ScienceWise - Spring 2010

Tractor-beam one step closer to reality

New optical vortex pipeline transports matter

If you put your hand into a shaft of sunlight, you can easily feel its warming effect and anyone who’s been foolish enough to put their finger into a powerful laser beam will be very aware of how much heat electromagnetic radiation can generate. Such radiative heating not only warms any object in the light path, it also warms any gas, such as air, that’s in contact with that object. The air molecules move a little faster when warmed and the increased force of their impacts with the surface of the object imparts a tiny thrust to it. This is known as photophoretic force.

It’s a tiny force that you could never hope to feel on something as massive as your hand, but if the object is light enough, it can have a very noticeable effect. The famous Crooke’s radiometer, a partially evacuated glass vessel with alternate silver and black vanes, operates on just this principle. When sunlight strikes the blackened sides of the vanes the air is heated slightly compared to the silver opposite side and the differential force makes the vanes spin round.
Although this phenomena has been known for over a century, it has recently found a new application in the world of nanotechnology.  A group of scientists at the Australian National University have developed a system for the manipulation of small particles in air using a sophisticated optical vortex.

As its name suggests, an optical vortex is a beam of light that propagates as a very fast and tight spiral about a central axis. One of the properties of such vortices is that in the centre, the beams destructively interfere creating a dark core. If you were to project such a beam onto a piece of paper, you would see a ring of light with a dark centre.
If a very small particle is trapped in this dark core, interesting things begin to happen. As gravity, air currents and random motions of air molecules around the particle push it out of centre, one side becomes illuminated by the laser whilst the other lies in darkness. This creates a small photophoretic force that effectively pushes the particle back into the darkened core. The net result is that any particle in the vortex is pushed towards the dark core. In addition to the trapping effect, a portion of the energy from the beam and the resulting photophoretic force, pushes the particle along the hollow laser pipe.

If you replace the single vortex with two that are concentric but propagate in opposite directions, it becomes possible to move the particle back and forth along the pipeline by adjusting the brightness of either vortex.

The choice of particle to move is also important to some extent though the system will manipulate almost any fine particle in almost any gas. “Ideally you want a surface that absorbs as much radiation as possible, like the black side of the vanes in a Crook’s radiometer,” Professor Andrei Rode explains, “and you also need something light and with low thermal conductivity so the local heating stays local. We’ve done a lot of work in the past with carbon nanofoam, an agglomerate of carbon nano-particles. This material has excellent properties in all these areas, so it was our first choice when setting up the experiment.”

The initial results were quite spectacular. The nanofoam particles remained securely trapped within the vortex whilst they were transported the full length of the optical bench. The system allows the manipulation of these particles with a few microns accuracy whilst being transported over distances of a meter or more. That’s the same precision as being able to throw a rock from Sydney and land it in a skip in Canberra.

Although the nanofoam performed extremely well, working with alternative particles widens the potential applications and has other scientific benefits.

“The physical properties of nanofoam are ideal for this experiment but because it is irregularly shaped, it makes development of a good theoretical model quite difficult.” Professor Rode says, “And we really need an exact mathematical treatment of this process if we’re going to unlock its full potential.”

To achieve this, the researchers decided to use another type of particle, a tiny hollow glass sphere about one tenth of a millimetre across. However because the technique relies on absorption of laser light and local heating of the air around the particle, to work really well, the transparent glass had to be first coated with a thin layer of graphite.

The size of the coated glass shell can be relatively easily measured using an electron microscope but working out its mass is a little more complicated as there are no effective balances for measuring the weight of things this small. So having completed a series of measurements on a given particle, the researchers broke it and measured the wall thickness again with an electron microscope. From this they were able to calculate the volume of glass and hence the mass.

There are a number of practical applications for this technology such as micro manipulation of objects, sampling of atmospheric aerosols, and low contamination, non touch handling of samples. But Professor Rode believes it’s hard to predict all the potential applications for any new technology. He tells a story about the Lebedev Physics Institute of the Russian Academy of Sciences, where he completed his PhD.

“There was a framed page from a PhD thesis in the library at Lebedev Physical Institute in Moscow, that of Nikoly Basov. Basov’s thesis was on a very early form of laser and a very eminent scientist, Giuzburd who was one of the examiners had written, ‘this work is very interesting but I see no practical application for it.’ Of course everyone knows how important lasers are today and Basov ultimately received the Nobel prize for his work, but this shows how difficult it can be to envisage what any new technology will bring to the world.”

This work is supported by a grant from the National Health and Medical Research Council

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