Researchers have demonstrated the world’s first metasurface laser that produces “super-chiral light”: light with ultra-high angular momentum. The light from this laser can be used as a type of “optical spanner” to or for encoding information in optical communications.

“Because light can carry angular momentum, it means that this can be transferred to matter. The more angular momentum light carries, the more it can transfer. So you can think of light as an ‘optical spanner’,” Professor Andrew Forbes from the School of Physics at the University of the Witwatersrand (Wits) in Johannesburg, South Africa, who led the research. “Instead of using a physical spanner to twist things (like screwing nuts), you can now shine light on the nut and it will tighten itself.”

The new laser produces a new high purity “twisted light” not observed from lasers before, including the highest angular momentum reported from a laser. Simultaneously the researchers developed a nano-structured metasurface that has the largest phase gradient ever produced and allows for high power operation in a compact design. The implication is a world-first laser for producing exotic states of twisted structured light, on demand.

Nature Photonics today published online the research that was done as a collaboration between Wits and the Council for Scientific and Industrial Research (CSIR) in South Africa, Harvard University (USA), the National University of Singapore (Singapore), Vrije Universiteit Brussel (Belgium) and CNST – Fondazione Istituto Italiano di Tecnologia Via Giovanni Pascoli (Italy).

In their paper titled: High-purity orbital angular momentum states from a visible metasurface laser, the researchers demonstrate a new laser to produce any desired chiral state of light, with full control over both angular momentum (AM) components of light, the spin (polarisation) and orbital angular momentum (OAM) of light.

The laser design is made possible by the complete control offered by new nanometer-sized (1000 times smaller than the width of a human hair) metasurface – designed by the Harvard group – within the laser. The metasurface is made up of many tiny rods of nanomaterial, which alters the light as it passes through. The light passes through the metasurface many times, receiving a new twist everytime it does so.

“What makes it special is that to the light, the material has properties impossible to find in Nature, and so is called a “metamaterial” – a make-believe material. Because the structures were so small they appear only on the surface to make a metasurface.”

The result is the generation of new forms of chiral light not observed from lasers until now, and complete control of light’s chirality at the source, closing an open challenge.

“There is a strong drive at the moment to try and control chiral matter with twisted light, and for this to work you need light with a very high twist: super-chiral light,” says Forbes. Various industries and research fields require super-chiral light to improve their processes, including the food, computer and biomedical industries.

“We can use this type of light to drive gears optically where physical mechanical systems would not work, such as in micro-fluidic systems to drive flow,” says Forbes. “Using this example, the goal is to perform medicine on a chip rather than in a large lab, and is popularly called Lab-on-a-Chip. Because everything is small, light is used for the control: to move things around and sort things, such as good and bad cells. Twisted light is used to drive micro-gears to get the flow going, and to mimic centrifuges with light.”