Determination of optical forces in the proximity of a nanoantenna

We have used the Maxwell stress tensor method to calculate the optical forces acting upon a dielectric nanosphere in the proximity of gold nanoantenna structure optically excited by a plane wave. We have explored the dependence of optical forces for the full range of excitation angles with the conclusion that the maximum force occurs for the excitation at critical angle. The large force at this angle is, however, at the expense of greatly increased intensity in the volume of the particle from which we conclude that the important measure for the trapping efficiency in the case of plasmonic nanostructures is not the incident intensity of the plane wave, but rather the local intensity averaged over the volume of the particle. Our calculations further show multiple trapping sites with similar trapping properties, which leads to uncertainty in the trapping position. Furthermore, our calculations show that the heating effects might play a significant role in the experimentally observed trapping.