What is a plasmon?

A plasmon is the physical phenomenon characterizing plasma oscillation. More precisely, in physics, in the same manner as photons are quantization of light waves, a plasmon is a quantum of plasma oscillation. The plasmon is the quasiparticle resulting from the quantization of plasma oscillations. Thus, plasmons are collective oscillations of the free electron gas density, often at optical frequencies. They can couple with a photon to create a third quasiparticle called a plasma polariton.