Tailored multipolar optical trapping with higher-order structured light

Abstract

Dipolar optical forces can confine Rayleigh-sized dielectric particles towards or away from intensity maxima, commonly known as bright and dark trapping. Structured light illumination opens up opportunities for customized trapping landscapes, unveiling the possibility of gray trapping, especially for larger Mie particles that can excite a higher-order multipolar response. While previous studies have primarily focused on calculations of the gradient force for some exemplary, focused scalar beams, we not only extend these analyses by combining vectorial focusing with generalized Lorenz-Mie theory, but also consider more complex trapping fields. Motivated by their unique tight focusing behavior, shaping strong transverse intensity gradients and three-dimensional electric field components, we explore higher-order cylindrical scalar and vector beams. We theoretically investigate multipolar trapping of Mie-regime silicon nanospheres in water, calculating the three-dimensional net radiation force and assess local trapping stability through the force Jacobian. Near an octupolar Mie resonance, we predict stable gray trapping with enhanced efficiency for vector beams. Further, we find field geometries enabling configurable, particle-size-dependent gray/dark trapping with subwavelength inter-particle spacing. These results establish polarization, illumination structure and particle parameters as complementary controls for radius-dependent multipolar trapping, providing a basis for potential applications in sub-wavelength particle sorting, trapping-assisted nanopatterning and optical binding investigations.