ABSTRACT
Phase gradients across the beam-for example, arising from divergence-mean that at any one position the Poynting vector generally has transverse as well as axial components. Integrating the axial components of this vector over the beam cross section gives the linear momentum in the propagation direction, whereas integration of the transverse components, multiplied by a radius vector defined with respect to a defined axis, gives the angular momentum of the beam. Just as the transfer of linear momentum from light to a particle results in optical trapping, so the transfer of angular momentum from light to a particle results in rotation. As is now well recognized, light’s angular momentum can be of a spin (manifest as the polarization state) or orbital (associated with phase cross section) type [2].
