Metasurface opens world of polarization
Device may have far-reaching applications in polarization imaging,
quantum optics, and more

Date:
June 3, 2020
Source:
Harvard John A. Paulson School of Engineering and Applied Sciences
Summary:
Researchers have designed a metasurface that can be continuously
tuned from linear to elliptical birefringence, opening up the
entire space of polarization control with just one device. This
single metasurface can operate as many birefringent materials in
parallel, enabling more compact polarization manipulation, which
could have far-reaching applications in polarization imaging,
quantum optics, and other areas.



FULL STORY ========================================================================== Polarization, the direction in which light vibrates, is invisible to
the human eye. Yet, so much of our optical world relies on the control
and manipulation of this hidden quality of light.


========================================================================== Materials that can manipulate the polarization of light -- known as birefringent materials -- are used in everything from digital alarm
clocks to medical diagnostics, communications and astronomy.

Just as light's polarization can vibrate along a straight line
or an ellipse, materials can also be linearly or elliptically
birefringent. Today, most birefringent materials are intrinsically
linear, meaning they can only manipulate the polarization of light in
a limited way. If you want to achieve broad polarization manipulation,
you need to stack multiple birefringent materials on top of one another,
making these devices bulky and inefficient.

Now, researchers from the Harvard John A. Paulson School of Engineering
and Applied Sciences have designed a metasurface that can be continuously
tuned from linear to elliptical birefringence, opening up the entire space
of polarization control with just one device. This single metasurface can operate as many birefringent materials in parallel, enabling more compact polarization manipulation, which could have far-reaching applications
in polarization imaging, quantum optics, and other areas.

The research is published in Science Advances.

"It is a new type of birefringent material," said Zhujun Shi, a
former graduate student at SEAS and first author of the paper. "We are
able to tailor broad polarization behavior of a material beyond what
naturally exists, which has a lot of practical benefits. What used to
require three separate conventional birefringent components now only
takes one." "The ability to manipulate a fundamental property of light
like polarization in completely new ways with a device that is compact
and multifunctional will have important applications for quantum optics
and optical communications," said Federico Capasso, Robert L. Wallace
Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS and senior author of the paper.



========================================================================== Metasurfaces are arrays of nanopillars spaced less than a wavelength
apart that can perform a range of tasks, including manipulating the
phase, amplitude and polarization of light. In the past, Capasso and
his team have designed these highly ordered surfaces from the ground up,
using simple geometric shapes with only a few design parameters.

In this research, however, the team turned to a new type of design
technique known as topological optimization.

"Topological optimization is an inverse approach," said Shi. "You start
with what you want the metasurface to do and then you allow the algorithm
to explore the huge parameter space to develop a pattern that can best
deliver that function." The result was surprising. Instead of neatly
ordered rectangular pillars standing like toy soldiers, this metasurface
is composed of nested half circles reminiscent of crooked smiley faces --
more like something a toddler would draw than a computer.

But these odd shapes have opened up a whole new world of
birefringence. Not only can they achieve broad polarization manipulations
like transforming linear polarization into any desired elliptical
polarization but the polarization can also be tuned by changing the
angle of the incoming light.

"Our approach has a wide range of potential applications across industry
and scientific research, including polarization aberration correction
in advanced optical systems," said Capasso.

This research was co-authored by Alexander Y. Zhu, Zhaoyi Li, Yao-Wei
Huang, Wei Ting Chen, and Cheng-Wei Qiu of the National University of Singapore. It was supported in part by the Air Force Office of Scientific Research under award number FA9550-19-1-0135.


========================================================================== Story Source: Materials provided by Harvard_John_A._Paulson_School_of_Engineering_and_Applied
Sciences. Original written by Leah Burrows. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Zhujun Shi, Alexander Y. Zhu, Zhaoyi Li, Yao-Wei Huang, Wei
Ting Chen,
Cheng-Wei Qiu, Federico Capasso. Continuous angle-tunable
birefringence with freeform metasurfaces for arbitrary polarization
conversion. Science Advances, 2020; 6 (23): eaba3367 DOI:
10.1126/sciadv.aba3367 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200603194419.htm https://www.sciencedaily.com/releases/2020/06/200603194419.htm

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