Laser Technology: Changing the Orbital Angular Momentum of Laser Beams
Nature Photonics published research by a team from South Africa and Italy demonstrating a new type of laser that is able to produce laser beams ‘with a twist’ as its output. These so-called vector vortex beams are represented on a higher-order Poincare sphere. Using geometric phase inside lasers for the first time, the work opens the way to novel lasers for optical communication, laser machining and medicine.
The idea was conceived by Prof. 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 CSIR. The team members of 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. The custom geometric phase optics, without which the realisation of the idea would not be possible, was produced by the Italian team from the University of Naples, Prof. Lorenzo Marrucci and Prof. Bruno Piccirillo.
Angular momentum is a familiar concept in our everyday lives: the spinning Earth carries spin angular momentum while the orbiting Earth carries orbital angular momentum (OAM). Light can also carry angular momentum: through its polarisation (spin), and through its pattern and phase (OAM). 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 carrying OAM is created by twisting the phase of light into a helical shape, forming a spiral. 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. 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. 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.
Darryl Naidoo, Filippus S.
Roux, Angela Dudley, Igor Litvin, Bruno Piccirillo, Lorenzo Marrucci & Andrew Forbes: Controlled generation of higher-order Poincaré sphere beams from a laser, Nature Photonics (2016) doi: 10.1038/nphoton.2016.37