Subwavelength propagation-invariant surface plasmon polaritons

Unless deliberately confined by waveguide structures, surface plasmon polaritons (SPP) quickly diverge and disperse.  By correlating spatial and temporal frequencies of a wave packet propagating along a metal dielectric surface we show that it is possible to construct an SPP that propagates invariantly in space and time without any waveguide confinement.  Peak transverse dimensions less than the carrier wavelength as well as natural SPP localization normal to the surface are possible leading to strong localized electric fields.  We also show that the group velocity of space-time SPP wave packets can be readily tuned to subluminal, superluminal, and even negative values by tailoring the spatiotemporal field structure independently of any material properties.