Vortex-Free Intrinsic Orbital Angular Momentum

Abstract: For three decades, optical orbital angular momentum (OAM) has been synonymous with phase vortices. Here, our research challenges this paradigm by demonstrating a vortex-free intrinsic OAM arising from three-dimensional caustic trajectories in structured light fields, distinct from traditional vortex-based mechanisms. Optical OAM was traditionally considered rare, predominantly manifesting during high-order atomic and molecular transitions. The conceptual shift initiated by Allen et al. in 1992, which showed that laser beams with optical vortex--characterized by their twisted wavefronts--naturally carry intrinsic OAM, laid the groundwork for subsequent studies. However, the exploration of intrinsic OAM in vortex-free laser modes remains a largely untapped area. Addressing this gap, our research proposes a third category of optical intrinsic angular momentum--termed "vortex-free OAM"--that complements spin angular momentum (SAM) in circularly polarized beams and OAM in vortex beams. This new form of OAM is governed by a hydrodynamic analogy between light propagation and fluid dynamics, linking the orbital motion of energy streamlines to quantifiable angular momentum. Through numerical simulations, experimental measurements, and investigations of the mechanical transfer of OAM to trapped particles using optical tweezers, we establish vortex-free OAM as a universal property of structured fields with three-dimensional caustic trajectories. Furthermore, we propose a unified hydrodynamic framework that transcends traditional wavefront-centric models, facilitating the generalization of OAM analysis to both vortex and vortex-free regimes. This framework not only enhances our understanding of the physical origin of optical angular momentum but also unlocks new opportunities in optical manipulation, classical and quantum communications, and light-matter interaction technologies.