Optical backflow for the manipulations of dipolar nanoparticles

Backflow is a counterintuitive phenomenon that is widely predicted in the fields of quantum physics and optics. In contrast to quantum backflow, which is challenging to be observed, optical backflow is prevailing in structured lights. For instance, the azimuthal backflow has been recently observed experimentally in optics via the superposition of two beams carrying different orbital angular momentum topological charges. In this paper, we investigate the spin-momentum characteristics of the superimposed orbital angular momentum beams to confirm the optical azimuthal backflow, which is closely related to off-axis vortex flow and super-oscillations. Furthermore, we extend our study to axial backflow, characterized by a reversed axial energy flow in tightly focused cylindrical vector vortex beams. Then, we explore the application of optical backflow in the manipulation of dipolar nanoparticles. By optimizing material parameters, we achieve on-demand control of optical forces in both azimuthal and axial backflow scenarios. Our findings provide in-depth insights into the optical backflow phenomena with potential applications in optical manipulations.