  Plasmonics is based on surface plasmon polariton (SPP) modes which can be
laterally confined below the diffraction limit, thereby enabling ultracompact
optical components. In order to exploit this potential, the fundamental
bottleneck of poor light-SPP coupling must be overcome. In established SPP
sources (using prism, grating} or nanodefect coupling) incident light is a
source of noise for the SPP, unless the illumination occurs away from the
region of interest, increasing the system size and weakening the SPP intensity.
Back-side illumination of subwavelength apertures in optically thick metal
films eliminates this problem but does not ensure a unique propagation
direction for the SPP. We propose a novel back-side slit-illumination method
based on drilling a periodic array of indentations at one side of the slit. We
demonstrate that the SPP running in the array direction can be suppressed, and
the one propagating in the opposite direction enhanced, providing localized
unidirectional SPP launching.
