We generate tightly focused optical vector beams whose electric fields spin around an axis transverse to the beams propagation direction. We experimentally investigate these fields by exploiting the directional near-field interference of a dipole-lik
e plasmonic field probe, placed adjacent to a dielectric interface, which depends on the transverse electric spin density of the excitation field. Near- to far-field conversion mediated by the dielectric interface enables us to detect the directionality of the emitted light in the far-field and, therefore, to measure the transverse electric spin density with nanoscopic resolution. Finally, we determine the longitudinal electric component of Belinfantes elusive spin momentum density, a solenoidal field quantity often referred to as virtual.
