Attraction-repulsion modulation of resonant Casimir-Polder forces between silver nanoparticles and atoms

Based on macroscopic quantum electrodynamics, the resonant Casimir-Polder (C-P) force between a silver nanoparticle and a two-level atom is investigated. For atoms in the non-retarded region, the C-P force originates from interactions with different surface plasmon modes, with each interaction exhibiting distinct spectral and spatial control characteristics. We will show that the force can be tuned from attraction to repulsion by controlling the atomic transition frequency, the direction of polarization and the atom-surface distance. For atoms in the retarded region, we will show that only the dipole plasmon mode makes a decisive contribution to the C-P force, leading to the force oscillating between attraction and repulsion with the atom-surface distance due to resonant dipole-dipole interaction. Furthermore, we reveal how the atomic polarization vector governs both the magnitude and directionality of the C-P force. This work has potential applications in quantum trapping and nanoscale manipulation.