This question might have been asked several times but I am confused on this topic. What actually are the plasmons? Do they only exist in the case of metals or they can be found even in semiconductors? They are even classified as bulk and surface plasmons. What is the difference between them?
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Think of an electromagnetic wave reaching a bunch of electric charges. The oscillating EM field will couple to the movement of free carriers, making them oscillate. Of course, moving charges also generate an electromagnetic field, and since these carriers are oscillating, the EM field will oscillate too: that is, we have another EM wave.
In the case of bulk plasmons (I prefer calling them plasma oscillations, to distinguish them from Surface Plasmons), this happens in general. Light hits a metal and carriers in the metal start oscillating. If carriers can move at the same frequency of the incident EM field, they will screen the field inside the material, and that's why metals have high reflectivity for certain frequencies. Back to the previous explanation: when the light hits the metal, bulk electrons oscillate and generate a field that propagates in the opposite direction, which is the reflected light. This is how reflection works in an atomic level. However, note that carriers don't move freely, there is some resistance. Due to this, the frequency at which they can oscillate is limited from above by what we call the plasma frequency. Above this frequency, the metal becomes more transparent. This is a cool way of understanding why optical experiments are interesting to "shed light" on the electronic properties of materials, and the reason why the optical conductivity is important to study Strongly Correlated Matter like Mott insulators, Charge density wave states and Superconductors.
But let's go back to the light hitting the sample. At the surface the story changes. When people refer to surface plamsons they normally talk about surface plasmon polaritons (SPP). The fundament is the same, oscillation of carriers coupled to an incident EM wave, but the effects are different. Here the momentum of the light influences these SPP, and we tend to refer to the cool plasmons that can be used as wave guides and to confine light as those with high momentum, where light-matter interaction is strong (for a formal description I recomend you Principles of Nano-Optics, by Novotny). These oscillations can even create standing waves at the surface that can be observed using near field optical microscopy (SNOM), here's a recent beautiful example: https://doi.org/10.1038/s41467-021-21193-5
As for semiconductors, they can hold plasmons, but here the temperature of course plays an important role. Insulating materials are normally described by the Lorenz model and metals by Drude Model; semiconductors use a mixing of both.
