  When a beam of light is laterally confined, its field distribution can
exhibit points where the local magnetic and electric field vectors spin in a
plane containing the propagation direction of the electromagnetic wave. The
phenomenon indicates the presence of a non-zero transverse spin density. Here,
we experimentally investigate this transverse spin density of both magnetic and
electric fields, occurring in highly-confined structured fields of light. Our
scheme relies on the utilization of a high-refractive-index nano-particle as
local field probe, exhibiting magnetic and electric dipole resonances in the
visible spectral range. Because of the directional emission of dipole moments
which spin around an axis parallel to a nearby dielectric interface, such a
probe particle is capable of locally sensing the magnetic and electric
transverse spin density of a tightly focused beam impinging under normal
incidence with respect to said interface. We exploit the achieved experimental
results to emphasize the difference between magnetic and electric transverse
spin densities.
