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Tweezer relies on self-healing beam

12 Sep 2002

Scientists that have made a new type of optical tweezer using a self-healing light beam reveal their results in today's Nature.

Scottish scientists have created an optical tweezer that can simultaneously grab particles in separate locations and manipulate all the trapped objects in unison.

The tweezer is the first to exploit a non-diffracting light beam, called a Bessel beam, which "self-heals" itself after encountering an obstruction such as a trapped particle. This feature ensures that the beam can continue trapping at several locations along its length. (Nature 419 145)

The team from St Andrews University, UK, say this result, which could not have been achieved with a standard optical tweezer, will lead to advanced micromachining and control of arrays of lab-on-a-chip devices.

A Bessel beam is made up of many wavefronts arranged on a cone. When all the wavefronts interfere, this leads to a bright central maximum surrounded by a series of concentric bright and dark rings.

"When we distort the central maximum by trapping a particle in it, this creates a shadow," explains group leader Kishan Dholakia. "But because the wavefronts are on a cone, the outer parts of the beam pass the particle unhindered and reconstruct the central maximum some distance from the original particle. This new maximum can trap more particles."

Dholakia and colleagues use a Bessel beam in two different tweezer geometries. In the first instance, they pass the beam upward through the sample cells. "This allows us to levitate, align, stack and guide particles," Dholakia told Optics.org.

Using this setup, the researchers have trapped a low-refractive index particle in one cell. This particle blocks the beam, which then reforms, allowing three 5 micron spheres to be stacked in a cell some 3 mm above the lower cell.

They have also aligned 8 micron glass fragments in the bottom layer whilst simultaneously trapping chromosomes in the top layer.

In the second configuration, the beam is past downward through the sample cells. "Objects are trapped in the rings surrounding the maximum forming 2D arrays of particles in each cell," says Dholakia.

This method allows three sample cells to form one above the other. Dholakia says his team can trap different particles in all three traps simultaneously and manipulate them.

The group is now working towards rotating particles in individual cells and use of other non-diffracting light patterns. "We also believe we can get the sample cell separation distance up to 10 mm," said Dholakia.

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

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