ABSTRACT
We revisit the mechanisms governing the subwavelength spatial
localization of light in surface plasmon polariton (SPP) modes by
investigating both local and global features in optical powerflow
at SPP frequencies. Close inspection of the instantaneous Poynting
vector reveals formation of optical vortices-localized areas of
cyclic powerflow-at the metal-dielectric interface. As a result,
optical energy circulates through a subwavelength-thick ‘conveyor
belt’ between the metal and dielectric where it creates a high
density of optical states (DOS), tight optical energy localization,
and low group velocity associated with SPP waves. The formation
of bonding and anti-bonding SPP modes in metal-dielectric-metal
waveguides can also be conveniently explained in terms of different
spatial arrangements of localized powerflow vortices between two
metal interfaces. Finally, we investigate the underlying mechanisms
of global topological transitions in metamaterials composed of
multiple metal and dielectric films, that is, transitions of their iso-
frequency surfaces from ellipsoids to hyperboloids, which are not
accompanied by the breaking of lattice symmetry. Our analysis
reveals that such global topological transitions are governed by the
dynamic local re-arrangement of local topological features of the
optical interference field, such as vortices and saddle points, which
reconfigures global optical powerflow within the metamaterial.
These new insights into plasmonic light localization and DOS
manipulation not only help to explain the well-known properties
of SPP waves but also provide useful guidelines for the design
of plasmonic components and materials for a variety of practical
applications.
