Optical Conveyor Belt Transports Submicron Objects

Daniel S. Burgess

Investigators at the Academy of Sciences of the Czech Republic in Brno and at the University of St. Andrews in the UK have used a sliding standing wave formed by counterpropagating Bessel beams to trap and move series of tiny polystyrene beads. The optical conveyor belt, they suggest, may be used to precisely deliver a variety of microscale biological and colloidal particles.

Using a sliding standing wave formed by counterpropagating Bessel beams, researchers trapped and transported submicron particles — in this example, a series of six 410-nm-diameter polystyrene beads. Courtesy of Pavel Zemánek, Academy of Sciences of the Czech Republic.

The setup splits the linearly polarized output of a 10-W ytterbium-doped fiber laser operating at 1.070 μm. Axicons transform the resulting p- and s-polarized beams into Bessel beams, which interfere in a 5-mm-long sample cell filled with heavy water to form a standing wave. Because one beam reflects from a mirror connected to a stepper motor, a phase shift can be introduced to slide the structure of the standing wave as desired.

The use of Bessel beams enables the simultaneous handling of several particles over relatively large distances (see “Bessel Tweezers Manipulate Multiple Objects,” Photonics Spectra, October 2002, page 19). The central maximum of such beams re-forms beyond an obstruction, so longitudinally aligned objects can be trapped in the sliding standing wave and transported along with it.

In the experiments, the researchers used the optical conveyor belt to move polystyrene beads 410 to 1000 nm in diameter more than 250 μm. They found that the 490- and 800-nm-diameter beads tended to jump between traps in the standing wave and to remain confined for shorter periods than did beads of other sizes, consistent with theoretical predictions.

Their calculations suggest that beads of certain sizes — specifically, 378 and 708 nm in diameter — would not be trapped at all, a phenomenon that could be exploited to optically sort objects by size.

Applied Physics Letters, April 25, 2005, 174101.