Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities

Thirty years ago, Coullet et al. proposed that a special optical field exists in laser cavities bearing some analogy with the superfluid vortex. Since then, optical vortices have been widely studied, inspired by the hydrodynamics sharing similar mathematics. Akin to a fluid vortex with a central flow singularity, an optical vortex beam has a phase singularity with a certain topological charge, giving rise to a hollow intensity distribution. Such a beam with helical phase fronts and orbital angular momentum reveals a subtle connection between macroscopic physical optics and microscopic quantum optics. These amazing properties provide a new understanding of a wide range of optical and physical phenomena, including twisting photons, spin–orbital interactions, Bose–Einstein condensates, etc., while the associated technologies for manipulating optical vortices have become increasingly tunable and flexible. Hitherto, owing to these salient properties and optical manipulation technologies, tunable vortex beams have engendered tremendous advanced applications such as optical tweezers, high-order quantum entanglement, and nonlinear optics. This article reviews the recent progress in tunable vortex technologies along with their advanced applications. Commemorating the 30th anniversary of the prediction of optical vortices by theoretical physicist Pierre Coullet and colleagues, researchers in China review the development of understanding and applications of these intriguing phenomena. Xing Fu at Tsinghua University, Xiaocong Yuan at Shenzhen University and co-authors, explain how the concept of optical vortices emerged from observed similarities between the behaviour of fluid vortices and some forms of laser light. The light waves of optical vortices are twisted around their direction of travel, with a point of zero intensity at their centre. The authors survey the steady refinement of techniques used to create optical vortices, and explore their applications. Prominent applications include sophisticated optical computing processes, novel microscopy and imaging techniques, the creation of ‘optical tweezers’ to trap particles of matter, and optical machining using light to pattern structures on the nanoscale.