Boosting the Force of Light
The force exerted by light can be sufficient to manipulate nano- and micron-sized objects. These effects can be exploited to trap and move atoms and particles in devices such as optical tweezers, or to generate displacements in nanoscale optomechanical systems. Writing in Physical Review Letters, Vincent Ginis at the Vrije Universiteit Brussel, Belgium, and colleagues demonstrate theoretically that metamaterials can be used to dramatically enhance the optically induced mechanical forces between two coupled waveguides.
In optical waveguides the electric field decays exponentially with distance, with a peak at the center of the waveguide. If two waveguides are placed in such proximity that the decaying fields overlap, this may cause an optically induced attractive or repulsive force between the two structures. The authors propose a device in which two optical waveguides, cladded with a metamaterial layer, are separated by a thin air gap. The presence of the metamaterial enhances the electric field at the edges of the modified waveguides, which in turn increases the magnitude of the optical force.
Ginis et al. modeled two types of metamaterials: single-negative (in which only the relative permittivity is negative) and double-negative (in which the permittivity and permeability are both negative, resulting in a negative index of refraction). In both cases the optical forces are enhanced, but the effect is stronger for single-negative metamaterial cladding, in which certain resonant losses can be avoided. Single-negative metamaterials can be formed from a stack of thin metal sheets, a configuration easily realizable experimentally. These results open the way for the design of new actuation devices, which can generate optical forces with magnitudes not currently achievable. – Katherine Thomas