Tunable plasmonic binding via transverse dipole–quadrupole Fano coupling

In this study, we conducted theoretical and numerical analyses of the near-field optical binding force acting on a nanocrescent dimer subjected to a normally incident plane wave. Our study reveals that Fano resonance induces wavelength-dependent force reversal (attraction to repulsion) near the dimer axis, governed by the relative phase of the transverse electric dipole modes, which are derived from bright-dark plasmonic hybridization. Systematic geometric tuning (tip spacing, size, and height) can control force spectra by modulating the reversal wavelength and amplitude via near-field coupling. Furthermore, polarization-angle tuning introduces a unique dimension—decoupling the force amplitude from the spectral position—that enables polarization-multiplexed manipulation. Remarkably, the same Fano-mediated phase coherence enabling force reversal also achieves unidirectional scattering (forward/backward ratio > 20 dB), merging the near-field assembly with far-field emission. These findings establish a unified plasmonic framework that bridges nanomechanics and photonics, with implications on self-assembling metamaterials, reconfigurable sensors, and optomechanical systems.