Optical Binding Forces on Gold or Silver Nanoparticles

This study theoretically investigates that plasmon-mediated optical forces, exerted on metal dimers, consisting of two gold or silver nanoparticles (NPs), induced by the normal illumination of a linearly polarized plane wave or Gaussian beam. Using the multiple multipole method, we analyzed the optical force in terms of Maxwell’s stress tensor versus the inter-particles distance for some specific wavelengths. Numerical results show that for a given wavelength there are several stable equilibrium distances between NPs, which is slightly shorter than some integer multiples of the wavelength in medium, for metal dimer acting as bonded together. At these specific distances, the optical force between dimer will become a repulsive or an attractive force when the two NPs are close to or away from each other. The spring constant of the restoring force at the first stable equilibrium is always the maximum, indicating that the first stable equilibrium distance is the most stable one. Moreover, the central line of a dimer tends to be perpendicular to the polarization of light. For a heterodimer, which consists of two NPs of different sizes or different materials, the phenomenon of stable equilibrium distance still exists but with a net driving photophoretic force to move the heterodimer. In addition, a Gaussian beam provides a gradient force reducing the stability of these equilibriums. For a trimer, consisting of three gold NPs, there still exists stable distance between NPs and also a photophoretic force. Moreover, the average scattering and absorption efficiencies as well as depolarization ratio of a randomly oriented gold nanorod are also studied. Our results indicate that the depolarization spectrum is broader than those of scattering and absorption efficiencies. The maximum depolarization ratio can be raised and the corresponding wavelength (not at the longitudinal SPR) is red-shifted by increasing the aspect ratio.