  In addition to longitudinal spin angular momentum (SAM) along the axis of
propagation of light, spatially structured electromagnetic fields such as
evanescent waves and focused beams have recently been found to possess
transverse SAM in the direction perpendicular to the axis of propagation. In
particular, the SAM of SPPs with spatial structure has been extensively studied
in the last decade after it became clear that evanescent fields with spatially
structured energy flow generate threedimensional spin texture. Here we present
numerical calculations of the space-time surface plasmon polariton (ST-SPP)
wave packet, a plasmonic bullet that propagates at an arbitrary group velocity
while maintaining its spatial distribution. ST-SPP wave packets with complex
spatial structure and energy flow density distribution determined by the group
velocity are found to propagate with accompanying three-dimensional spin
texture and finite topological charge density. Furthermore, the spatial
distribution of the spin texture and topological charge density determined by
the spatial structure of the SPP is controllable, and the deformation
associated with propagation is negligible. ST-SPP wave packets, which can
stably transport customizable three-dimensional spin textures and topological
charge densities, can be excellent subjects of observation in studies of
spinphotonics and optical topological materials.
