Chapter 6 - Plasmonics

Abstract

This chapter introduces the principles of the surface plasmon resonance (SPR) phenomenon and its applications, particularly in sensing. The SPR is a quantum electromagnetic (EM) phenomenon arising from the interaction of light with free electrons at a metal–dielectric interface emerging as a longitudinal EM wave in a two-dimensional gas of charged particles such as free electrons in metals. Under certain conditions, the energy carried by the photons is transferred to collective excitations of free electrons, called surface plasmons (SPs), at that interface. This transfer of energy occurs only at a specific resonance wavelength of light when the momentum of the photon matches that of the plasmon. The SPs excited are strongly localized across the interface, and may be considered classically as EM surface waves that propagate along the interface and decay exponentially with distance normal to the interface. SPR is responsible for a dip in reflectance at the specific wavelength, the dip resulting from the absorption of optical energy in the metal. Since SP waves are tightly bound to metal–dielectric interfaces penetrating around 10 nm into the metal and typically more than 100 nm into the dielectric, they concentrate EM waves in a region that is considerably smaller than their wavelength, a feature that suggests the possibility of using SPs for the fabrication of nanoscale photonic circuits operating at optical frequencies. This constitutes an important area of research, since surface-plasmon-based circuits are known to merge the fields of photonics and electronics at the nanoscale, thereby enabling them to overcome the existing difficulties related to the large size mismatch between the micrometer-scale bulky components of photonics and the nanometer-scale electronic chips.