Shack-Hartmann measurement of OAM in highly focused light beams

The angular momentum of light beams has been widely used in optical tweezers applications. Spin angular momentum has been used to rotate birefringent objects such as Vaterite microspheres, while orbital angular momentum has been used to rotate trains of particles around a central beam axis. However, while orbital angular momentum can be used to turn micro scale objects, the exact amount of torque created in a set of optical tweezers is not well known. The spin and orbital components of torque become hard to separate in highly focused beams. In order to resolve this difficulty we evaluate the torques created in highly focused light beams using a Spatial Light Modulator based Shack-Hartmann system to examine the torques exerted on glass microrods.