An Optical Conveyor Belt

An optical conveyor belt for moving sub-micron objects has been achieved by collaborating physicists at the Institute of Scientific Instruments in Brno, Czech Republic and at the University of St. Andrews in Scotland. Their set-up used a special type of non-diffracting laser light that forms a very narrow beam existing over long distance without changing its width.

Two such counter-propagating laser beams establish up a lace-like standing wave pattern which can suspend and hold tiny polystyrene spheres of just the right size. The balls, which range in size from 400 nm to one micron, have a density comparable to water. Previously, scientists have used such non-diffracting "optical lace" beams to move particles with the force of radiation pressure, but without the ability to stop them using only a single beam.

The Czech and Scottish researchers, by contrast, set up a light lace pattern with numerous knots, corresponding to intensity maxima (antinodes) of the standing wave. Furthermore a particle can be confined near a knot and all the knots can then be moved simultaneously over large distances by changing the relative phases of the counter-propagating laser beams.

Moreover thanks to the self-healing property of the non-diffracting beams, many particles can be confined simultaneously in the standing wave structure (near the knots) without significantly spoiling the beam properties. The positioning accuracy, related to the precision of the phase shift and the optical trap depth (the size of the knots), is at the micron level and will get better.

Pavel Zemanek (zemanek@isibrno.cz) says that possible applications for his device include the delivery of biological or colloidal microparticles or even ultracold atoms. (Cizmar et al., Applied Physics Letters, 25 April 2005; lab site at http://www.isibrno.cz/omitec/index.php?swt.html ) (A few years we wrote about a different kind of photon conveyor belt: http://www.aip.org/pnu/1997/split/pnu321-1.htm )