The optical Earnshaw theorem states that a small particle cannot be trapped solely by scattering forces. This limitation is overcome in a novel differential all-optical manipulator. It utilizes four collimated laser beams arranged along the axes of a tetrahedron to confine and move a microscopic sample in an aqueous medium. By adjusting the intensity of each beam individually the magnitude and direction of the optical forces acting on the sample, and via these its position, are controlled. Since only scattering forces are exploited the system is not confined to trapping near a geometrical focus, and therefore enables three-dimensional manipulation over ultra-long working distances. Latex beads 20μm in diameter can be positioned arbitrarily within a volume defined by the overlap of the four 100μm diameter beams. The sample is observed from four directions simultaneously, demonstrating the instrument’s potential as a universal manipulator in connection with high- and isotropic-resolution light microscopy.
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