Lasers set micro-objects in a spin

17 Jun 2002

A new laser-trapping technique moves and rotates microscopic objects.

UK scientists have developed a new laser-trapping technique to rotate micron-sized particles. By trapping objects within the interference pattern of a helix-shaped "Laguerre-Gaussian" (LG) laser and a plane wave, Kishan Dholakia and colleagues from St Andrews University, Scotland, were able to rotate objects ranging from microscopic silica balls and glass rods to a hamster chromosome.

"Previous methods have proven useful in specific applications, but have serious shortcomings for general applications in rotating optical microcomponents and realizing optical micromachines", report the research group in Science 292 912-914.

The group's latest development builds on the well established "optical tweezers" technique. Here, scientists harness the optical-gradient force by directing a focused laser at a particle. Differences in the refractive index of the particle and its surroundings force the particle into the region of highest light intensity within the beam. However, Dholakia and colleagues combined two specialized lasers to form the helix-shaped LG laser and found that by manipulating the orientation of one of the light beams, they could "spiral" the helix laser and the trapped particle with it.

"The beauty of our technique is that we can dictate how far we want the spiral pattern to go round and at what speed," Dholakia said. "This means that we can fully control the rotation of that one particle."

The group believes that their technique could be used to drive cogs in micromachines and even in biological machines that might function in living cells. "We have only just begun to realize the possibilities of what we can do with this technology," added Dholakia.