I don't exactly agree with the previous answer. Specifically, plasmons are a quantum of free-electron oscillation (ie plasma), while polaritons are hybridizations of photons and excitons. Excitons are a bound electron-hole pair (a composite boson), and not directly relevant to the concept of plasma oscillations. My understanding is that excitons are more relevant to gapped semiconductor systems, where the conduction band is mostly empty of electrons, while plasmons are more relevant to metals where you intrinsically have a sea of free electrons that can oscillate as a plasma.

However, plasmons, excitons, and polaritons are all bosons of sorts. On top of that, photons can couple to plasmons and excitons (and optical phonons) giving rise to different flavors of polaritons: plasmon-polaritons, exciton-polaritons, and phonon-polaritons.

I don't exactly agree with the previous answer. Specifically, plasmons are a quantum of free-electron oscillation (ie plasma), while polaritons are hybridizations of photons and excitons. Excitons are a bound electron-hole pair (a composite boson), and not directly relevant to the concept of plasma oscillations. My understanding is that excitons are more relevant to gapped semiconductor systems, where the conduction band is mostly empty of electrons, while plasmons are more relevant to metals where you intrinsically have a sea of free electrons that can oscillate as a plasma.

However, plasmons, excitons, and polaritons are all bosons of sorts. On top of that, photons can couple to plasmons and excitons (and optical phonons) giving rise to different flavors of polaritons: plasmon-polaritons, exciton-polaritons, and phonon-polaritons.