Tech Briefs Magazine - November 2021 - PIT-24

member Mikhail Shalaginov. " That's why
having this kind of platform is like a holy
grail for optical engineers, that allows
[the metalens] to switch focus efficiently
and over a large range. "
In conventional lenses, glass is precisely
curved so that an incoming light
beam refracts off the lens at various
angles, converging at a point a certain
distance away, known as the lens' focal
length. The lenses can then produce a
sharp image of any objects at that particular
distance. To image objects at a
different depth, the lens must be physically
moved.
Rather than relying on a material's
fixed curvature to direct light, the
researchers looked to modify the GSSTbased
metalens in a way that the focal
length changes with the material's phase.
In their study, they fabricated a 1-micronthick
layer of GSST and created a " metasurface "
by etching the GSST layer into
microscopic structures of various shapes
that refract light in different ways.
According to Gu, " By knowing how
the material will behave, we can design a
specific pattern which will focus at one
point in the amorphous state and
change to another point in the crystalline
phase. "
They tested the new metalens by placing
it on a stage and illuminating it with
a laser beam tuned to the infrared band
of light. At certain distances in front of
the lens, they placed transparent objects
composed of double-sided patterns of
horizontal and vertical bars, known as
resolution charts, which are typically
used to test optical systems. The lens, in
its initial, amorphous state, produced a
sharp image of the first pattern. The
team then heated the lens to transform
the material to a crystalline phase. After
the transition, and with the heating
source removed, the lens produced an
equally sharp image, this time of the second,
farther set of bars.
These experiments show how a meta -
lens can actively change focus without
any mechanical motions. The re search -
ers say that a metalens could be potentially
fabricated with integrated microheaters
to quickly heat the material with
short millisecond pulses. By varying the
heating conditions, they can also tune to
intermediate states, enabling continuous
focal tuning.
For more information, contact Abby
Abazorius at abbya@mit.edu.
Making Measurements with a Fine-Toothed Comb
National Institute of Standards and Technology (NIST), Gaithersburg, MD
o many people, a measurement
sounds mundane, like marking ticks
on a ruler or reading the line on a thermometer.
It's a piece of data. And they
tend to think that improved measurements
look like finer and finer ticks on a
ruler. But making new measurements is
more than just making finer marks on a
ruler. To measure something is to understand
it, pull it apart and see how it
works. New measurements can unlock
possibilities that even scientists never
thought of when they started out.
Perhaps there is no better example than
the optical frequency comb. Very simply,
this device is a ruler for light. Yet it's so
much more than a ruler.
T
Radio waves, microwaves, visible light,
X-rays, and infrared are all part of the
spectrum of electromagnetic frequencies.
They're all waves, traveling at the
speed of light, but the distance between
the peaks of those waves can be kilometers
apart, like some radio waves, or
nanometers apart, like visible light and
ultraviolet.
In the 1970s, scientists at the National
Institute of Standards and Technology
(NIST) were stuck. They wanted more
precise and accurate atomic clocks, ones
based on the very high optical frequencies
of light released by atoms as their
electrons jump between energy states, as
opposed to the lower microwave frequencies
they were using. Better clocks
24
Cov
ToC
Optical frequency combs allow scientists to measure light - and our world - with great precision
and accuracy. This device has led to innovations that scientists never imagined when it was created.
(Credit: J. Wang/NIST)
would give them a more precise definition
of the second. A more precise second
would give them a better definition
of the meter, which is the distance light
travels in a vacuum in a tiny fraction of a
second. But all that relied on being able
to measure these frequencies of light
accurately and precisely.
There was a gap in measurement
between the two ends of the electromagnetic
spectrum. Scientists could measure
radio and microwave frequencies accurately,
but there were no electronics that
could count fast enough to keep up with
the atom's optical frequencies. They
could use a laser with a matching frequency
to read the atom's optical frequency.
Scientists had lasers with known,
exact frequencies, but they could only
produce a single frequency or color.
Without knowing the exact frequency of
the atom, finding the right frequency
laser to read the atom would take a lot of
trial and error. NIST scientists tried
daisy-chaining several lasers of different
frequencies together to make a rudimentary
optical ruler. That worked well
enough to redefine the meter but wasn't
a long-term solution.
Enter the frequency comb, a Nobel
Prize-winning device, and the result of
decades of research from NIST and othPhotonics
& Imaging Technology, November 2021

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Tech Briefs Magazine - November 2021

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