Flat metagrating axicons for optical gradient force enhancement

Abstract

Nanostructured dielectric metasurfaces offer unprecedented control over electromagnetic wavefronts, enabling both ultrathin analogues of classical optical components and entirely novel light-matter interactions. Recently, significant interest has emerged in utilizing these structures for optomechanical applications, specifically tailoring the momentum transfer of an impinging wavefront to drive and control mechanical motion. In this work, we utilize dielectric metagratings, sparse metasurfaces engineered for highly efficient anomalous diffraction, to design flat optomechanical axicons. By precisely redirecting the incident wavefront into a targeted diffraction order, these structures exhibit gradient-following behaviour within spatially bounded fields, such as Gaussian beams. To evaluate and realize these designs, we employ Finite Element Method (FEM) simulations coupled with Particle Swarm Optimization (PSO), inversely designing geometric parameters to maximize the optomechanical momentum exchange. The resulting optimized structures demonstrate highly efficient anomalous diffraction and generate strong lateral forces, indicating viability for long-distance optical transport and the development of light-driven machinery. To validate these theoretical models, the optomechanical dynamics of the fabricated structures were experimentally characterized. While optomechanical interaction was confirmed, the particles consistently exhibited an off-axis, edge-aligned trapping equilibrium rather than the predicted beam-centered stabilization. Physical analysis indicates that this divergence from idealized two-dimensional models is likely driven by optical torque, which induces a physical tilt and creates asymmetric, repelling Fresnel reflections at the dielectric interface. However, isolating the magnitude of these tilt-induced forces remains challenging, as potential structural degradation during the fabrication process may concurrently reduce or eliminate the anomalous diffraction efficiency.