![\"david\"](\"http:\/\/physics.as.nyu.edu\/props\/IO\/2774\/42\/grier.david.jpg\")
<\/p>\nProfessor David Grier and David Ruffner (graduate student), working at the Department of Physics and Centre for Soft Matter Research have published a paper announcing that they have experimentally demonstrated a class of tractor beams[1]. They were able to move microscopic spheres of silica suspended in water over distances of about 30 micrometres.<\/p>\n
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David G. Grier<\/em><\/p>\n The definition of a tractor beam (quoted from the paper) is: A tractor beam is a traveling wave that can transport illuminated material along its length back to its source<\/em>.<\/p>\n By this definition things like optical tweezers<\/a> or optical conveyor belts<\/a> are not technically tractor beams.<\/p>\n The problem<\/strong><\/p>\n If you think about it, in order for a particle to be pulled by a beam of light, rather than pushed, it has to redirect the momentum of enough photons to overcome the force due to the radiation pressure of the beam. <\/p>\n This can happen if the intensity of light beam changes sufficiently along the axis of the beam. For example, one can use a beam with tightly focused areas.<\/p>\n The method<\/strong><\/p>\n The tractor beam demonstrated by Ruffner and Grier uses a special kind of laser beam called a Bessel beam laser<\/a>– the amplitude is given by a Bessel function of the first kind. A perfect Bessel beam would not diffract and spread out as it propagates. (For those that are interested, there is a lot of classical mathematical physics behind Bessel functions) <\/p>\n The team produced two Bessel beams side by side using a lens to angle them so they overlapped. Then by varying the relative phase<\/a> of the two beams, the particle becomes trapped in an “optical conveyor” which allows the particle to be transported in three dimensions.<\/p>\n Cross-section of the Bessel beam and graph of intensity<\/em><\/p>\n Applications<\/strong><\/p>\n No-one is about to start pulling space craft about using this technique. However, it may find applications in biology rather quickly. <\/p>\n Reference<\/strong><\/p>\n [1] Optical conveyors: A class of active tractor beams, D. B. Ruffner and D. G. Grier, Physical Review Letters<\/em>, in press (2012) (download<\/a>)<\/p>\n Link<\/strong><\/p>\n David Grier’s Home Page<\/a><\/p>\n","protected":false},"excerpt":{"rendered":" Professor David Grier and David Ruffner (graduate student), working at the Department of Physics and Centre for Soft Matter Research have published a paper announcing that they have experimentally demonstrated a class of tractor beams[1]. They were able to move microscopic spheres of silica suspended in water over distances of about 30 micrometres. David G. … Continue reading One step closer to a Star Trek tractor beam<\/span> ![\"beam\"](\"http:\/\/upload.wikimedia.org\/wikipedia\/commons\/thumb\/a\/ac\/Bessek_beam_intensity.svg\/711px-Bessek_beam_intensity.svg.png\")
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