Light-induced forces on small particles

Light-trapping is generally associated with the phenomenon that small particles are driven toward the intensity maxima of a carefully sculpted laser beam. There is another type of inter-particle optical force arising from the coherent multiple-scattering of light between the particles, previously observed in various experiments. This thesis is mainly devoted to the theoretical study of such optical binding force for a cluster of dielectric particles. In contrast to the intensity-driven light-trapping, the optical binding force is present even when a plane wave (with homogeneous intensity) is incident upon a collection of microparticles.; Firstly, the theory of optical force for neutral dielectric Rayleigh particles is presented. We then show through rigorous calculations that the optical binding force (with the incident intensity ∼106 W/cm2) can dominate other interactions and bind dielectric microspheres in stable structures that behave like "molecules." Such photonic clusters can exhibit a multiplicity of static and drifting equilibrium configurations, with some having remarkable geometries such as a quasicrystal-like arrangement. The photonic clusters exhibit exotic dynamics, and the equilibrium configurations can correspond with either stable or a type of quasi-stable states in which the cluster maintains an average shape, with individual particles executing periodic motion in the presence of frictional dissipation. Photonic clusters consist of Rayleigh particles are also investigated. A stable one-dimensional lattice is found and analyzed, localized vibration modes are observed.; Finally, we consider an interesting type of resonant inter-particle optical force. We shall see that tuning of the incident light's frequency to the morphology-dependent resonances of a cluster of transparent microspheres induces a strong, resonant optical force between the spheres. The resonant force can be enhanced by orders of magnitude so that it dominates other interactions (at a modest incident intensity of ∼104 W/cm2). We also proposed various ways to utilize the resonant force in the binding of a microsphere cluster.