"Our novelty was to realise that by using custom-geometric phase optics to map polarisation to OAM, the laser could be designed to tell the difference between the clockwise and anticlockwise light," says Forbes. Moreover, combining spin and orbital components to produce general beams from a single laser that are mixtures of the two momenta, have not been demonstrated before. The problem is that usually lasers cannot tell the difference between light that is twisted clockwise and light that is twisted anti-clockwise, and so the laser simply gives a combination of both in an uncontrolled manner. Because the twisting of the pattern gets tighter and tighter as you move towards the centre of the beam, the light disappears and such beams are often called doughnut beams or vortex beams.
Light carrying OAM is created by twisting the phase of light into a helical shape, forming a spiral. Producing light with a controlled spin in a laser has been known for decades, but producing OAM beams inside a laser is not so simple. Light can also carry angular momentum: through its polarisation (spin), and through its pattern and phase OAM," says Forbes. "We are all familiar with angular momentum in our everyday lives: the spinning Earth carries spin angular momentum while the orbiting Earth carries orbital angular momentum (OAM). The custom geometric phase optics, without which the realisation of the idea would not be possible, were produced by the Italian team from the University of Naples, Prof. Stef Roux (Wits and the CSIR), Dr Angela Dudley (Wits and the CSIR) and Dr Igor Litvin (CSIR) all contributed significantly to the work.
Andrew Forbes of the University of the Witwatersrand (Wits), who also led the collaboration while all the key experiments were performed by Dr Darryl Naidoo of the Council for Scientific and Industrial Research (CSIR).
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