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Light journeys unimpeded along material’s surface

Exotic etched glass could improve optics-based communications

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Throw some electrons onto the surface of a topological insulator and they seemingly become invincible, effortlessly bypassing obstructions along their route. Now researchers have crafted a structure that empowers particles of light to do the same thing. The first demonstration of a topological insulator for photons, reported April 10 in Nature, could lead to improved optical transmissions that are crucial for global communication.

“I think it’s wonderful,” says Michal Lipson, a physicist at Cornell University who was not involved with the study. “The light goes right around any obstacles, which is pretty remarkable.”

Topological insulators have been a burgeoning area of condensed-matter physics since they were proposed in 2005 (SN: 5/22/10, p. 22). Typical materials are either conductors or insulators, but topological insulators such as bismuth telluride are exotic hybrids: They block electric current yet allow electrons to flow along their surfaces.

What’s more, these surface electrons can move unimpeded through bumps and grooves that would normally block their path. That useful property makes topological insulators intriguing candidates for future electronics.

The ability to enable electrons to surf along the surface and avoid obstacles is so enticing that some physicists have investigated whether other particles, particularly photons, could do the same thing. Along with electrons, photons are an essential element of modern technology. Electrons flow through chips in our computers and smartphones, while photons are the information carriers that enable high-speed communication over fiber-optic cables. The key to faster, more efficient communication networks is minimizing the scattering of photons when they encounter obstacles.

With that goal in mind, physicist Mordechai Segev and his team at the Technion-Israel Institute of Technology in Haifa, along with colleagues from Friedrich Schiller University Jena in Germany, set out to demonstrate the first photonic topological insulator. They started with a block of glass and etched in hundreds of helical waveguides, which are essentially wires for light. The waveguides were tightly packed in a honeycomb-like structure so that light trying to make its way through one waveguide interfered with light in the others and canceled out.

The only part of each waveguide that did not cancel out light was its outer edge. As a result, photons got steered along the outside of the bundle of waveguides, confining them to the surface of the glass block.

When the researchers shined a beam of red light on one face of the glass, the photons moved along the surface of the glass, easily made a turn once they reached an edge of the glass and then continued on their way along the surface. None of the light got scattered by surface imperfections.

Segev says his team’s photonic topological insulator will lead to improved optical transmissions. Jacob Taylor, a physicist at the University of Maryland’s Joint Quantum Institute, adds that the impressive light-harnessing properties of the group's creation could allow people to send more data over a popular type of wire known as multi-mode optical fiber.

Taylor has similar applications in mind for a photonic topological insulator his team is creating. While the structure of his team’s material is not as simple as Segev’s, it is made of silicon-based components like the ones widely used in the telecommunications industry.

Segev expects many more examples in coming months. “There were a number of groups competing to try to achieve this,” he says. “We’re happy to have won the race.”

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