Reduction of optical forces exerted on nano-particles covered by scattering cancellation based plasmonic cloaks

Scattering cancellation approach has been recently proposed as a promising route to design invisibility cloaks. However, reduced observability of an object is just one of the potential applications of this technique. In this paper, we investigate the possibility to reduce optical forces exerted on a given nanoparticle by covering it with a properly designed plasmonic cloak. We show, in fact, that conditions similar to those used to make spherical and cylindrical nanoparticles invisible to the electromagnetic field by using the scattering cancellation approach, may be straightforwardly applied also to minimize both gradient and scattering optical forces exerted by the illuminating radiation on the same covered nanoparticles. These results are then validated through full-wave simulations, properly considering both dispersion and losses of the plasmonic materials used to design the cloaks. We also extend our speculations to the case of optical torques exerted on spheroidal and cylindrical Rayleigh particles, deriving the conditions to obtain stable equilibrium positions. This investigation leads to the anomalous result that the usual unstable equilibrium positions of uncovered particles may result stable ones when properly designing the particle cover. Finally, in order to apply the proposed theoretical speculations to more complex cases, we derive the conditions for minimizing optical forces exerted on a cloaked Rayleigh particle placed above a dielectric half space. These results may find interesting applications in different fields of nanotechnology.