Zero-index waveguide enables researchers to directly observe infinitely long wavelengths: Page 2 of 3

October 10, 2017 // By Jean-Pierre Joosting
In 2015, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) developed the first on-chip metamaterial with a refractive index of zero, meaning that the phase of light could be stretched infinitely long. The metamaterial represented a new method to manipulate light and was an important step forward for integrated photonic circuits, which use light rather than electrons to perform a wide variety of functions.

However, after the initial 2015 breakthrough, the research team ran into a catch-22. Because the team used prisms to test whether light on the chip was indeed infinitely stretched, all of the devices were built in the shape of a prism. But prisms aren't particularly useful shapes for integrated circuits. The team wanted to develop a device that could plug directly into existing photonic circuits and for that, the most useful shape is a straight wire or waveguide.

The researchers – led by Eric Mazur, the Balkanski Professor of Physics – built a waveguide but, without the help of a prism, had no easy way to prove if it had a refractive index of zero.

Postdoctoral fellows Orad Reshef and Philip Camayd-Muñoz had an idea on how to create a standing wave.

Usually, a wavelength of light is too small and oscillates too quickly to measure anything but an average. The only way to actually see a wavelength is to combine two waves to create interference.

Imagine strings on a guitar, pinned on either side. When a string is plucked, the wave travels through the string, hits the pin on the other side and gets reflected back – creating two waves moving in opposite directions with the same frequency. This kind of interference is called a standing wave.

Reshef and Camayd-Muñoz applied the same idea to the light in the waveguide. They "pinned-down" the light by shining beams in opposite directions through the device to create a standing wave. The individual waves were still oscillating quickly but they were oscillating at the same frequency in opposite directions, meaning at certain points they canceled each other out and other points they added together, creating an all light or all dark pattern. And, because of the zero-index material, the team was able to stretch the wavelength large enough to see.

A zero-index waveguide compatible with current silicon photonic technologies. Image courtesy of Second Bay Studios/Harvard SEAS.


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