Rod of light traps particles

17 Jun 2002

A non-diffracting light beam has been used for the optical trapping and manipulation of particles.

Researchers from the University of St Andrews, UK, have used a non-diffracting laser beam to manipulate dielectric and biological particles. Kishan Dholakia and co-workers believe that this is the first demonstration of particle manipulation with a Bessel light beam (Opt. Comm. 197 239).

The center of a Bessel beam does not spread out and acts like a rod of light in which particles can be captured. "A variety of particles were used, including silica spheres, elongated glass fragments, chromosomes and E. coli bacteria cells," said Dholakia.

The researchers initially manipulated and tweezed a variety of particles in two dimensions. These included hamster chromosomes, E. coli bacteria cells and 5 µm-diameter silica spheres. The group then managed to show further manipulation by stacking, aligning and guiding these particles.

The central maximum of a Bessel beam was used to capture and stack particles one above another. Dholakia explains: "One particle is tweezed in the central portion of the beam. You then translate the beam and hovering particle over another particle (at a lower level) and it too gets pushed up into the beam just below the first particle. We have been able to stack a total of nine particles in this manner." Higher laser powers, typically greater than 60mW, were required compared to the 35mW used in the initial tweezing experiments.

The Bessel beam was also used to align elongated particles. E. coli bacteria and 50 µm long fragments of glass originally in the horizontal plane were rotated upright within the non-diffracting central maximum, aligning with the light intensity profile.

Finally, the researchers used a Bessel beam as an optical guide for particles. Spheres with dimensions of 1 µm were guided vertically upwards over a distance of 1 mm, the full height of the glass microscope slide in which the sample was held.

"We believe this method of optical manipulation will find several interdisciplinary uses including optical micromachines, colloid research and biological studies," said Dholakia.

Optical tweezers make use of the reflection and refraction of light at a dielectric surface. Light possesses momentum. An interaction between light and matter can result in an exchange of momentum. In the case of the linear momentum of light, this can be used to trap microscopic particles. If a dielectric sphere is placed in a focused light beam, it is drawn into the region of highest light intensity due to the change in momentum it experiences.

The Bessel beam was generated using a conical glass element known as an axicon. A standard Gaussian beam was incident on the axicon and the emergent beam consisted of light and dark rings, with a light central maximum. The non-diffracting centre had a propagation distance of a few millimeters, approximately 40 times the Rayleigh range of a Gaussian beam. This feature offered enhanced optical guiding, which was previously limited to the Rayleigh range of the Gaussian beam.