Orbital angular momentum of light

Summary

Light carries angular momentum in two forms: spin, tied to circular polarisation, and orbital angular momentum, tied to the spatial shape of the wavefront. A beam whose phase varies as $e^{i\ell\varphi}$ around its axis carries orbital angular momentum $\ell\hbar$ per photon, where $\hbar$ is the reduced Planck constant. Such beams have helical wavefronts and an optical vortex on the axis, as in the Laguerre–Gaussian modes. Allen and coworkers showed in 1992 that Laguerre–Gaussian laser modes carry this angular momentum. It can be transferred to small particles and make them rotate.