We study the energy and momentum of the surface plasmon-polariton (SPP) excited in a symmetric 3-layer insulator-metal-insulator structure, which is known to support the symmetric (S) mode with the negative group velocity as well as the antisymmetric (AS) mode with only positive energy flow. The electric and magnetic field vectors are calculated via both the phenomenological and the microscopic approach; the latter involves the hydrodynamic model accounting for the quantum statistical effects for the electron gas in metal. Explicit representation for the energy and momentum constituents in the dielectric and in the metal film are obtained, and the wavenumber dependences of the energy and momentum contributions for the whole SPP are analyzed numerically. The various energy and momentum constituents are classified with respect to their origin: field or material, and the physical nature: orbital (canonical) and spin (Belinfante) momentum contributions. The pictures characteristic for the S and AS modes are systematically compared. The results can be useful for the studies and applications of the SPP-induced thin-film effects, in particular, for the charge and spin dynamics in thin-film plasmonic systems.
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