Tech Briefs Magazine - March 2022 - PIT-22

eters, which produce images in the microscope
very similar to those of microdroplets.
Indeed, the scientists found
that when they used the plastic spheres
to calibrate their measurements of image
boundaries, the microdroplet volume
derived from microscopy precisely
matched that from gravimetry. (The researchers
found that the knife edges resulted
in a poorer match.) The scientists
also calibrated several other aspects of
the optical microscope, including focus
and distortion, maintaining the links to
the SI throughout.
With these improvements, optical microscopy
resolved the volume of microdroplets
to one trillionth of a liter. The
standards and calibrations are practical
and can be applied to many types of optical
microscopes employed in basic and
applied research, the researchers noted.
In fact, the less advanced the microscope
optics, the more a microscopy measurement
can benefit from standards and
calibrations to improve the accuracy of
image analysis.
In their main experiment, the researchers
used a printer to shoot a jet
of microdroplets of cyclopentanol, a viscous
alcohol that evaporates slowly. They
precisely controlled the jet to produce a
known number of microdroplets. As the
jet of microdroplets flew from the printer
into a container a few centimeters
away, they were backlit and imaged with
the optical microscope. The researchers
then weighed the container and its accumulation
of many microdroplets.
With the optical microscope calibrated
and checked by comparing it with the
gravimetry method, the team embarked
on another experiment, replacing the
cyclopentanol with water microdroplets
containing nanoparticles of polystyrene,
which are common but unofficial standards
for nanoplastic analysis. This system
more closely resembles the type of
sample that many scientists are interested
in, for instance in studying plastic pollution.
The researchers used the printer
to deposit rows of individual water microdroplets
on a surface one at a time.
After landing on the surface, the water
microdroplets evaporated, leaving behind
the nanoparticles. The team then
counted the nanoparticles, which were
labeled with a fluorescent dye. In this
way, the team recorded the number of
particles suspended within the volume
of each microdroplet, which provides
a measure of concentration. This measurement
is both a way to sample the
bulk liquid and study the properties of
microdroplets containing small numbers
of nanoparticles.
Using this method and an illumination
system that is faster than the one
employed by the team, scientists would
have the capability of measuring the volume,
motion and contents of a spray or
cloud of microdroplets, the researchers
said. Such measurements could play a
key role in future studies for epidemiological,
environmental, and industrial
applications.
For more information, contact Samuel L.
Davis at samuel.stavis@nist.gov.
New Device Modulates Visible Light with the Smallest Footprint and
Lowest Power Consumption
Columbia University, New York, NY
O
ver the past several decades, researchers
have moved from using
electric currents to manipulating light
waves in the near-infrared range for telecommunications
applications such as
high-speed 5G networks, biosensors on
a chip, and driverless cars. This research
area, known as integrated photonics, is
fast evolving, and investigators are now
exploring the shorter - visible - wavelength
range to develop a broad variety
of emerging applications. These include
chip-scale light detection and ranging
(LiDAR), augmented/virtual/mixed reality
(AR/VR/MR) goggles, holographic
displays, quantum information processing
chips, and implantable optogenetic
probes in the brain.
The one device critical to all these applications
in the visible range is an optical
phase modulator, which controls
the phase of a light wave, similar to how
the phase of radio waves is modulated
in wireless computer networks. With a
phase modulator, researchers can build
an on-chip optical switch that channels
light into different waveguide ports.
With a large network of these optical
switches, researchers could create sophisticated
integrated optical systems
that could control light propagating on
a tiny chip.
A visible-spectrum phase modulator (the ring at the center of a radius of 10 microns) is tinier than a
butterfly wing scale. (Photo: Heqing Huang and Cheng-Chia Tsai/Columbia Engineering)
22
But phase modulators in the visible
range are very hard to make: there
are no materials that are transparent
enough in the visible spectrum while
also providing large tunability, either
through thermo-optical or electro-optical
effects. Currently, the two most suitable
materials are silicon nitride and
lithium niobate.
Photonics & Imaging Technology, March 2022
PIT Tech Briefs 0322_2.indd 22
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Tech Briefs Magazine - March 2022

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