  The optical binding forces between guided lightwaves in dielectric waveguides
can be either repulsive or attractive. So far only attractive force has been
observed. Here we experimentally demonstrate a bipolar optical force between
coupled nanomechanical waveguides. Both attractive and repulsive optical forces
are obtained. The sign of the force can be switched reversibly by tuning the
relative phase of the interacting lightwaves. This tunable, bipolar interaction
forms the foundation for the operation of a new class of light force devices
and circuits.
