Particle clearance with Airy light beams

It has been likened to an optically driven snowblower for microscopic particles and could see practical use for the sorting of micrometre sized particles. Researchers from the University of Saint Andrews in the UK have reported on their work using Airy beams for particle clearing, in an advance online publication for Nature Photonics^†.

Micro-manipulation techniques which use light to control the movement of microscopic particles, either for the purposes of selection or processing, include optical tweezers and light patterning. These use optical force gradients to trap or steer the particles, with the motion typically confined to a two dimensional plane, either by steering a laser beam or using complex light patterns. This new technique, which uses an Airy beam, offers researchers another tool to work with, which is capable of guiding particles in the direction of the beam path.

Airy beams are produced by modulating the phase across the beam front of a laser in such a way that the components of the beam front interfere and construct the profile of a beam that appears to curve as it propagates out from the source. The result is that particles trapped and guided within the light beam follow a parabolic trajectory for a finite distance.

For the experiments, the researchers used an Argon ion laser (Ar⁺) operating at a wavelength of 514 nm, and a spatial light modulator to produce the Airy beam, which measured approximately 10 μm across and had a power of around 25 mW. The beam was projected upwards through a sample chamber containing colloidal glass spheres, 1.5 μm in diameter, in an aqueous solution. The beam was found to propagate about 75 μm before the gradient forces became too weak to trap the glass particles, at which point they would fall out under the influence of gravity.

Clearing from green zone to blue (a) and then reversing the field to clear from blue to green (b).
[Reprinted by permission from Macmillan Publishers Ltd: Nature Photonics, advance online publication, 28 Sep 2008 (doi: 110.1038/nphoton.2008.201)]

The results from one of the "snowblowing" experiments are shown in the adjacent figure. Each frame shows a sample of colloids with the shape of the Airy beam, which was projected upwards out of the page, shown superimposed in white. To help illustrate the effect, each frame has been split into four quadrants, each with a different colour. Frame A shows the result after the beam was on for 2 minutes and the white arrow points in the direction along which the particles were moved. It is evident that a greater number of particles appear in the blue quadrant than the green. Frame B shows the result after having reversed the direction of flow, by rotating the Airy beam 180°. The beam was on for only 1 minute this time, and the result was that a greater number of particles appeared in the green quadrant and less appeared in the blue. In both frames, the number of particles in the red and yellow quadrants did not appear substantially altered, although there are signs that the beam had some influence at the edges closest to the light beam, resulting in particle diffusion from overfilled regions, or clearing due to particles being dragged into the beam. A more quantitative analysis of these results was presented in the Nature Photonics paper, supporting the conclusion that the Airy beam was ejecting particles from one region into another.

The researchers went on to numerically model the process and discovered that the Airy beam exhibited self-healing properties, which explain the movement of particles caught in its path. When a particle is introduced into the beam path, it has the effect of both absorbing and scattering the light, which disrupts the intensity pattern of the Airy beam. The beam, however, reconstructs itself after a short distance known as the healing length, so the beam path and the motion of particles within it remain unaltered.

This is the first demonstration of Airy beams being used for particle manipulation. The optical snowblower could become an important tool in micro-fluidic sample processing, for example, where microscopic particles need to be selected and steered along given paths, or ejected altogether from the sample preparation area.

† Jörg Baumgartl, Michael Mazilu, Kishan Dholakia (2008). Optically mediated particle clearing using Airy wavepackets Nature Photonics DOI: 10.1038/nphoton.2008.201