The optical resonance between an absorbing particle and the trapping laser can enhance the radiation force exerted on micro/nanoscale objects. However, the exact mechanism behind this resonance is still elusive. To unravel the phenomenon, we studied the resonance between a single dye-doped polystyrene particle and a 1064 nm trapping laser under specifically designed optical conditions. The dye-doped particle was trapped at a water-glass interface while simultaneously being excited by a 488 nm widefield laser. In contrast with former reports (â 10-35% trapping stiffness enhancement), we obtained an unprecedented 4-fold trapping stiffness enhancement due to resonant excitation. When we photobleached the embedded dyes as a control, the trapping stiffness enhancement was no longer observed. Based on nonlinear resonant radiation force theory and the experimental data obtained with a three-dimensional multiplane microscope, we propose that the widefield laser excites the dye to S1 and the trapping laser induces a simultaneous ultrafast S1-S2-S1 cyclic transition, resonantly enhancing the induced dye polarization and, consequently, the radiation force. The elucidation of the optical resonance effect is expected to ultimately enable single molecule manipulation in solution at room temperature.