  Transverse spin angular momentum of light is a key concept in recent
nanophotonics to realize unidirectional light transport in waveguides by
spin-momentum locking. Herein we theoretically propose subwavelength
nanoparticle chain waveguides that efficiently sort optical spins with
engineerable spin density distributions. By arranging high-refractive-index
nanospheres of different sizes in a zigzag manner, directional optical spin
propagation is realized. The origin of the efficient spin transport is revealed
by analyzing the dispersion relation and spin angular momentum density
distributions. In contrast to conventional waveguides, the proposed asymmetric
waveguide can spatially separate up- and down-spins and locate one parity
inside and the other outside the structure. Moreover, robustness against
bending the waveguide and its application as an optical spin sorter are
presented. Compared to previous reports on spatial engineering of local spins
in photonic crystal waveguides, we achieved substantial miniaturization of the
entire footprint down to subwavelength scale.
