3 have to be employed. We explore the physics of localized surface plasmons by first considering the interaction of metal nanoparticles with an electromagnetic wave in order to arrive at the resonance condition. Subsequent sections discuss damping processes, studies of plasmon resonances in particles of a variety of different shapes and sizes, and the effects of interactions between particles in ensembles. Other important nanostructures apart from solid particles that support localized plasmons are dielectric inclusions in metal bodies or surfaces, and nanoshells. The chapter closes with a brief look at the interaction of metal particles with gain media. For gold and silver nanoparticles, the resonance falls into the visible region of the electromagnetic spectrum. A striking consequence of this are the bright colors exhibited by particles both in transmitted and reflected light, due to res