An optical conveyor belt

An optical conveyor belt for transporting submicron objects has been devised by collaborating physicists from the Institute of Scientific Instruments in Brno, Czech Republic, and the University of St. Andrews in Scotland. Rather than using a laser beam with a Gaussian profile, their setup had two counter-propagating nondiffracting beams with profiles described by a zero-order Bessel function. The beams established a standing-wave pattern in which steep changes in optical intensity could confine small particles. Unlike in an optical tweezer, however, the relative phases of the beams could be controlled to march the particles along the length of the beam while keeping them not only confined laterally but also trapped longitudinally in intensity maxima or minima. The “self-healing” property of the nondiffracting beams means that many particles can be confined simultaneously in the standing-wave structure because their presence does not degrade the beam. The figure shows a pair of 410-nm-diameter polystyrene spheres transported over a distance of a quarter of a millimeter in under three seconds. (The stacked images were acquired in 0.25 s increments.) The positioning accuracy—currently at the micron level—is related to the precision of the phase shift and the optical trap depth and will get better, according to Pavel Zemánek, the group’s leader. (T. Čižmár et al., Appl. Phys. Lett. 86, 174101, 2005 .)