The orbital angular momentum of light: A new source of optical torque in liquid crystals

We present a study on the flux of angular momentum from a monochromatic optical field into a nematic liquid crystal. In particular, we investigate, starting form first principles, the role played by the orbital part of the angular momentum of light in reorienting the sample. We demonstrated that, when the fluid motion is neglected, the photon spin couples with the director n directly, while the photon orbital angular momentum couples with the gradients of n. This work was motivated by the recent experiments where the transfer of the orbital part of the angular momentum carried by a laser beam into a nematic film has been claimed. A possible approach to model experimental results is also suggested.