World’s First Super-Chiral Light

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,” 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, you can now shine light on the nut and it will tighten itself.”

An artistic impression of the metasurface laser to produce super-chiral twisted light with OAM up to 100. (Source: Wits U.)

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. 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).

The laser design is made possible by the complete control offered by new nanometer-sized 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 metamaterial has pro­perties impossible to find in Nature. 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. 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.”

Light is chiral and has two forms: the spin and the OAM. Spin AM is similar to planets spinning around their own axis, while OAM is similar to planets orbiting the Sun. “Control­ling light’s chirality at the source is a challenging task and highly topical because of the many appli­cations that require it, from optical control of chiral matter, to metrology, to communi­cations,” says Forbes. “Complete chiral control implies control of the full angular momentum of light, polarisation and OAM.”

Because of design restric­tions and imple­mentation impe­diments, only a very small subset of chiral states has been produced to date. Ingenious schemes have been devised to control the helicity of OAM beams but they too remain restricted to this symmetric set of modes. It was not possible to write down some desired chiral state of light and have a laser produce it, until now. The laser used a meta­surface to imbue light with ultra-high angular momentum, giving it an unpre­cedented twist in its phase while also controlling the polari­sation. By arbitrary angular momentum control, the standard spin-orbit symmetry could be broke, for the first laser to produce full angular momentum control of light at the source.

The metasurface was built from carefully crafted nano­structures to produce the desired effect, and is the most extreme OAM structure so far fabricated, with the highest phase gradient yet reported. The nanometre resolution of the metasurface made possible a high-quality vortex with low loss and a high damage threshold, making the laser possible. The result was a laser that could lase on OAM states of 10 and 100 simul­taneously for the highest reported AM from a laser to date. In the special case that the metasurface is set to produce symmetric states, the laser then produces all prior OAM states reported from custom structured light lasers.

“What we find particularly exciting is that our approach lends itself to many laser archi­tectures. For instance, we could increase the gain volume and meta­surface size to produce a bulk laser for high-power, or we could shrink the system down onto a chip using a monolithic metasurface design,” says Forbes. “In both cases the lasing mode would be controlled by the pump’s polari­sation, requiring no intra-cavity elements other than the meta­surface itself. Our work represents an important step towards merging the research in bulk lasers with that of on-chip devices.” (Source: Wits U.)

Reference: H. Sroor et al.: High-purity orbital angular momentum states from a visible metasurface laser, Nat. Phot., online 27 April 2020; DOI: 10.1038/s41566-020-0623-z

Link: School of Physics, University of the Witwatersrand, Wits, South Africa

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