Optics

The light fantastic

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Optical tweezers use light to manipulate tiny particles — but only one at a time. If the light in the tweezers is a 'Bessel beam', this problem can be overcome, creating some interesting experimental possibilities.

Optical tweezers have become a standard tool in many areas of science, such as colloid research and biological studies. On page 145 of this issue, Garcés-Chávez et al.1 report an extension of this technique that makes it possible to manipulate ensembles of particles simultaneously.

Laser light can exert force on dielectric (polarizable) particles through radiation pressure and refraction2: light has momentum, and so light and matter can interact by exchanging momentum. Although these forces are small (of the order of 10−12 newtons), they are sufficient to trap and manipulate micrometre-sized particles in a liquid environment3. So far, optical manipulation of particles in different places at the same time has been possible only if the particles are in the same optical plane of view4. Otherwise, the problem is that the manipulating light beam will diverge around the first trapped particles, and then cannot be brought back to a focus within the short distance to the next set of particles. So experiments must be done in series, and this is very time-consuming. To speed things up, it would be better to work in parallel, manipulating particles held in multiple back-to-back compartments (such as a collection of biological-cell samples) at the same time with a single light beam.

Using a special kind of optical focusing to create a 'Bessel beam' of light, Garcés-Chávez et al.1 now provide the first demonstration of the manipulation of micrometre-sized particles in multiple planes. A Bessel beam (Fig. 1) is so called because the variation of its intensity follows the mathematical pattern known as a zero-order Bessel function. Thanks to its special optical properties, the bright central spot of the beam can re-form after passing an obstruction; the beam does not interfere with the obstruction and re-forms fully only a few micrometres further on. Garcés-Chávez et al. show that even when a Bessel beam of light is partially obstructed by an object in the first fluid-filled compartment, particles in a second, spatially separated, fluid compartment can be manipulated using the same light beam and with the same precision. This has been impossible with the conventional optical manipulation techniques used so far.

Figure 1: A Bessel beam.
figure1

Light passing through a particular scheme of lenses can form a beam of light with a bright central spot and concentric rings of decreasing intensity — known as a Bessel beam, as its varying intensity is described by the zero-order Bessel function shown here. Garcés-Chávez et al.1 have used a Bessel beam to create optical tweezers capable of manipulating many samples at once.

The authors created a diffraction-free Bessel beam5 using simple lenses and a special conical lens called an axicon. The bright spot at the beam's centre is a 'non-diffracting' focal line of light and is distinctly different from 'normal' optical trapping beams. The narrow beam has a propagation distance of a few millimetres, which is about 40 times the Rayleigh range (that is, the distance along which particles can be guided) of a normal, or 'gaussian', beam, and this means that optical manipulation can be done in multiple back-to-back compartments at the same time.

The Bessel beam can be imagined as a single rod of light, pushing the trapped particles along its axis. The trapping can extend over a few millimetres, and particles can be held firmly in two dimensions. In contrast, a gaussian beam can hold particles in a stable, three-dimensional configuration6,7, but the beam diverges quickly, so particles can only be guided for a few micrometres along the axis of such a beam.

Certainly, one of the most impressive applications of (gaussian) optical tweezers is in the study of molecular motors8,9 and polymer mechanics10. These experiments have provided fresh insight into biochemical processes at the single-molecule level and are of great relevance in biology. The key to such experiments is in immobilizing the biological molecule of interest on the surface of a bead and attaching its interacting partner molecule to a second bead, which can then be trapped in three dimensions using infrared laser light6. But particles or beads caught in a Bessel beam are constantly pushed along the beam axis, and so comparable studies using Bessel-beam tweezers are not yet possible.

It is foreseeable, however, that if a counter-propagating Bessel beam were applied, the particles' forward motion could be stopped6. Like balancing a rod on the tip of another rod, a stable three-dimensional trap between the beams could be established if the power of the beams were adjusted properly. Extending this idea to exploit the Bessel-beam tweezers' ability to work on many particles simultaneously over considerable distance, patterns could be created from interfering Bessel beams, forming arrays of light spots that could be used, for example, to rotate microsystems, or to control lab-on-a-chip microstructures.

In biology, Bessel-beam tweezers could be used to sort the oval-shaped particles produced in the micro-dissection of chromatin (made up of DNA and proteins) through tiny openings or pores. For cell sorting, these tweezers are capable of more accurate guiding than systems used so far11. The Bessel-beam technique will no doubt find application across a broad range of science.

References

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Correspondence to Martin Hegner.

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Hegner, M. The light fantastic. Nature 419, 125–127 (2002) doi:10.1038/419125a

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