Boosting the Force of Light
Enhancing Optical Gradient Forces with Metamaterials
Published January 28, 2013
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
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