Mapping out tractor beams: topological angular momentum and reduced axial flux; gradient versus non-conservative forces

We experimentally demonstrate continuous attraction of macroscopic targets (> 1 cm) towards the source, against a net momentum flux in the system. Use of a simple setup provides an easily understood illustration of the negative radiation pressure concept for tractor beam, and how these are distinct from the gradient force acting in conventional optical tweezers. Here, we map out regimes over which negative radiation forces dominate, and (favorably) compare the thresholds observed to those that emerge from simulations. Theoretical explorations of tractor beam action commonly invoke higher-order Bessel beams, and here we make clear that the reason for this is because of the reduction in axial momentum associated with such hollow-core beams, which allows effects associated with off-axis “skew” momentum to become dominant. Ultimately, there is interest in exploring the language used for describing such effects: radiation pressure versus gradient force (which we suggest might be better described in terms of non‐conservative versus conservative forces), and “orbital” angular momentum (which we suggest might be more appropriately termed “topological” angular momentum).

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