Advances in optical angular momentum

Some 16 years ago, Allen et al. [Phys. Rev. A 45, 8185 (1992)] recognised that laser beams which carried an angular momentum additional to photon spin, could be realized in the laboratory. Such beams-have helical phase fronts and so have an azimuthal component to the Poynting vector, which results in angular momentum along the beam axis. This orbital angular momentum, very often combined with spin to make optical angular momentum, has given rise to many developments. These range from optical spanners for driving micro-machines to high dimensional quantum entanglement and new opportunities in quantum information processing. The concept of orbital angular momentum is now leading to new understanding of a wide range of phenomena, including fundamental processes in Bose-Einstein condensates, while the associated technologies have led to new applications in optical tweezing and microscopy.