Harvard SEAS developers produce zero-index Si-photonics waveguide

10 Oct 2017

In doing so, Eric Mazur's team observed a physical phenomenon that is usually unobservable — a standing wave of light.

Artist's impression of a zero Index waveguide.

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.

That metamaterial, which was based on low-aspect-ratio silicon pillar arrays embedded in a polymer matrix clad by gold films, represented a new method to manipulate light and was an important step forward for the development of integrated photonic circuits, to perform a wide variety of functions.

Now, a team of SEAS researchers, again led by Eric Mazur, the Balkanski Professor of Physics, have pushed the technology further. They have developed a zero-index waveguide compatible with current silicon photonic technologies. In doing so, the team observed a physical phenomenon that they say is usually unobservable — a standing wave of light.

The research is published in the journal ACS Photonics. The Harvard Office of Technology Development has filed a patent application and is exploring commercialization opportunities. The paper was co-authored by Daryl I. Vulis, Yang Li and Marko Loncar, Tiantsai Lin Professor of Electrical Engineering at SEAS. The research was supported by National Science Foundation and was performed in part at the Center for Nanoscale Systems (CNS).

The SEAS announcement explains, “When the refractive index of a material is reduced to zero the light no longer behaves as a moving wave. Instead, the wave is stretched infinitely long, creating a constant phase. The phase oscillates only as a variable of time, not space.”

Infinite wavelength

The team added that the development of an “infinitely long wave” based on a material with a zero refractive index is interesting for integrated photonics because most optical devices use interactions between two or more waves, which need to propagate in sync as they move through the circuit. If the wavelength is infinitely long, matching the phase of the wavelengths of light is not an issue, since the optical fields are the same everywhere.

But after the initial 2015 breakthrough, the research team ran into a problem. 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 are not particularly useful shapes for integrated circuits. The team needed to develop a device that could plug directly into existing photonic circuits and to achieve that, the most useful shape is a straight wire or waveguide.

The SEAS silicon waveguide supports phase-free propagation in the telecom regime.

Mazur’s researchers developed and built such a waveguide but, without the help of a prism they had no easy way to prove if it had a refractive index of zero. Then, postdoctoral fellows Orad Reshef and Philip Camayd-Muñoz had an idea. 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 – such as achieved by a so-called standing wave.

Oscillate wildly

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 rapidly 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. Because of the zero-index material, the team was able to stretch the wavelength long enough to observe.

This may be the first time a standing wave with infinitely-long wavelengths has ever been seen. “We were able to observe a breath-taking demonstration of an index of zero,” said Reshef, who recently accepted a position at the University of Ottawa. “By propagating through a medium with such a low index, these wave features, which in light are typically too small to detect directly, are expanded so you can see them with an ordinary microscope.”

“This adds an important tool to the silicon photonics toolbox,” said Camayd-Muñoz. “There's exotic physics in the zero-index regime, and now we're bringing that to integrated photonics. It means we can plug directly into conventional optical devices, and find real uses for zero-index phenomena. In the future, quantum computers may be based on networks of excited atoms that communicate via photons. By making the wavelength large, we can enable long-range interactions to scale up quantum devices.”