Recently, the International Laboratory for Future Optics, led by Academician Zhuang Songlin at the University of Shanghai for Science and Technology, in collaboration with Professor Zhan Qiwen's Nanophotonics team, achieved a remarkable milestone in optics. By applying the Maxwell's equations and optical conformal mapping, they have, for the first time in optics, successfully created exquisite photonic toroidal vortex structure.
Toroidal vortices, also known as vortex rings, are intriguing propagating ring-shaped structures with whirling disturbances rotating about the ring. Toroidal vortices are not uncommon in nature and science. Indeed, aquarium visitors are often amazed at how dolphins master creating and playing with bubble rings, which are air-filled vortex rings propagating in water. The scientific investigation of vortex rings dates back to 1867, when Lord Kelvin proposed the vortex atom model. After one and a half centuries, vortex rings are still being actively investigated in a variety of disciplines. For example, in meteorology, vortex rings of wind, rain and hail are tightly related to the development of microbursts, which pose a great threat to aviation safety. In cardiology, asymmetric redirection of blood flow through the heart resembles a toroidal vortex. In magnetics, the experimental observation of vortex rings in a bulk magnet has only just been accomplished, opening up possibilities for studying complex three-dimensional (3D) solitons in bulk magnets. In photonics and light science, there have been studies on toroidal dipole and multipole excitation in metamaterials. However, the theory and experimental demonstrations of a propagating photonic toroidal vortex remain elusive.
This research work reports the experimental observation of a photonic toroidal vortex as a new solution to Maxwell’s equations, generated using conformal mapping. The resulting light field has a helical phase that twists around a closed loop, leading to an azimuthal local orbital angular momentum density. The preparation of such an intriguing state of light may offer insights for exploring the behavior of toroidal vortices in other disciplines and find important applications in light–matter interactions, optical manipulation, photonic symmetry and topology, and quantum information. The related research findings, titled 'Toroidal vortices of light,' were published in the top international journal 'Nature Photonics.' The work was supported by the Key Project of the National Natural Science Foundation of China (NSFC) under the Major Research Program on Novel Light Field Control Physics and Applications (Project ID: NSFC92050202).
