Optical Trapping with Integrated Near-Field Apertures

In recent years, optical micromachines based on forces exerted by strongly focused beams of light have started to provide unprecedented access to nonintrusive measurement and manipulation of matter on submicron length scales. However, the sharpness of the present optical tweezers is restricted by the spatial gradients of light attainable with diffraction-limited optics. Here, we demonstrate a technique for optical trapping of single particles in a fluid, which is based on the intense near-field gradients around small apertures in a metal film. Our scheme should be able to trap smaller particles with a fraction of the laser intensity required by conventional optical tweezers. Detailed simulations of the electromagnetic fields near apertures and the resulting forces they can produce are described. We also present a proof-of-principle experiment in which the trapping of latex beads is demonstrated by following the time evolution of their fluorescence. Our scheme allows containment of particles in free solution in nanometer sized “beakers ” opening the way for research on single nanoparticle chemistry, microrheology in confined environments, and ultrahigh sensitivity sensors