A team at St. Andrews University has demonstrated a variant of optical tweezers that can cause micron-size objects to rotate. The technique could be used to drive miniature cogs and motors in micromachines or to orient DNA strands for polymerase chain reaction amplification.

The researchers, who reported their findings in the May 4 issue of Science, split the laser beam with a holographic element to create an interferometer that generated an intensity pattern at the focus of the tweezers when the legs were recombined. By tilting a glass plate that they inserted in one arm of the interferometer, they could alter the pattern, creating a rotating spiral. Objects inserted into the pattern, including silica microspheres, a glass rod and a Chinese hamster chromosome, rotated at controlled speeds exceeding 5 Hz.

The team used a 300-mW Nd:YVO₄ laser in the experiments, which produced 13 mW in the interference pattern at the focus. Rotation rates of up to hundreds of hertz are possible with an optimized setup.