Special issue on orbital angular momentum

Abstract

It is now nearly 20 years since Allen et al published their seminal paper at Leiden: `Orbital angular momentum of light and the transformation of Laguerre–Gaussian laser modes' (L Allen, M W Beijersbergen, R J C Spreeuw and J P Woerdman 1992 Phys. Rev. A 45 8185).Although it had long been recognized that light could carry an angular momentum in addition to that associated with photon spin, this had previously only been associated with high-order and rare atomic/molecular transitions. The breakthrough of Allen et al was their recognition that laser beams carrying orbital angular momentum could be produced within a standard optics laboratory. The natural basis set for such beams were the Laguerre–Gaussian laser modes whose perfectly helical phasefronts give an azimuthal component to the Poynting vector and hence energy and momentum flow around the beam axis.In this original work from Leiden, they used a clever combination of cylindrical lenses to transform between Laguerre–Gaussian and Hermite–Gaussian laser modes. However, in parallel with these studies had been the use of diffractive optics (holograms) to introduce helical phasefronts to a normal laser beam. Although at the time the link to orbital angular momentum went unnoticed, this holographic technique is now the method of choice for both generation and measurement of light beams containing orbital angular momentum.In 1995, Rubinsztein-Dunlop and co-workers showed that the orbital angular momentum of a focused light beam could be transferred within optical tweezers to set small particles into rotation. However, these optical spanners were just the beginning of what orbital angular momentum could do.The paper of Allen et al in 1992 undoubtedly created a new field of optics research, producing many papers which themselves have impacted various fields ranging from optical manipulation, non-linear and quantum optics to imaging. Beyond optical wavelengths, orbital angular momentum is becoming an important concept in electron, x-ray and radio frequency beams. Whatever the future holds, the story is not yet over.