Generally speaking of plasma or plasma, everyone thinks of ionized air or high-temperature matter on the sun. However, if you carefully consider the free electron model of metals, the "gas" of free electrons moving in the background of positive ions is also a kind of "plasma".

The collective oscillation of the positive ion lattice in the metal can be quantized into the generation, propagation and annihilation of phonons; similarly, the collective oscillation of the free electron gas in the metal can also be regarded as a collective excitation, or it can be quantized as a kind of quasiparticle The emergence, spread and annihilation of This quasiparticle is plasmon, or "plasmon". It is "extreme", not "primitive." "Excimer" means "collectively excited element" or "exciting element"; "plasma element" implies that plasmon is the most basic unit of plasma-but in fact it is completely different. The basic units of plasma in metals are obviously free electrons and metal positive ions. There are two kinds of collective oscillations of electrons in metals: one is that the electrons of the entire metal sway synchronously under the action of an external electric field, and the other is that they collectively sway only on the metal surface. Obviously, due to the efficient shielding of the metal to the external electric field, the first collective swing cannot be realized in the macroscopic metal bulk material (so it is not easy to be confused with the high-temperature bulk plasma of the controlled nuclear fusion research), but only It can only exist in nanoparticles. The second one is more common, and the "metallic luster" we often say is also related to this. Since this second kind of swing is on the surface, of course it is called "surface plasmon."

In addition, this plasmon is not fixed in one place but will propagate, and wherever it propagates, since only the electrons vibrate but the metal positive ions do not move, it will inevitably cause the local area to be no longer electrically neutral, but "polarized" In order to emphasize the locality and quasi-particle characteristics of this polarization, it finally became the name "Surface Plasmon Polaron" (SPP). A little bit of paraphrase, it can be called "surface plasmon excimer", which means that the characteristic of this excimer is "polarized".

In summary, "surface plasma" can also be regarded as a kind of "plasma". However, this "plasma" is not the kind of high-temperature plasma that most people imagine alongside "solid, liquid, and gas".