Full-gray optical trapping by high-order multipole-resonant gradient forces in structured light

The optical gradient force trap lies at the heart of optical tweezers, with applications in a wide spectrum of sciences. It is conventionally perceived that the trap is either bright or dark, capturing small particles towards the intensity maxima or minima. Here, we demonstrate that there may exist an intermediate trapping state, which we refer to as the full-gray optical trapping, to permit distinct equilibrium positions where the light intensity incident on each part of the particle is neither maximized nor minimized. The appearance of this third-type trap is attributed to the excitation of high-order multipole resonances, which strongly couples nonlocal intensity inhomogeneity to the gradient force, yielding a unique force-vector field that converges on the gray region. By expanding the classification of optical traps and generalizing the gradient force theory to arbitrary-order multipoles, our findings underscore the impact of Mie responses on optomechanics and will facilitate the development of nanoparticle cooling, patterning, and ultrasensitive sorting.