Space-Time Surface Plasmon Polaritons: A New Propagation-Invariant Surface Wave Packet

We introduce the unique class of propagation-invariant surface plasmon polaritons (SPPs) representing pulsed surface wave packets propagating along unpatterned metal-dielectric interfaces and are localized in all dimensions, with potentially subwavelength transverse spatial widths. The characteristic features of such linear diffraction-free, dispersion-free plasmonic bullets stem from tight spatiotemporal correlations incorporated into the SPP spectral support domain, and we thus call them space-time SPPs. We 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. We present an analytical framework and numerical simulations for the propagation of space-time SPPs in comparison with traditional pulsed SPPs whose spatial and temporal degrees of freedom are separable, thereby verifying the propagation-invariance of the former.