Plasmonic Spin-Hall Effect in Surface Plasmon Polariton Focusing

Surface plasmon polaritons (SPPs) are fundamental collective charge density excitations at surfaces of solid-state plasmas. They can energize physical and chemical processes at the nanoscale, but control of the spatiotemporal evolution of their energy, momentum, and spin is necessary to effectively transfer their quantum properties to molecules and other nanoscopic objects. Thus, to design the coupling of SPPs into other modes of an electronic system, it is important to describe and control their nanofemto spatiotemporal distributions. We investigate the spatial SPP field and spin distributions when launched by illuminating a converging lens coupling structure in an Ag film with obliquely incident linearly or circularly polarized femtosecond light pulses. The propagation and focusing of SPPs are recorded as movies by scanning the pump-probe pulse delay and recording the evolving interference patterns between the SPP and optical fields by ultrafast interferometric time-resolved two-photon photoemission electron microscopy. The observed PEEM images of SPPs for p-polarized excitation light are symmetric with a distinct focal spot, but with circularly polarized excitation the coupling structure also acts as a plasmonic spin-Hall device exhibiting light helicity dependent asymmetric launching and focusing with respect to the optical plane. We show that the anisotropy arises because the SPP excitation efficiency depends on matching of the spin angular momenta between the incident vacuum light and the SPP field. Because the coupling structure generates distinct plasmonic field texture, the generated chiral density of the SPP field is spatially dependent on the focusing conditions and helicity of the excitation light. We demonstrate that the SPP fields excited by circularly polarized light (CPL) converge to a focal point of high chiral density with the location and sign defined by the light-helicity, which can be utilized to design and enhance quantum interactions with chiral objects, such as molecules.