The new metasurface laser produces the world’s first super chiral light

Researchers have demonstrated the world’s first meta-surface laser to produce “superchiral light”: light with ultra-high angular momentum. The light from this laser can be used as a type of “optical key” to or encode information in optical communications.

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

The new laser produces a new high purity “twisted light” that was not observed from lasers before, including the highest angular momentum reported by a laser. Simultaneously, the researchers developed a nanostructured meta-surface that has the largest phase gradient ever produced and enables high-power operation in a compact design. Engagement is the world’s first laser to produce exotic states of twisted structured light, on demand.

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

In their article titled: High Purity Orbital Momentum States of a Visible Metasurface Laser, the researchers demonstrate a new laser to produce any desired chiral light state, with full control over both components of angular momentum (AM) light. , the gyration (polarization) and orbital angular momentum (OAM) of light.

The design of the laser is possible thanks to the complete control offered by the new meta-surface the size of a nanometer (1000 times smaller than the width of a human hair), designed by the Harvard group, inside the laser. The metasurface is made up of many small nanomaterial bars, which alter light as it passes. Light passes through the meta-surface many times, receiving a new twist every time it does so.

“What makes it special is that, for light, the material has properties that are impossible to find in nature, and that is why it is called” metamaterial “, a fantasy material. Because the structures were so small that they only appeared on the surface to create a meta-surface. “

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

“There is a strong push right now to try to control chiral matter with twisted light, and for this to work you need light with a very high spin – superchiral 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, as in microfluidic systems to drive flow,” says Forbes. “Using this example, the goal is to perform medicine on a chip rather than in a large laboratory, and it is popularly called Lab-on-a-Chip. Because everything is small, light is used for control: to move things and sort things like good and bad cells. Twisted light is used to drive micro gears to make the flow work and to mimic centrifuges with light. “

The Chiral Challenge

“Chirality” is a term often used in chemistry to describe compounds that are found as mirror images of each other. These compounds have a “hand” and can be considered right-handed or left-handed. For example, lemon and orange flavors are the same chemical compound, but only differ in their “feel”.

Light is also chiral but has two forms: spin (polarization) and OAM. Spin AM is similar to planets that rotate around their own axis, while OAM is similar to planets that orbit around the Sun.

“Controlling light chirality at the source is a challenging and highly topical task due to the many applications that require it, from optical control of chiral matter to metrology and communications,” says Forbes. “Full chiral control involves controlling the full angular momentum of light, polarization, and OAM.”

Due to design constraints and implementation impediments, to date only a very small subset of chiral states have occurred. Ingenious schemes have been devised to control the helicity (the combination of twist and linear motion) of the OAM beams, but they also remain restricted to this symmetric set of modes. It was not possible to write down some desired chiral light state and have it produced by a laser, until now.

Metasurface laser

The laser used a meta-surface to imbue the light with ultra-high angular momentum, giving it an unprecedented “turn” in its phase while controlling polarization. By arbitrary control of angular momentum, the standard symmetry of the rotating orbit could be broken, so that the first laser produces full control of the angular momentum of light at the source.

The metasurface was constructed from carefully designed nanostructures to produce the desired effect, and is the most extreme OAM structure manufactured to date, with the highest phase gradient reported so far. The nanometer resolution of the meta-surface made possible a high quality vortex with low loss and a high threshold of damage, making the laser possible.

The result was a laser that could achieve OAM states of 10 and 100 simultaneously for the highest AM reported from a laser to date. In the special case that the meta-surface is configured to produce symmetrical states, the laser produces all previous OAM states reported from the custom structured light lasers.

Going forward

“What we found particularly exciting is that our approach lends itself to many laser architectures. For example, we could increase the gain volume and meta-surface size to produce a high-power bulk laser, or we could shrink the system to a chip using a monolithic meta-surface design, “says Forbes.

“In both cases, the laser mode would be controlled by the polarization of the pump, which requires no elements within the cavity other than the meta-surface itself. Our work represents an important step toward merging laser research into bulk with on-chip devices. ”