A Zero-Index
Waveguide
Researchers directly observe infinitely long wavelengths for the first time
By Leah Burrows
Researchers directly observe infinitely long wavelengths for the first time
By Leah Burrows
October 9, 2017 -- 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, SEAS researchers 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 research is published in ACS
Photonics. 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
a ratio called the refractive index — the higher the index, the more squished
the wavelength.
When the refractive index is reduced to zero the light no
longer behaves as a moving wave, traveling through space in a series of crests
and troughs, otherwise known as 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.
But 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.
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.
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.
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
“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.”
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