A plasmon is a quantum of the collective oscillation of a conductive electron gas in a metal, or generally speaking in a conductor (Berini et al. 2016) or a semiconductor where a conductive electron gas can form under appropriate conditions, as plasmonic phenomena have been observed and studied with diverse materials far beyond metals and alloys (Zhong et al. 2015). To excite a plasmon on a metal surface, a photon needs to be coupled to the surface either by a carefully selected combination of incident medium (a coupling prism) and angle or with properly designed fine surface structures. It is worth mentioning that an accelerated electron can also be used to excite a plasmon on a metal surface, but the following context will focus on optically excited plasmons.44