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22nd of November 2017

Science



Newly Developed Metasurface Generates Structured Light Beams | Materials Science, Physics | Sci-News.com

A team of researchers has developed a new tool that can produce complex states of light. The research appears in the journal Science.

A metasurface uses circularly polarized light to generate and control new and complex states of light, such swirling vortices of light. Image credit: Second Bay Studio / Harvard John A. Paulson School of Engineering and Applied Sciences.

A metasurface uses circularly polarized light to generate and control new and complex states of light, such swirling vortices of light. Image credit: Second Bay Studio / Harvard John A. Paulson School of Engineering and Applied Sciences.

The new metasurface connects two aspects of light: orbital angular momentum and circular polarization (or spin angular momentum).

Polarization is direction along which light vibrates. In circularly polarized light, the vibration of light traces a circle. Think about orbital angular momentum and circular polarization like the motion of a planet. Circular polarization is the direction in which a planet rotates on its axis while orbital momentum describes how the planet orbits the Sun.

The fact that light can even carry orbital momentum is a relatively recent discovery, but it’s this property of light which produces strange new states, such as beams in the shape of corkscrews.

Previous research has used the polarization of light to control the size and shape of these exotic beams but the connection was limited because only certain polarizations could convert to certain orbital momentums.

“We have developed a metasurface which is a new tool to study novel aspects of light,” said senior author Professor Federico Capasso, from the Harvard John A. Paulson School of Engineering and Applied Sciences.

“This optical component makes possible much more complex operations and allows researchers to not only explore new states of light but also new applications for structured light.”

“This metasurface gives the most general connection, through a single device, between the orbital momentum and polarization of light that’s been achieved so far,” added first author Dr. Robert Devlin, also from the Harvard John A. Paulson School of Engineering and Applied Sciences.

The device can be designed so that any input polarization of light can result in any orbital angular momentum output — meaning any polarization can yield any kind of structured light, from spirals and corkscrews to vortices of any size.

And, the multifunctional device can be programmed so that one polarization results in one vortex and a different polarization results in a completely different vortex.

“This is a completely new optical component. Some metasurfaces are iterations or more efficient, more compact versions of existing optical devices but, this arbitrary spin-to-orbital conversion cannot be done with any other optical device,” said co-author Dr. Antonio Ambrosio, of the Harvard Center for Nanoscale Systems.

“There is nothing in nature as well that can do this and produce these states of light.”

One potential application is in the realm of molecular manipulation and optical tweezers, which use light to move molecules. The orbital momentum of light is strong enough to make microscopic particles rotate and move.

Another application is high-powered imaging. The black hole in the center of the vortex, known as the zero-light intensity region, can image features smaller than the diffraction limit, which is usually half of the wavelength of light. By changing the polarization of light, the size of this center region can be changed to focus different-sized features.

But these beams can also shed light on fundamental questions of physics.

“There is interest in these beams in quantum optics and quantum information,” said co-author Noah Rubin, from the Harvard John A. Paulson School of Engineering and Applied Sciences.

“On the more applied side, these beams could find application in free-space optical communication, especially in scattering environments where this is usually difficult.”

“Moreover, it has been recently shown that similar elements can be incorporated into lasers, directly producing these novel states of light. This may lead to unforeseen applications.”

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Robert C. Devlin et al. Arbitrary spin-to-orbital angular momentum conversion of light. Science, published online November 2, 2017; doi: 10.1126/science.aao5392

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