What the..? The world of optical trapping would look quite different if we were shrunk to the size of a photon.
Picture this: You are standing on the edge of a chasm that rends the otherwise featureless plain surrounding you asunder. It's not particularly wide - you can see the other side - but, like a terrifying crevasse in glacial ice, you can't see the bottom for its dizzying height. Suddenly, a huge beam of light erupts from these murky depths. Emerging from the abyss, riding the beam of light, is a giant glass sphere. It's coming for you quicker and quicker and, just as you think it's about to fly into space it stops dead in its tracks and just hovers in front of you. Then it starts dancing…
The description above sounds like something from a stark science fiction vision of some alien planet, flung far into the future and the distant reaches of the universe. Yet it's happening right here on Earth.
"Ease up on the peyote," you may be thinking. "There's no such thing as a tractor beam that can suspend giant glass spheres at great heights!" Well if in the above scenario you are the size of a photon (1/100th the width of human hair) that is exactly what you would see in a new device that utilises a thirty-year old phenomenon to use light to control light.
I work at the CUDOS, the Centre of Excellence for Ultrahigh-bandwidth Devices for Optical Systems at the University of Sydney. AS you'd suspect, we're interested in light: specifically we are interested in controlling light and making it go places and do things that it might not otherwise do.
I am part of a team that built the device described above that uses a light beam to levitate and hold a glass sphere 1/5th the width of a human hair. Why would I want to do this? Because the glass sphere acts like a lens and, once it is held firmly by the beam of light, I can move it around by changing the position of light beam that is holding it. The focusing properties of the sphere mean I can image parts of the micro world and, further, I can manipulate beams of light in this micro world (other than the one holding the sphere) all without actually touching anything. Light controlling light…
The phenomenon I'm utilising to levitate and control the sphere is called 'optical trapping' and was discovered in 1976 by Arthur Ashkin from Bell Laboratories.
An optical trap is basically a laser beam, strongly focused using a microscope lens. The shape of a focused beam is kind of like an hour-glass: as it emerges from the microscope lens, it shrinks down to its smallest point (the beam waist) after which it expands again.
When a small particle, like our glass sphere, gets near the beam it feels a force pulling it into the beam waist. Once there, these same forces hold the sphere in a stable way, meaning that if the sphere tries to escape it is pulled back into the waist of the beam - just like the tractor beams from science fiction.
Once the sphere is held in the optical trap, it is a simple matter to move it around. All I have to do is change the position of the trapping beam waist and the glass sphere obediently follows.
So now I have a levitating glass sphere at my command, what shall I do with it? I mentioned that at CUDOS we like controlling light. So what I do is bring my sphere near the end of an optical fibre: long thin glass 'wires' used for transporting light. The beam of light coming out of the end of the fibre is spreading rapidly. Remember that the glass sphere acts like a lens? With the sphere positioned on the end of the optical fiber the beam is now brought to a focus. Furthermore, I can make the sphere "bob" from side to side and in doing so I can control where the focused light from the optical fiber lands. Light controlling light…
The idea of using optical traps to manipulate things in the micro world is very powerful. Already scientists routinely use this technique to manipulate cells, sort them, hold them, rotate them even perform surgery on them, all without touching them. I hope that this demonstration will lead to further work where independently trapped objects will interact on a microchip, performing a large number of functions combining light, electronics and maybe even biological matter and fluids. The sky is the limit when you're able to levitate.
Peter Domachuk is a researcher at the Centre for Ultrahigh bandwidth Devices for Optical Systems (CUDOS) in Sydney. CUDOS is a collaborative project between the University of Sydney, the Australian National University in Canberra, Macquarie University, Melbourne's Swinburne University and the University of Technology, Sydney. This story won first prize in a CUDOS writing competition.
