A light bright and tiny: Scientists build a better nanoscale LED
New design overcomes long-standing LED efficiency problem -- and can
transform into a laser to boot

Date:
August 14, 2020
Source:
National Institute of Standards and Technology (NIST)
Summary:
A new design for light-emitting diodes achieves a dramatic
increase in brightness as well as the ability to create laser
light - - characteristics that could make it valuable in a range
of applications.

The device shows an increase in brightness of 100 to 1,000 times
over conventional submicron-sized LED designs.



FULL STORY ==========================================================================
A new design for light-emitting diodes (LEDs) developed by a team
including scientists at the National Institute of Standards and Technology (NIST) may hold the key to overcoming a long-standing limitation in the
light sources' efficiency. The concept, demonstrated with microscopic
LEDs in the lab, achieves a dramatic increase in brightness as well as
the ability to create laser light -- all characteristics that could make
it valuable in a range of large-scale and miniaturized applications.


==========================================================================
The team, which also includes scientists from the University of Maryland, Rensselaer Polytechnic Institute and the IBM Thomas J. Watson Research
Center, detailed its work in a paper published today in the peer-reviewed journal Science Advances. Their device shows an increase in brightness
of 100 to 1,000 times over conventional tiny, submicron-sized LED designs.

"It's a new architecture for making LEDs," said NIST's Babak Nikoobakht,
who conceived the new design. "We use the same materials as in
conventional LEDs.

The difference in ours is their shape." LEDs have existed for decades,
but the development of bright LEDs won a Nobel prize and ushered in a
new era of lighting. However, even modern LEDs have a limitation that frustrates their designers. Up to a point, feeding an LED more electricity makes it shine more brightly, but soon the brightness drops off, making
the LED highly inefficient. Called "efficiency droop" by the industry,
the issue stands in the way of LEDs being used in a number of promising applications, from communications technology to killing viruses.

While their novel LED design overcomes efficiency droop, the researchers
did not initially set out to solve this problem. Their main goal was
to create a microscopic LED for use in very small applications, such
as the lab-on-a-chip technology that scientists at NIST and elsewhere
are pursuing.

The team experimented with a whole new design for the part of the LED
that shines: Unlike the flat, planar design used in conventional LEDs,
the researchers built a light source out of long, thin zinc oxide strands
they refer to as fins. (Long and thin are relative terms: Each fin is
only about 5 micrometers in length, stretching about a tenth of the way
across an average human hair's breadth.) Their fin array looks like a
tiny comb that can extend to areas as large as 1 centimeter or more.



==========================================================================
"We saw an opportunity in fins, as I thought their elongated shape and
large side facets might be able to receive more electrical current,"
Nikoobakht said.

"At first we just wanted to measure how much the new design could
take. We started increasing the current and figured we'd drive it until
it burned out, but it just kept getting brighter." Their novel design
shone brilliantly in wavelengths straddling the border between violet
and ultraviolet, generating about 100 to 1,000 times as much power as
typical tiny LEDs do. Nikoobakht characterizes the result as a significant fundamental discovery.

"A typical LED of less than a square micrometer in area shines with about
22 nanowatts of power, but this one can produce up to 20 microwatts,"
he said. "It suggests the design can overcome efficiency droop in LEDs
for making brighter light sources." "It's one of the most efficient
solutions I have seen," said Grigory Simin, a professor of electrical engineering at the University of South Carolina who was not involved
in the project. "The community has been working for years to improve
LED efficiency, and other approaches often have technical issues when
applied to submicrometer wavelength LEDs. This approach does the job
well." The team made another surprising discovery as they increased
the current. While the LED shone in a range of wavelengths at first,
its comparatively broad emission eventually narrowed to two wavelengths
of intense violet color. The explanation grew clear: Their tiny LED had
become a tiny laser.

"Converting an LED into a laser takes a large effort. It usually requires coupling a LED to a resonance cavity that lets the light bounce around
to make a laser," Nikoobakht said. "It appears that the fin design can
do the whole job on its own, without needing to add another cavity."
A tiny laser would be critical for chip-scale applications not only for chemical sensing, but also in next-generation hand-held communications products, high-definition displays and disinfection.

"It's got a lot of potential for being an important building block,"
Nikoobakht said. "While this isn't the smallest laser people have made,
it's a very bright one. The absence of efficiency droop could make it
useful." The research was supported in part by the U.S. Army Cooperative Research Agreement.


========================================================================== Story Source: Materials provided by National_Institute_of_Standards_and_Technology_(NIST).

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Babak Nikoobakht, Robin P. Hansen, Yuqin Zong, Amit Agrawal,
Michael Shur
and Jerry Tersoff. High-brightness lasing at submicrometer enabled
by droop-free fin light-emitting diodes (LEDs). Science Advances,
2020 DOI: 10.1126/sciadv.aba4346 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200814142950.htm

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