Within the context of spin-related optical phenomena, the near-field
directionality is generally understood from the quantum spin Hall effect of
light, according to which the transverse spin of surface or guided modes is
locked to the propagation direction. So far, most previous works have been
focused on the spin properties of circularly polarized dipolar sources.
However, in near-field optics, higher-order multipole sources (e.g.,
quadrupole, octupole, and so on) might become relevant, so a more in-depth
formulation would be highly valuable. Building on the angular spectrum
representation, we provide a general, analytical, and ready-to-use treatment in
order to address the near-field directionality of any multipole field,
particularizing to the electric quadrupole case. Besides underpinning and
upgrading the current framework on spin-dependent directionality, our results
may open up new perspectives for engineering light-matter coupling at the
nanoscale.Comment: 7 pages, 2 figures. Supplemental Material (19 pages). Supplemental
tools (calculator of angular spectra and animation) available at
https://doi.org/10.5281/zenodo.267790