By controlling the optical force, optical tweezers can manipulate many kinds of small particles without mechanical contact. In the theoretical analysis of the optical force, conventional methods are based on the integration of the Maxwell stress tensor over the outer surface of the particle, while the Maxwell stress tensor is determined by the electromagnetic field distribution around the particle itself. However, we find that this conventional method may not be appropriate in most situations, as two main issues arise, especially for plasmonics nanoparticles because of the metal involved. The first is the selection of the relative permittivity on the interface between the particle and the background medium, while the second is the use of the divergence theorem. Here, we present an improved and more correct technique to compute the optical force of optical tweezers on the plasmonics nanoparticle. The analysis of an Au-Ag core–shell nanostructure, conducted by adopting this revised method, shows that the negative force is located not only at the Fano resonance but also at longer wavelengths.
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