Near-field Optical Excitation and Detection of Surface Plasmons

By definition surface plasmons are the quanta of surface-charge-density oscillations, but the same terminology is commonly used for collective oscillations in the electron density at the surface of a metal. Because the surface charge oscillations are intimately coupled to electromagnetic fields, surface plasmons are polaritons. In the past, surface plasmons have attracted considerable attention due to their application in optical sensor devices [1, 2]. Because of their localized nature, surface plasmons have recently also been explored in integrated optical circuits and optical waveguides [3, 4]. However, one of the key properties of surface plasmons is the associated light localization which can be explored for localized photon sources in optical spectroscopy and microscopy [5, 6]. Surface enhanced Raman scattering (SERS) is a prominent example for the latter application. Recently, it was demonstrated that the SERS effect can be spatially controlled with a laser-irradiated metal tip. This combination of SERS and microscopy provides high spatial resolution and simultaneous chemical information in the form of vibrational spectra [7]. Starting with experiments on planar metal surfaces we will discuss in this article general properties of surface-plasmon polaritons (SPPs) and then concentrate on light localization associated with laser-irradiated