Mohammad M. Salary and Hossein Mosallaei
Optical forces can be used to manipulate small particles through various mechanisms. In this work, we present a comprehensive analysis of optical forces acting onto the nanoparticles located over a substrate using different manipulation techniques, as well as the conditions of the optimization of these forces. In particular, we study optical trapping, acceleration, and binding. Calculations are carried out using the exact multipole expansion method combined with Maxwell stress tensor formalism, providing a general framework to study optical forces on particles for arbitrary incident fields using closed-form expressions. The method takes into account multiple-scattering between the particles and substrate, and allows clear predictive abilities well beyond the dipole model We consider the interaction of dielectric and metallic nanoparticles with various substrates. The presence of substrate is shown to have a significant impact on the nanoparticles resonances and provides an additional degree of freedom in tailoring the optical forces. We explore different physical processes contributing to the optical force and their interplay on the mobility of the particle. It is established that engineering layered substrates can broaden the scope of trapping and acceleration, and enhance the binding forces. It can also provide a high tunability of the acceleration direction. The analysis presented in this paper provides key physical insights to identify optimum setup for nanoparticles manipulation in various applications.
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