Transforming optical forces with metamaterials

In the interaction between light and matter, linear momentum can be transferred from electromagnetic waves to particles to generate so-called optical forces. Recently, it has been suggested that optical gradient forces can be used to actuate nanophotonic devices. The typical setup consists of two waveguides positioned in each other's vicinity such that they are coupled through the interaction of the evanescent tails. Although the gradient forces between these waveguides can be enhanced considerably with the use of electromagnetic resonators, the resulting displacements remain relatively small.
In this contribution, we demonstrate how transformation optics can be used to calculate and design optical gradient forces inside complex media. The technique of transformation optics allows approaching electromagnetic problems from a geometric perspective as it offers an equivalence relation between a transformation of the underlying coordinate system and the material parameters of a metamaterial [2]. Here, we design metamaterials to create the illusion for the electromagnetic fields that the two slab waveguides are positioned at a smaller separation distance from each other. Indeed, optical forces between two waveguides decay exponentially with the separation distance between the waveguides. To enhance the forces between two waveguides, the distance should be diminished. In Fig. 1, we show the optical forces generated between two waveguides where the electromagnetic distance is transformed by different implementations of metamaterial claddings and demonstrate optical force enhancement over several orders of magnitude.