Tunable photon-recoil forces and negative torque at flat-top beam edges

Tightly focused Gaussian beams are the cornerstone of traditional optical tweezers. Flat-top beams also enable consummate control of particles over a two-dimensional plane. The former depends on the intensity gradient, while the latter the phase gradient. Here we present a promising alternative for micro/nano-manipulation that complement the phase gradient force in a flat-top beam: utilizing the light-recoiling, particle can be reversibly manipulated or trapped, even along directions without phase or intensity gradients. Typically, these photon-recoil forces are dependent heavily on the details of the microscopic structures of matter, thus limiting both their tunability and reversibility. The photon-recoil-based manipulation technique (PMT) we develop utilizes polarization modulation to exert tunable and reversible lateral forces on simple nanospheres by shaping the imaginary Poynting momentum (IPM) in a flat-top beam. By harnessing recoil forces arising from IPM, our PMT creates edge-specific pathways, enabling tunable driving forces for nanoparticle transport and the formation of stable potential wells. Furthermore, PMT makes it possible to achieve negative optical torque on single nanowires, thereby overcoming previous limitations and opening different avenues in optical manipulation.