  We investigate the evolution of transverse spin in tightly focused circularly
polarized beams of light, where spin-orbit coupling causes a local rotation of
the polarization ellipses upon propagation through the focal volume. The effect
can be explained as a relative Gouy-phase shift between the circularly
polarized transverse field and the longitudinal field carrying orbital angular
momentum. The corresponding rotation of the electric transverse spin density is
observed experimentally by utilizing a recently developed reconstruction
scheme, which relies on transverse-spin-dependent directional scattering of a
nano-probe.
