Spin-to-orbital angular momentum conversion via light intensity gradient

Besides a linear momentum, optical fields also carry angular momentum (AM), which has two intrinsic components: one is spin angular momentum related to the polarization state and the other is orbital angular momentum (OAM) caused by the helical phase due to the existence of a topological azimuthal charge. The two AM components of the optical field may not be independent of each other, especially if spin-to-orbit conversion (STOC) under high focusing creates a spin-dependent optical vortex in the longitudinal field. However, it would be very exciting to experimentally manifest and control the local OAM density. Here, we present a strategy for achieving STOC via a radial intensity gradient. The linearly varying radial phase provides an effective way to control the local AM density, which induces a counterintuitive orbital motion of the isotropic microparticles in optical tweezers without intrinsic OAM. Our work not only provides fundamental insights into the STOC of light, but could also have applications in optical micromanipulation.