Tech Briefs Magazine - November 2021 - PIT-22

Nanoscale Machines Convert Light into Work
Optica, Washington, DC
esearchers at the University of Chicago
have developed a tiny new machine
that converts laser light into work. These
optically powered machines self-assemble
and could be used for nanoscale manipulation
of tiny cargo for applications such
as nanofluidics and particle sorting. It
has been a long-standing goal in the
nanoscience community to create selfassembling
nanoscale machines that can
perform work in conventional environments
such as room temperature liquids.
The machines are based on a type of
matter known as optical matter in which
metal nanoparticles are held together by
light rather than the chemical bonds that
hold together the atoms that make up
typical matter.
Both the energy for assembling the
machine and the power to make it work
come from light. Once the laser light is
introduced to a solution containing
nanoparticles, the entire process occurs
on its own. Although the user does not
a)
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b)
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need to actively control or direct the outcome,
this could readily be done to tailor
the machines for various applications,
according to the researchers.
In optical matter, a laser light field creates
interactions between metal nanoparticles
that are much smaller than the
wavelength of light. These interactions
cause the particles to self-assemble into
ordered arrays. This is a similar principle
to optical trapping, in which light is used
to hold and manipulate particles, biological
molecules, and cells.
In previous work, the researchers discovered
that when optical matter is
exposed to circularly polarized light, it
rotates as a rigid body in a direction opposite
to the polarization rotation. In other
words, when the incident light rotates one
way the optical matter array responds by
spinning the other. This is a manifestation
of " negative torque. " The researchers
speculated that a machine could be developed
based on this new phenomenon.
c)
In the new work, they created an optical
matter machine that operates much like a
mechanical machine based on interlocking
gears. In such machines, when one
gear is turned, a smaller interlocking gear
will spin in the opposite direction. The
optical matter machine uses circularly
polarized light from a laser to create a
nanoparticle array that acts like the larger
gear by spinning in the optical field. This
" optical matter gear " converts the circularly
polarized light into orbital, or angular,
momentum that influences a nearby
probe particle to orbit the nanoparticle
array (the gear) in the opposite direction.
The researchers made two machines
based on this design using laser light with
a 600-nanometer wavelength and sliver
nanoparticles just 150 nanometers in
diameter in water. They found that using a
gear made of eight nanoparticles created a
more efficient machine than a sevennanoparticle
gear, suggesting that the
machine's efficiency could be altered by
building different gears.
The researchers said that their simulations
show that a much larger machine
made of many more particles should be
able to exert more power to the probe, so
that is an aspect of refinement that they
anticipate pursuing.
-π
phase
Researchers created an optical matter machine that operates much like a mechanical machine in which
if one gear is turned, a smaller interlocking gear will spin in the opposite direction (a). The optical matter
machine (b) uses circularly polarized light to create a nanoparticle array that acts like the larger gear by
spinning in the optical field. This makes a probe particle - analogous to the second smaller gear - orbit
the nanoparticle array in the opposite direction. (Credit Norbert F. Scherer, University of Chicago)
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They are now experimenting with making
machines with many more particles or
with particles of different materials. The
practicality of the machine could also be
improved by creating patterned gears
where the nanoparticles are immobile.
This would enable the ability to optically
address and combine several gears to
make a more complex machine.
For more information, contact Norbert F.
Scherer at nfschere@uchicago.edu
New " Metalens " Shifts Focus Without Tilting or Moving
Massachusetts Institute of Technology (Cambridge, MA)
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olished glass has been at the center of
imaging systems for centuries. Their
precise curvature enables lenses to focus
light and produce sharp images, whether
the object in view is a single cell, the page
of a book, or a far-off galaxy. Changing
focus to see clearly at all these scales typically
requires physically moving a lens,
by tilting, sliding, or otherwise shifting
the lens. This is usually done with the
help of mechanical parts that add to the
bulk of microscopes and telescopes.
Now MIT engineers have fabricated a
tunable " metalens " that can focus on
objects at multiple depths, without
changes to its physical position or shape.
The lens is made not of solid glass, but
of a transparent " phase-changing " material
that, after heating, can rearrange its
atomic structure and thereby change the
way the material interacts with light.
The researchers etched the material's
surface with tiny, precisely patterned
structures that work together as a " metasurface "
to refract or reflect light in
unique ways. As the material's property
changes, the optical function of the metaPhotonics
& Imaging Technology, November 2021

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Tech Briefs Magazine - November 2021

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