Tech Briefs Magazine - September 2022 - PIT-26

" Metalenses take advantage of the fact
that the properties of light change when
it hits a surface, " Arndt said. " For example,
light travels faster through air than
it does through water. That's why you get
reflections on the surface of a pond. The
surface of the water is the interface, and
when sunlight hits the interface, a little
of it reflects off. "
Their prior work showed a metasurface
could produce VUV by upconverting
long-wave UV via a frequency-doubling
process called second-harmonic
generation. But VUV is costly, in part,
because it is expensive to manipulate
after it's produced. Commercially available
systems for that can fill cabinets as
large as refrigerators or compact cars
and cost tens of thousands of dollars,
she said.
" For a metalens, you're trying to both
generate the light and manipulate it, "
Arndt said. " In the visible wavelength regime,
metalens technology has become
very efficient. Virtual reality headsets use
it. Metalenses have also been demonstrated
in recent years for visible and
infrared wavelengths, but no one had
done it at shorter wavelengths. And a lot
of materials absorb VUV. So, for us it was
just an overall challenge to see, 'Can we
do this?' "
Tests at Rice showed the metalens they
made could focus its 197-nanometer output
onto a spot measuring 1.7 microns
in diameter, increasing the power density
of the light output by 21 times.
Arndt said it's too early to say whether
the technology can compete with stateof-the-art
VUV systems. " It's really fundamental
at this stage, but it has a lot of
potential. It could be made far more efficient.
With this first study, the question
was, 'Does it work?' In the next phase,
we'll be asking, 'How much better can
we make it?' "
For more information, contact Jade
Boyd at jadeboyd@rice.edu.
Upside-Down Design Expands Wide-Spectrum Super-Camera Abilities
Duke University, Pratt School of Engineering, Durham, NC
y turning a traditional lab-based
fabrication process upside down,
researchers at Duke University have
greatly
B
expanded
the
abilities
of
light-manipulating metasurfaces while
also making them much more robust
against the elements.
The combination could allow these
quickly maturing devices to be used in
a wide range of practical applications,
such as cameras that capture images in a
broad spectrum of light in a single shutter
snap.
Plasmonics is a technology that essentially
traps the energy of light in groups
of electrons oscillating together on a
metal surface. This creates a small but
powerful electromagnetic field that interacts
with incoming light.
Traditionally, these groups of electrons
- called plasmons - have been
excited on the surfaces of metal nanocubes.
By controlling the size of the
nanocubes and their spacing from each
other as well as the metal base below, the
system can be tuned to absorb specific
wavelengths of light.
These so-called plasmonic metasurfaces
consist of three layers - a metal
base is coated in a nanometer-thin
transparent substrate and topped with
silver nanocubes. While this configuration
has worked well for laboratory
demonstrations, it leaves little room
for creativity. Because an area of the
nanoparticle must be within a few
nanometers of the metal surface below,
researchers couldn't use a wide variety
of shapes.
To get around this need for flatness,
the team decided to try to put each
nanoparticle in its own dimple or well.
This would surround the entire lower
halves of the nanoparticles with metal,
allowing the sides as well as the bottoms
to host plasmons. But because of incredibly
tight tolerances, this is easier said
than done.
To meet this challenge, the researchers
essentially flipped the traditional
fabrication process upside down. Rather
than starting with a metal surface and
putting a thin transparent substrate on
top followed by nanocubes, they start
with the nanocubes, which they cover
with a precisely thin spacer coating that
follows the underlying shape, and top
off with a metal coating. It's almost like a
pineapple upside-down cake, where the
nanocubes are the pineapples that get
covered in caramelized sugar and baked
into a thin bottom.
Because more than one surface of the
nanocubes could now trap plasmons between
gaps, they could experiment in 3D
with new nanoparticle shapes. In their
paper, the team tried out solid spheres
and cuboctahedra - a shape consisting
of eight triangular faces and six square
Sputter Metal
Assemble NPs
NP
Cubes
Cuboctahedra
Spheres
Transparent
Substrate
A new upside-down fabrication method for light-manipulating metamaterials envelops nanoparticles with a transparent spacing layer followed by a coating
of metal. The way the metal coating envelops part of the nanoparticle while maintaining tight, nanometer tolerances, allows for a much larger design space
than was previously possible. (Image: Duke University)
26
Photonics & Imaging Technology, September 2022
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Tech Briefs Magazine - September 2022

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