Surface plasmons at a metal-insulator interface

In the context of this experiment, the term plasmon denotes a collective, longitudinal excitation of conduction electrons in a metal. More generally, a plasmon is the quantum of a charge carrier density fluctuation in a plasma, a neutral gas of mobile particles which are partly or fully ionized. Following the particle-wave dualism the term plasmon is associated with both quantum of density fluctuation and the wave-like density oscillation. Within the Drude model the conduction electrons in a metal are quasi-free particles which are only slightly damped by the remaining ions and therefore can be treated as a plasma. Note that this would not be valid in the Drude-Lorentz model, since it includes the restoring force created by those ions. When such density oscillations are bound to a metal-insulator interface, they are referred to as surface plasmons (SPs). They go along with a light field coupled to the interface and are therefore also referred to as plasmon-polaritons. In solid-state physics the term polariton accounts for all elementary excitations which couple to light. Surface plasmons have considerable commercial relevance, e.g. in biosensing applications and surface-enhanced Raman spectroscopy (5–8, 11, 19). In addition, SPs attract the attention of contemporary research being touted as an interface between optics and nanoelectronics. In the Fortgeschrittenen Praktikum entitled ”Surface plasmons at a metal-insulator interface” a basic understanding of SPs is achieved by determination of the dispersion relation of a surface plasmon at a metal-air interface. The dispersion relation of SPs can be derived from Maxwell’s equations. This derivation will be presented in section 1 of this instruction. In the experiment, the reflection of a monochromatic light beam on a thin metal layer is measured as a function of the angle of incidence of the light beam. When the SP is excited, the reflection from the metal layer is attenuated. From the angle, under which the attenuation is strongest, the momentum of the SP at a given energy is determined. In section 2, the experimental setup and the measurement technique are introduced. In section 3 you are given detailed instructions how to measure the SP absorption.