  The quantization of plasmons has been analyzed mostly under the assumption of
an infinite-sized bulk medium interacting with the electromagnetic field. We
reformulate it for finite-size media, such as metallic or dielectric
nano-structures, highlighting sharp differences. By diagonalizing the
Hamiltonian by means of a Lippmann-Schwinger equation, we show the contribution
of two sets of bosonic operators, one stemming from medium fluctuations, and
one from the electromagnetic field. The results apply to general models
including dissipative and dispersive responses.
