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Optical tweezers make 3D structures

17 Jun 2002

Research published in today's Science reveals a simple method to manipulate 3D structures with light beams.

By interfering two laser beams, scientists have successfully created three-dimensional structures with optical tweezers for the first time. The behaviour of these structures will give researchers insights into photonic-bandgap materials, electron and atom transport and superconductivity. (Science 296 1101)

After stacking particles in a single column, Kishan Dholakia and colleagues from St Andrews University, UK, set about extending this effect. "Particles get drawn into the bright region, near the focus of the beam," explains Dholakia. "We realised if we had many, individual bright sites, essentially multiple tweezers, we could replicate this effect and make 3D structures such as cubes and cuboids."

To generate a pattern with several bright spots in which to trap particles, the team interferes two so-called "Laguerre-Gaussian" hollow light beams each having a characteristic annular form. The resulting pattern mimics two spirals, each in opposing directions, like a double helix.

The tweezer setup comprises an Nd:YAG laser emitting 200 mW continuous-wave at 1064nm. "Passing the beam through an interferometer, we can readily make 3D cubic structures from high-refractive index silica spheres," Dholakia told Optics.org. "We now want to create an extended crystalline structure and study its form using diffraction."

Dholakia says that studying the behaviour of the predetermined 3D structures will also find uses in colloid physics. "Observing the way other particle aggregate around this central structure under a variety of conditions is of industrial importance," he said.

Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.

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