Tech Briefs Magazine - April 2023 - 30

Optics
Microscopic Color Converters May Yield Small
Laser-Based Devices
An atomically thin material is used to build a device that can change the color of laser beams.
Columbia University, New York, NY
N
ew research suggests that laser-based
devices are poised to become a lot
smaller. Researchers at Columbia University
and Politecnico di Milano studied a 2D
material called molybdenum disulfide
(MoS2
less than
) and characterized how efficiently
devices built from stacks of MoS2
one micron thick - 100 times thinner
than a human hair - convert light frequencies
at telecom wavelengths to produce
different colors.
" Nonlinear optics is currently a macroscopic
world, but we want to make it microscopic, "
said Giulio Cerullo, a Nonlinear
Optics Researcher at Politecnico di Milano
in Italy.
Lasers radiate a special kind of coherent
light, which means all the photons in
the beam share the same frequency and
color. Lasers operate only at specific frequencies,
but devices often need to be
able to deploy different colors of laser
light. For instance, a green laser pointer
is actually produced by an infrared laser
that's converted to a visible color by a
macroscopic material. Researchers use
nonlinear optical techniques to change
the color of laser light, but conventional
materials need to be relatively thick for
color conversion to occur efficiently.
With 3R-MoS2 in hand, researchers began
peeling off samples of varying thickness
to test how efficiently they converted
the frequency of light and saw extremely
large enhancement almost immediately.
The new crystal is as efficient but 100
times smaller than lithium niobate - currently
the most-popular crystal for waveguided
conversion and generating entangled
photons - and flexible enough that it
can be combined with silicon photonic
platforms to create optical circuits on
chips, following the trajectory of ever-smaller
electronics.
Researchers have used an atomically thin material
to build a device that can change the color of
laser beams. (Image: engineering.columbia.edu)
is one of the most-studied examples
of an emerging class of materials -
transition metal dichalcogenides - which
can be peeled into atomically thin layers.
Single MoS2
MoS2
layers can convert light frequencies
efficiently but are too thin to be
used to build devices. Larger crystals of
MoS2
, however, tend to be more stable in a
non-color converting form. To fabricate
the necessary crystals, 3R-MoS2
, the team
worked with the commercial 2D-material
supplier HQ Graphene.
Optical Signal Amplifying Technique
This discovery opens many avenues in fields like telecommunications, bioimaging, and
remote sensing.
Institut de la Recherche Scientifique, Quebec City, Canada
W
eak optical signals are common in
many science and technology applications.
However, they are difficult to detect
or process due to the incoherent
noise that is inherently present in any system.
PhD student Benjamin Crockett and
colleagues, working under the supervision
of Professor José Azaña of the Institut
national de la recherche scientifique
(INRS), have conceived a technique for
the recovery of weak, noise-dominated op30
tical
signals. Their research was published
in the journal Optica.
This achievement is relevant to various
fields like telecommunications, bioimaging,
and remote sensing. For example,
there is considerable loss from long-distance
propagation through optical fiber. In
light-based biomedical imaging applications,
there are limitations for the light power
to avoid damaging the living tissue. Similarly,
in LiDAR applications (laser remote
www.techbriefs.com
sensing), high optical powers can pose a
threat to the eyes of a passerby. In all these
cases, the recovery of weak, noise-dominated
signals remains a key challenge.
Lasers have enabled tremendous advances
in so many fields of research. Yet
the problem of sensitivity and noise
eventually leads to an obstacle for further
development.
Most common approaches are ill suited
to retrieve a weak signal because they eiTech
Briefs, April 2023
With the result, the bottleneck toward
real-life applications is large-scale production
of 3R-MoS2
and high-throughput
structuring of devices - at which point the
industry will need to take over.
" I've been working on nonlinear optics
for more than 30 years now, " said Cerullo.
" Research is most often incremental,
slowly building on what came before.
It's rare that you do something completely
new with big potential. I have a
feeling that this new material could
change the game. "
For more information, contact Ellen
Pullekines Neff at en2505@columbia.
edu; 212-854-2993.

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Tech Briefs Magazine - April 2023

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