Spheres of a diameter of about 1 micrometer in a fluid are flowing from right to left in a fluidic channel (defined by the two yellow lines) that sits atop a chip with waveguides. The waveguides are about 800 nm wide. The light propagating from bottom to top in the waveguide (red) has an evanescent field which interacts with the particles as they flow near the waveguide, and this results in the trapping and forced motion from bottom to top of the particles by the optical field.