What's a surface plasmon?

 
Note: A model of a conductor, e.g. a specific metal, consists of ions surrounded by a free electron sea or free electron gas.
 
"The free electron gas is part of a quantum system, subject to the periodic potential of the metal ions that make up the lattice.
"A plasmon" is a quasi particle - a particle that can be described by a collection of interacting particles - in this case, the electrons. Plasmons occur on the surface of a metal, are quantized, and consist of collective oscillations of the free electron gas." Hornyak et al."
 
The electrons of the surface, "they all kind of talk to each other if you will and move together and respond to external stimuli as a system."
 

 

 

 

 

Surface plasmon resonance (SPR)

 
https://youtu.be/sM-VI3alvAI (Images from video - slight modification)