Optical vortex generation by magnons with spin-orbit-coupled light

Light possesses both spin and orbital angular momentum, which can spontaneously couple in spatially asymmetric optical fields. This phenomenon is referred to as optical spin-orbit coupling. This coupling is pivotal in modern optics due to its broad applications in communications, sensing, and quantum control. A central challenge is to elucidate how spatial asymmetries in optical fields facilitate this coupling. Previous research has primarily addressed spatial asymmetry using materials and devices such as lenses, interfaces, inhomogeneous media, and metasurfaces. However, Maxwell's equations indicate that matter can also introduce temporal asymmetry to optical fields. For instance, magnetic ordering can break time-reversal symmetry via the magneto-optical effect, resulting in nonreciprocal optical phenomena. Despite its importance, the combined effects of spatial and temporal asymmetries in optical fields remain unexplored. This study demonstrates that breaking time-reversal symmetry via magnons and spatial symmetry via light focusing enables the nonreciprocal transformation of a Gaussian beam into an optical vortex beam. This effect is attributed to the interplay between magnon-induced Brillouin light scattering and optical spin-orbit coupling. The results indicate that total angular momentum, including contributions from both magnons and photons, is conserved, suggesting that magnons can control both the spin and orbital angular momentum of light.