Tech Briefs Magazine - October 2021 - 26

Optics
Graphene-Based Photonics Devices for Remote Sensing
Growing large-area graphene on optical substrates enables use in photonics devices.
Goddard Space Flight Center, Greenbelt, Maryland
C
urrently, the dominant method to
generate ultrafast laser pulses passively
is to use semiconductor saturable
absorber mirrors (SESAMs). This type of
passive mode locker produces exceptional
results but is difficult to fabricate,
expensive, and limited in bandwidth. In
contrast, a graphene-based saturable
absorber is easier to produce and has
the advantages of much wider bandwidth,
lower saturation intensity, tunable
modulation depth, ultrafast recovery
time, and much higher optical damage
threshold, thus producing higher
energies.
A process for transferring graphene to
suitable optical substrates, as well as tips
of optical fiber, was developed by growing
large-area graphene using a low-pressure
chemical vapor deposition (LPCVD)
technique. In addition, a second process
was developed for depositing multiple
graphene layers on the same substrate.
Using these new processes, engineers
developed a process to make a saturable
absorber with single-layer as well as multilayer
graphene.
High energy (10s nJ) and femtosecond
pulses with repetition rate on the order of
hundreds of MHz have been demonstrated
using graphene as a saturable absorber
in developing a graph ene-based modelock
device for a laser transmitter. This
work has also led to the development of
other novel photonics devices. Aside from
the applications as a laser transmitter, the
saturable absorber can be used as a scalable
graphene-based bolometer.
NASA is actively seeking licensees to commercialize
this technology. Please contact
NASA's Licensing Concierge at Agency-PatentLicensing@mail.nasa.gov
or call us at 202358-7432
to initiate licensing discussions.
Follow this link for more information: https://
technology.nasa.gov/patent/GSC-TOPS-27.
Material Increases Efficiency of LED Devices
The approach achieves near 100% light emission efficiency at all brightness levels.
Lawrence Berkeley National Laboratory, Berkeley, California
S
martphones, laptops, and lighting ap -
plications rely on light emitting di -
odes (LEDs) to shine brightly. But the
brighter they shine, the more inefficient
they become, releasing more energy as
heat instead of light. Researchers have
demonstrated an ap proach for achieving
near 100% light emission efficiency at all
brightness levels.
The approach focuses on stretching or
compressing a thin semiconductor film
in a way that favorably changes its electronic
structure. The team identified
how the semiconductor's electronic
structure dictated interaction among the
energetic particles within the material.
Those particles sometimes collide and
annihilate each other, losing energy as
heat instead of emitting light in the
process. Changing the material's electronic
structure reduced the likelihood
for annihilation and led to a near-perfect
conversion of energy to light, even at
high brightness.
The discovery was made using a single,
3-atom-thick layer of a type of semiconductor
material, a transition metal
di chalcogenide, that was subjected to
mechanical strain. These thin materials
26
Cov
Applying mechanical strain on this atomically thin, transparent, monolayer semiconductor results
in a material with near 100% light emission efficiency. (Credit: Ali Javey/Berkeley Lab)
have a unique crystal structure that gives
rise to unique electronic and optical
properties: When their atoms are excited
either by passing an electric current
or shining light, energetic particles
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ToC
called excitons are created. Excitons can
release their energy either by emitting
light or heat. The efficiency with which
excitons emit light as opposed to heat is
an important metric that determines the
Tech Briefs, October 2021

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Tech Briefs Magazine - October 2021

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