Abstract

The torque exerted by an astigmatic optical beam on small transparent isotropic particles was dynamically measured observing the angular motion of the particles under a microscope.  The data confirmed that torque was originated by the transfer of orbital angular momentum associated with the spatial changes in the phase of the optical field induced by the moving particle.  This mechanism for angular momentum transfer works also with incident light beams with no net angular momentum. 

Nine frames of a trapped glass rods showing the alignment motion along the major axis of the trapping beam shape.  As the optical torque acting on the particle depends on its orientation [see Eq. ( 8)], the rotation speed is not constant.  The frames are 200 ms apart.  The scale bar is 10 μm.