A Zero-Index Waveguide

Real-time, unprocessed video of standing waves of light in a 15-micrometer-long, zero-index waveguide taken with an infrared camera. The perceived motion is caused by atmospheric disturbances to the free- standing fibers that couple light onto the chip, changing the relative phase between the two incoming beams. (Source: Harvard SEAS)

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. Now, they have pushed that technology further developing a zero-index wave­guide compatible with current silicon photonic technologies. In doing so, the team observed a physical pheno­menon that is usually unob­servable: a standing wave of light. The Harvard Office of Tech­nology Deve­lopment has filed a patent appli­cation and is exploring commerciali­zation oppor­tunities.

When a wavelength of light moves through a material, its crests and troughs get condensed or stretched, depending on the properties of the material. How much the crests of a light wave are condensed is expressed as the refractive index. When the refrac­tive index is reduced to zero the light no longer behaves as a moving wave, traveling through space in phases. Instead, the wave is stretched infi­nitely long, creating a constant phase. The phase oscil­lates only as a variable of time, not space.

This is exciting 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 isn’t an issue, since the optical fields are the same everywhere. After the initial 2015 break­through 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 around Eric Mazur built a wave­guide but, without the help of a prism, 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 to get 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. 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. “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. That’s an important step, because 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. The inter­action range of the atoms is roughly equal to the wave­length of light. By making the wave­length large, we can enable long-range interactions to scale up quantum devices.” (Source: Harvard SEAS)

Reference: O. Reshef et al.: Direct Observation of Phase-Free Propagation in a Silicon Waveguide, ACS Phot., online 9 October 2017; DOI: 10.1021/acsphotonics.7b00760

Link: John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, USA

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