Constructing spin-structured focal fields for chiral-sensitive trapping with dielectric metalens

Engineering the chiral field is crucial for the flexible manipulation of chiral particles. Some complex optical setups for constructing spin-structured fields have been well developed to sort particles with opposite chiralities toward opposite transversal directions. In this work, we demonstrate the robust construction of a class of focal fields that possess laterally variant optical spin angular momentum by using the monolayer dielectric metalens. By utilizing the simultaneous modulation capacity of the phase and polarization of the dielectric metalens, we can establish a line focus with laterally tailored gradient optical helicity. The focusing property of this metalens and the polarization structure of the focal field are theoretically analyzed using a hybrid vector-focusing model and experimentally demonstrated by NA = 0.2 and 0.5 samples. We illustrate that this type of gradient helicity offers opportunities to induce a chirality-sensitive lateral force for chiral particles.