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Guiding light is not just a soap opera

Photonics Spectra
Oct 2009
David L. Shenkenberg,

Computers of the future may use light rather than electricity for logic functions. “There is this dream of the all-photonic circuit to guide light and perform functions,” commented Willie Padilla, a professor at Boston College. Such light-based computers could run much faster than ordinary ones.

An all-photonic network also could make the Internet speedier. “It is estimated that an all-photonic Internet could transmit data at a couple hundred gigabits per second,” Padilla added. “This is ten thousand times faster than current high-speed connections.”

One of the challenges in developing all-photonic computers and an all-photonic Internet is navigating light around curves and crossing points. Waveguides for this purpose could be made out of metamaterials – man-made materials with unusual physical properties.

In the Aug. 17, 2009, issue of Optics Express, Padilla and Nathan Landy, now at Duke University in Durham N.C., laid out the theoretical framework for a metamaterial waveguide that travels around a region with many irregular curves. To demonstrate the flexibility of their method, the researchers used a map of the Eastern seaboard of the US as an example of a pathway with irregular curves for light to travel around.

To bend the light around the numerous twists and turns of the coastline, they imagined a grid of squares within a large rectangle. The researchers used this rectangle to represent a waveguide for light to travel through. Then they used mathematics to transform this rectangular “waveguide” and its internal grid lines to conform to the map of the coastline.

The grid lines represent a physical property called the permittivity. When the rectangle and its internal grid lines are conformed to the map of the coastline, the permittivity varies spatially with regard to the coastline. Because the permittivity is related to the refractive index, the refractive index varies with respect to the coastline.

In a theoretical analysis using a map of the Eastern coastline of the US as an example, researchers describe a method for navigating light around complex curves. Courtesy of Nathan Landy.

The light generally travels straight ahead even though it is distorted. “The light … doesn’t know that it is traveling on this complicated path,” Padilla said. “The light sees normal waveguide straight space.”

The fact that the light moves on an overall linear path, the researchers noted, will enable the creation of waveguides without resonant losses even for a three-dimensional waveguide. They added that the conformal mapping technique could be used to make a concentrator. “This demonstrates how complex or versatile this method is for demonstrating devices that we cannot comprehend yet,” Padilla said.

They referenced epsilon-near-zero tunneling, an idea advanced by Nader Engheta, who is the subject of this month’s profile on page 16. They wrote that their method is broadband and avoids complex impedance-matching techniques, both advantages over epsilon-near-zero tunneling. However, they conceded that their method requires calculating the refractive index for each curve.

Since this theoretical demonstration, the researchers in the Padilla lab have constructed a metamaterial device that functions at microwave frequencies. They have not formally published this work in a peer-reviewed journal.

Basic ScienceBoston Collegeconformal mappingDavid L. Shenkenbergepsilon-near zerometamaterialsnanoNathan Landyphotonic circuitsphotonic computersResearch & TechnologyTech PulsetheoreticalwaveguidesWillie Padilla

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