Opt. Lett. 36, 4638–4640 (2011)

When a dielectric particle is exposed to a strong optical gradient it becomes electrically polarized and subject to an attractive force in the direction of increasing field strength. This is the principle by which optical tweezers are able to trap and manipulate living cells and other microscopic objects.

A similar force can be generated between two light-carrying microscopic (and nanoscopic) waveguide structures. Moreover, the sign of the force can be tuned from attractive to repulsive by shifting the phase of the light travelling in one waveguide relative to the other — which is potentially useful for optomechanical sensing and switching applications. Unfortunately, the magnitude of such forces is much less than the force exerted by optical tweezers. And at small separations the sign is invariably attractive.

Ardavan Oskooi and colleagues propose a way to increase the repulsion between waveguides. Their calculations suggest that the key is to cut each waveguide in half, as semicircular waveguides perform much better than circular. This works even better in the case of photonic crystal waveguides, which show a 30-fold increase in repulsive force that remains repulsive even at small separations.