Upside-down light: Scientists invert optical waves

Scientists call the unique lightwaves hyperbolic surface polaritons.
By Brooks Hays  |  Feb. 23, 2018 at 9:36 AM
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Feb. 23 (UPI) -- Scientists in Spain have developed a new material, a so-called hyperbolic metasurface, that inverts light waves. The technology could give researchers more precise control of optical waves and could be incorporated into a variety of optical devices.

Normally light waves propagate outward in circular, or convex, wavefronts from its source, like the ripple from a stone tossed into water. The behavior is explained by light's normal medium, homogeneous and isotropic, the same in all directions.

Scientists theorized that light waves propagated along specially engineered surfaces would produce inverted optical waves.

"On such surfaces, called hyberbolic metasurfaces, the waves emitted from a point source propagate only in certain directions, and with open, or concave, wavefronts," Javier Alfaro, a PhD student at the nanoGUNE Cooperative Research Center in Basque, said in a news release.

Because the metasurface constrains the paths of the lightwaves, the wavelengths are much smaller and only propagate in certain directions. Scientists call the unique lightwaves hyperbolic surface polaritons.

As part of their latest research efforts, scientists at nanoGUNE created a new kind of hyperbolic metasurface made from boron nitride. The 2D graphene-like material is designed to convert infrared lightwaves into hyperbolic surface polaritons. The technology could be used in tiny sensors and other nanoscale optoelectronic devices.

To build their new hyperbolic metasurface, scientists used electron beam lithography technology to finely etch nanoscale textures onto ultra thin flakes of boron nitride.

"The same fabrication methods can also be applied to other materials, which could pave the way to realize artificial metasurface structures with custom-made optical properties," said researcher Saül Vélez.

Scientists used a scattering-type scanning near-field microscope to image the waves created when light was propagated across their newly fabricated hyperbolic metasurface.

"It was amazing to see the images," said nanGUNE professor Rainer Hillenbrand. "They indeed showed the concave curvature of the wavefronts that were propagating away form the gold nanorod, exactly as predicted by theory."

Scientists described their efforts in a new paper published this week in the journal Science.

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