  Besides a linear momentum, optical fields carry an angular momentum (AM),
which have two intrinsic components: one is spin angular momentum (SAM) related
to the polarization state of the field, and the other is orbital angular
momentum (OAM) caused by the helical phase due to the existence of topological
azimuthal charge. The two AM components of the optical field may not be
independent of each other, and the Spin-to-Orbital AM conversion (STOC) under
focusing will create a spin-dependent optical vortex in the longitudinal filed.
Here we demonstrate a new mechanism (or novel way, new way, specific process)
for the STOC based on a radial intensity gradient. The radial phase provides an
effective way to control the local AM density, which induce counterintuitive
orbital motion of isotropic particles in optical tweezers without intrinsic
OAM. Our work not only provides fundamental insights into the spin-orbit
interaction of light, but also push towards possible applications in optical
micro-manipulation.
