Light, all tied up in knots!

Physics

Light, the weightless, electromagnetic wave, which can yet behave as a particle, to buffet atoms into place, is proving versatile in the world of the very small. Dr Mark Dennis and colleagues at the University of Bristol report in Nature their progress in manipulating light, literally to “knot light waves like a shoelace.”

Light and leverage
A stream of water or a blast of air is a movement of physical things, the molecules of water or air, which have mass, can exert pressure and flow smoothly or with eddies and turbulence. But light is just a ‘push-pull’ of electric and magnetic effects, which are caused by and cause each other, rushing forward like radiating circles of water bobbing up and down in a pond where a stone has been thrown. But unlike the ripples, there is nothing material that moves in a light wave.

And yet, as electric and magnetic effects spread out, these can have effects at distant places and they carry away energy as they move. And, as energy and mass are equivalent, a light wave has momentum too, and exerts force when it is absorbed or is reflected. Recent discoveries about light say it behaves like a stream of air or water not only in pushing and eroding things that come in its way, but also in forming eddies and whirlpools, which could be designed in specific shapes, to manipulate tiny things in a sensitive way!

The light wave
In an ocean wave moving in some direction, water is not actually flowing in that direction; it is just a sequence of up-down movement of water that is seen to move. This kind of wave, where the action is not in the direction of the wave, is called a ‘transverse wave’.

Light is also this kind of wave, with the magnetic and electric effects zig-zagging, growing one way, then shrinking, growing the opposite way, shrinking again, and so on, transverse to the direction of the light beam. But there is a difference – the ocean wave is made of water, which has weight, and must move up-down, because of gravity – in the vertical plane. In the case of light, gravity has no importance and the electric and magnetic effects can ‘swing’ in any plane.   

Light and interference
Because light consists of waves, the waves can also be ‘in step’ and help each other or can be ‘out of step’ and cancel each other. We may have seen this in action at the sea shore, where there are waves coming in and waves going back. If both waves are ‘in step’ we get a large one, and if they are not, then the wave becomes a ‘dud’. The same thing happens with light and this is the reason that sharp images form, at the focus, where the waves reinforce, after passing through a lens.
It is this property of light, with the wavelength being very small, that helps us develop microscopes, to see very small objects. This is also the property that comes in the way of seeing things smaller than the wavelength of light, that light is able to simply ‘pass over’ such things, without forming images at all.

Hologram
This is also the property that helps capture detailed pictures of 3D objects, when a light source and the light bouncing off the objects are allowed to interfere. The complex pattern of light and dark areas then captures the entire shape of the object, which can be retrieved by viewing the light source through the interference pattern.
The only requirement is that the source should be a laser, which produces a consistent wave chain, like the ocean waves, and not a jumble of waves that emerge from ordinary sources, like an electric bulb or the sun. This interference pattern, when photographed and frozen, to recreate the original, is called a hologram.

The light vortex
With the help of a lens which has a gradually increasing, spiralling thickness, the slowing down of light as it traverses the lens describes a circle. A lens like this can be made so that light twists like a corkscrew and light at the axis is extinguished by interference. Such a lens will work for one frequency and will display an annulus of light, or a ring with a dark centre.

The character of light manipulated like this is that the electric effects are controlled and small electrically sensitive objects, even molecules can be moved at will. A focused laser beam itself was seen to draw such material to its centre, and devices based on this were known as ‘optical tweezers’. The development of vortices in light beams is a large step forward, as it enables more sensitive movement of nano particles with the help of controlled light beams.

While the development of glass or plastic lenses for complex vortex forms may be challenging, an alternative is to make use of holograms, to create optical vortices. The University of Briston group made use of an abstract form of mathematics called ‘knot theory’, a formal examination of knotty situations that arise when shoelaces get messed up. With this, they were able to generate holograms which resulted in very complex optical control.