We revisit the mechanisms governing the sub-wavelength 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, i.e., 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.Comment: 25 pages, 9 figures, Ch. 8 of Singular and Chiral Nanoplasmonics
(S.V. Boriskina and N.I. Zheludev Eds.) Pan Stanford, Singapore, 201
