Metasurface design methods can make LED light act more like lasers


Date:
June 3, 2020
Source:
University of California - Santa Barbara
Summary:
Researchers continue to push the boundaries of LED design a little
further with a new method that could pave the way toward more
efficient and versatile LED display and lighting technology.



FULL STORY ==========================================================================
UC Santa Barbara researchers continue to push the boundaries of LED design
a little further with a new method that could pave the way toward more efficient and versatile LED display and lighting technology.


==========================================================================
In a paper published in Nature Photonics, UCSB electrical and computer engineering professor Jonathan Schuller and collaborators describe
this new approach, which could allow a wide variety of LED devices --
from virtual reality headsets to automotive lighting -- to become more sophisticated and sleeker at the same time.

"What we showed is a new kind of photonic architecture that not only
allows you to extract more photons, but also to direct them where
you want," said Schuller. This improved performance, he explained, is
achieved without the external packaging components that are often used
to manipulate the light emitted by LEDs.

Light in LEDs is generated in the semiconductor material when excited, negatively charged electrons traveling along the semiconductor's crystal lattice meet positively-charged holes (an absence of electrons) and
transition to a lower state of energy, releasing a photon along the
way. Over the course of their measurements, the researchers found that
a significant amount of these photons were being generated but were not
making it out of the LED.

"We realized that if you looked at the angular distribution of the emitted photon before patterning, it tended to peak at a certain direction that
would normally be trapped within the LED structure," Schuller said. "And
so we realized that you could design around that normally trapped light
using traditional metasurface concepts." The design they settled upon
consists of an array of 1.45-micrometer long gallium nitride (GaN)
nanorods on a sapphire substrate, in which quantum wells of indium
gallium nitride were embedded, to confine electrons and holes and thus
emit light. In addition to allowing more light to leave the semiconductor structure, the process polarizes the light, which co-lead author Prasad
Iyer said, "is critical for a lot of applications." Nanoscale Antennae


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The idea for the project came to Iyer a couple of years ago as he was completing his doctorate in Schuller's lab, where the research is focused
on photonics technology and optical phenomena at subwavelength scales.

Metasurfaces -- engineered surfaces with nanoscale features that interact
with light -- were the focus of his research.

"A metasurface is essentially a subwavelength array of antennas,"
said Iyer, who previously was researching how to steer laser beams with metasurfaces. He understood that typical metasurfaces rely on the highly directional properties of the incoming laser beam to produce a highly
directed outgoing beam.

LEDs, on the other hand, emit spontaneous light, as opposed to the
laser's stimulated, coherent light.

"Spontaneous emission samples all the possible ways the photon is allowed
to go," Schuller explained, so the light appears as a spray of photons traveling in all possible directions. The question was could they, through careful nanoscale design and fabrication of the semiconductor surface,
herd the generated photons in a desired direction? "People have done patterning of LEDs previously," Iyer said, but those efforts invariably
split the into multiple directions, with low efficiency. "Nobody had
engineered a way to control the emission of light from an LED into a
single direction." Right Place, Right Time


==========================================================================
It was a puzzle that would not have found a solution, Iyer said, without
the help of a team of expert collaborators. GaN is exceptionally difficult
to work with and requires specialized processes to make high-quality
crystals. Only a few places in the world have the expertise to fabricate
the material in such exacting design.

Fortunately, UC Santa Barbara, home to the Solid State Lighting and Energy Electronics Center (SSLEEC), is one of those places. With the expertise
at SSLEEC and the campus's world-class nanofabrication facility, the researchers designed and patterned the semiconductor surface to adapt
the metasurface concept for spontaneous light emission.

"We were very fortunate to collaborate with the world experts in making
these things," Schuller said.

Research on this project also was conducted by Ryan A. DeCrescent
(co-lead author), Yahya Mohtashami, Guillaume Lhereux, Nikita Butakov,
Abdullah Alhassan, Claude Weisbuch, Shuji Nakamura and Steven P. DenBaars,
all from UCSB.


========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Barbara. Original written by Sonia
Fernandez. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Prasad P. Iyer, Ryan A. DeCrescent, Yahya Mohtashami, Guillaume
Lheureux,
Nikita A. Butakov, Abdullah Alhassan, Claude Weisbuch, Shuji
Nakamura, Steven P. DenBaars, Jon. A. Schuller. Unidirectional
luminescence from InGaN/GaN quantum-well metasurfaces. Nature
Photonics, 2020; DOI: 10.1038/s41566-020-0641-x ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200603151151.htm https://www.sciencedaily.com/releases/2020/06/200603151151.htm

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