Comprehensive microscopic theory for coupling of individual and collective excitations via longitudinal and transverse fields

A plasmon is a collective excitation of electrons due to the Coulomb interaction. Both plasmons and single-particle excitations (SPEs) are eigenstates bulk metallic systems they orthogonal each other. In nontranslationally symmetric such as nanostructures, SPEs coherently interact. It has been well discussed that SPEs, respectively, can couple with transverse (T) electric fields in systems, also coupled other via longitudinal (L) field. However, there missing link previous studies: coherent coupling between mediated by T Herein, we develop theoretical framework describe self-consistent relationship through both L fields. The described terms charge current densities constitutive equation nonlocal susceptibility, where include components. electromagnetic originating from Green's function Maxwell's equations. field generated densities, whereas component attributed density only. We introduce four-vector representation incorporating vector scalar potentials gauge, which separated explicitly. eigenvalues matrix for equations appear poles system excitations. numerical demonstration spectrum performed rectangular nanorod. indicates non-negligible shift an enhancement energy transfer T-field-mediated developed formulation enables approach unknown mechanisms hot carriers radiative