Flat metagrating axicons for optical gradient force enhancement

Sammanfattning

Nanostructured dielectric metasurfaces offer unprecedented control over electromagneticwavefronts, enabling both ultrathin analogues of classical optical componentsand entirely novel light-matter interactions. Recently, significant interesthas emerged in utilizing these structures for optomechanical applications, specificallytailoring the momentum transfer of an impinging wavefront to drive and control mechanicalmotion. In this work, we utilize dielectric metagratings, sparse metasurfacesengineered for highly efficient anomalous diffraction, to design flat optomechanicalaxicons. By precisely redirecting the incident wavefront into a targeted diffractionorder, these structures exhibit gradient-following behaviour within spatially boundedfields, such as Gaussian beams. To evaluate and realize these designs, we employ FiniteElement Method (FEM) simulations coupled with Particle Swarm Optimization(PSO), inversely designing geometric parameters to maximize the optomechanicalmomentum exchange. The resulting optimized structures demonstrate highly efficientanomalous diffraction and generate strong lateral forces, indicating viabilityfor long-distance optical transport and the development of light-driven machinery.To validate these theoretical models, the optomechanical dynamics of the fabricatedstructures were experimentally characterized. While optomechanical interaction wasconfirmed, the particles consistently exhibited an off-axis, edge-aligned trappingequilibrium rather than the predicted beam-centered stabilization. Physical analysisindicates that this divergence from idealized two-dimensional models is likely drivenby optical torque, which induces a physical tilt and creates asymmetric, repellingFresnel reflections at the dielectric interface. However, isolating the magnitude ofthese tilt-induced forces remains challenging, as potential structural degradationduring the fabrication process may concurrently reduce or eliminate the anomalousdiffraction efficiency.