Phys. Rev. Lett. 109, 163903 (2012)
Optical tractor beams pull illuminated material in a direction along the beam, towards the light source. Such upstream transportation has been realized with solenoidal waves, although it is accompanied by circular motion, which is not always desirable. David Ruffner and David Grier from New York University in the USA have now experimentally demonstrated active optical tractor beams that can push or pull micrometre-scale objects with purely linear motion. The researchers formed an 'optical conveyor belt' by superimposing several coherent co-axial Bessel beams to create a series of periodic optical traps along the beams' common axis. By systematically varying their axial wave numbers and relative phase, the researchers were able to transport a trapped particle in either direction along the axis. The beams were formed by imprinting an appropriate phase profile onto the wavefronts of a Gaussian beam using a computer-addressable spatial light modulator. A linearly polarized bean with a wavelength of 532 nm and an estimated power of 17 mW was sent through a 100x high-numerical-aperture oil-immersion objective lens into a sample of 1.5-μm-diameter colloidal silica particles dispersed in water. The researchers observed both up- and downstream particle motion, moving a distance of around 30 μm in 80 s. Numerical analysis suggests that the simultaneous manipulation of multiple objects and the transportation of irregularly shaped objects or objects with inhomogeneous optical properties should also be possible.
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