Upper bound to optical forces through the multipolar control of optical beams
Optical tweezers enable the manipulation of microscopic objects using light, yet the fundamental limits to the optical forces that can be exerted on matter remain unknown. Here we derive a general upper bound to the maximum optical force that can be applied to a particle, based on an expansion of electromagnetic fields into well-defined helicity multipolar modes. This method finds the optimal force for any kind of fields external to the particle, including evanescent fields. We apply the method to homogeneous spherical particles in a stable trap and identify the field distributions that saturate this bound for the trapping stiffness. We further provide experimentally accessible strategies to approach these optimal conditions, including configurations using counterpropagating and single-beam traps. Experiments demonstrate a threefold enhancement of trapping forces relative to conventional designs, while theoretical predictions indicate that order-of-magnitude improvements are achievable for larger particles and high angular momentum beams. Our results establish fundamental design principles for maximizing optical forces and define the ultimate limits of optical manipulation.