An international team of physicists has reported that it has produced super-chiral light, which is light with an ultrahigh angular momentum; the highest yet reported for a laser.

The light from this laser can be used as a type of “optical spanner” to or for encoding information in optical communications, it says in a paper published in the Nature Photonics science journal.

The team includes scientists from the University of the Witwatersrand (Wits); the Council for Scientific and Industrial Research; Harvard University, in the US; the National University of Singapore; Vrije Universiteit Brussel, in Begium; and CNST – Fondazione Istituto Italiano di Tecnologia Via Giovanni Pascoli, in Italy.

The articles states that the researchers have developed a nanostructured 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.

In their paper '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 the orbital angular momentum (OAM) of light.

"The new laser produces a new high-purity 'twisted light' not observed from lasers before. Our work represents an important step towards merging the research in bulk lasers with that of on-chip devices," says Wits School of Physics Professor Andrew Forbes, who led the research.

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, he explains.

“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 microgears to get the flow going and to mimic centrifuges with light.”

"Chirality” is a term often used in chemistry to describe compounds that are found as mirror images of one another. These compounds have a “handedness” and can be thought of as either left- or right-handed. For example, lemon and orange flavours are the same chemical compound, but only differ in their “handedness”, explains Forbes.

Light is also chiral, but has two forms: the spin (polarisation) and the OAM. Spin AM is similar to planets spinning around their own axes, 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 and communications. Complete chiral control implies control of the full angular momentum of light, polarisation and OAM,” says Forbes.

This is what the team achieved, he adds.

"The laser design is made possible by the complete control offered by new nanometer-sized (1 000 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 every time it does so."

The laser uses a metasurface to imbue light with ultrahigh 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 can be broken 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," says Forbes.

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, he highlights.

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. Various industries and research fields require super-chiral light to improve their processes, including the food, computer and biomedical industries, says Forbes.

“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,” he says.

“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," he notes.