A team of scientists from the University of the Witwatersrand, Harvard University and the National University of Singapore has come up with a new metasurface-enhanced laser that efficiently produces any desired chiral state of light, with full control over both angular momentum components of light: spin (polarization) and orbital angular momentum.

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.

Light is also chiral, but has two forms: the spin and the orbital angular momentum. Spin angular momentum is similar to planets spinning around their own axis, while orbital angular momentum 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,” said Professor Andrew Forbes, a researcher in the School of Physics at the University of the Witwatersrand.

“Complete chiral control implies control of the full angular momentum of light, polarization and orbital angular momentum.”

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 (the combination of spin and linear motion) of orbital angular momentum 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.

Professor Forbes and colleagues used nanometer-sized metasurface in their new 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.

“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,” Professor Forbes said.

The team’s laser used a metasurface to imbue light with ultra-high angular momentum, giving it an unprecedented twist in its phase while also controlling the polarization.

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 laser could lase on orbital angular momentum states of 10 and 100 simultaneously for the highest reported angular momentum from a laser to date.

In the special case that the metasurface is set to produce symmetric states, the laser then produces all prior orbital angular momentum states reported from custom structured light lasers.

“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,” Professor Forbes said.

“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.”

“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.”

The new laser is described in a paper in the journal Nature Photonics.

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