Multiple scattering forces and “photonic clusters”

While matter can alter the propagation of light (e.g., scattering, diffraction, photonic crystals, etc.), light can also exert forces on matter (i.e. the Lorentz force). The most well-known type of light-induced force is probably radiation pressure, which originates from the transfer of momentum during the scattering and absorption of photons by matter. The gradient force is also a very important light induced force, where small particles are attracted toward the high intensity region. With the lasers that are available today, light induced forces can be applied to, and are particularly suited to the manipulation of small systems with size on the micron-scale or below.

We investigate the interaction of a cluster of dielectric particles under the illumination of a laser beam; each particle in the cluster scatters light and thus exerts forces on each other. This phenomenon is known as optical binding. We showed that, through this multiple scattering-induced binding mechanism, a cluster of dielectric particles can be bound into a stable “photonic cluster” structure.

We also investigate the resonant interaction of a cluster of spheres. A transparent sphere with diameter larger than a few wavelengths supports Whispering Gallery Modes (WGM’s). When two such spheres are near each other, the WGM’s of the sphere are coherently coupled in a way analogous to the hybridization of atomic orbitals in molecules. We found that the light induced force can be enhanced by orders of magnitude at the resonances of the sphere cluster and that this resonant light force may lead to stable binding.