Low-Power Optical Traps using Anisotropic Metasurfaces: Asymmetric Potential Barriers and Broadband Response

We propose lateral optical trapping of Rayleigh particles using tailored anisotropic and hyperbolic metasurfaces illuminated with a linearly polarized Gaussian beam. This platform permits optical traps to be engineered at the beam axis with a response governed by nonconservative and giant lateral recoil force coming from the directional excitation of confined surface plasmons during the light-scattering process. Compared to optical traps set over uniform metals, either in bulk or thin-layer configurations, the proposed traps are broadband in the sense that they can be set with beams oscillating at any frequency within a wide range in which the metasurface supports surface plasmons. Over that range, the metasurface dispersion evolves from an anisotropic elliptic to a hyperbolic regime going through a topological transition and enables optical traps with distinctive spatially asymmetric potential distribution, local potential barriers arising from the momentum imbalance of the excited plasmons, and an enhanced potential depth that permits stable trapping of nanoparticles using low-intensity laser beams. To investigate the performance of this platform, we develop a rigorous formalism based on Lorentz force within the Rayleigh approximation combined with anisotropic Green's functions and calculate the trapping potential of nonconservative lateral forces using the Helmholtz decomposition method. Tailored anisotropic and hyperbolic metasurfaces, commonly implemented by nanostructuring thin metallic layers, permit to use low-intensity laser sources operating in the visible or infrared frequencies to trap and manipulate particles at the nanoscale, and may enable a wide range of applications in bioengineering, physics, and chemistry.