Plasmon Polaritons in 2-D Nanoparticle Arrays

The hybrid plasmonic-electromagnetic modes-or plasmon polaritons-in 2-D square arrays of noncontacting noble-metal nanoparticles are studied via an exactly solvable model. The approach accounts self-consistently for diffractive losses out of the plane and also for the vector nature of the electromagnetic field, and is thus in essence a 3-D theory of the 2-D nanoparticle array. The resulting polarization mixing between modes leads to a coupling between longitudinal and transverse excitation off special directions in the Brillouin zone. This is manifested as warping of the plasmon-polariton dispersion surfaces. Modes with group and phase velocities of opposite signs are observed.