Optical Trapping Expands to 3-D

Since the mid-80s, optical tweezers have helped study the physics behind a myriad of microscopic phenomena ranging from DNA elasticity to micromachines. However, the effectively planar nature of the optical trapping effect -- a clear object being pushed toward the focal point of a laser beam -- has limited the vast majority of the investigations to two-dimensional force evaluations perpendicular to the laser beam.

Researchers from Instituto Nacional de Astrofisica in Puebla, Mexico, and St. Andrews University and Edinburgh University, both in the UK, have found that the interference pattern created by two Laguerre-Gaussian light beams can be used in an optical tweezers setup to create multiple trapping sites within the interference pattern. As reported in the May 10 issue of Science, they used the pattern to stack as many as 16 microscopic spheres in each of the sites, thus creating 3-D structures. By changing the frequency of one of the beams, they induced an angular Doppler effect that rotated the interference pattern, resulting in a rotation of the stack.

Using these techniques, the researchers also have constructed asymmetrical 3-D structures of spheres, which they believe will be used someday to create predetermined nuclei for novel, self-assembled crystalline structures.