Colloidal crystallization induced by optical gradient forces exerted by optical tweezers

摘要

Optical tweezers were used to crystallize colloidal dispersions without manipulating colloids on a single particle level. Using "optical gradient" or "high-frequency dielectrophoretic" forces, we demonstrate control over local particle concentration. This control can be used to induce crystallization and melting in two- and three-dimensional colloidal dispersions using single beam gradient optical tweezers. In our setup, two microscope objectives (one above and one below the sample) allow independent three-dimensional manipulation and imaging of the structure formation inside the sample. We demonstrate crystallization near a wall for a range of particles sizes, refractive index contrasts and numerical apertures. In a colloidal mixture of tracer particles with a high refractive index core and low refractive index host spheres dispersed in a host refractive index matching solvent, control over the tracers can indirectly lead to controlled crystallization of the host particles.