Tech Briefs Magazine - May 2022 - PIT-25

5G Strobe Light: NIST Imaging System Spotlights the Tiny Mechanical
Hearts at the Core of Every Cellphone
National Institute of Standards and Technology, Gaithersburg, MD
nside every cellphone lies a
tiny mechanical heart, beating
several billion times a second.
These micromechanical
resonators play an essential
role in cellphone communication.
Buffeted by the cacophony
of radio frequencies in
the airwaves, these resonators
select just the right frequencies
for transmitting and receiving
signals between mobile
devices.
I
that pulses between 20 and
250 times more slowly than the
frequency at which the micromechanical
resonator vibrates.
That strategy enabled the laser
pulses illuminating the resonator
to, in effect, slow down the
acoustic vibrations, similar to
the way that a strobe light appears
to slow down dancers in
a nightclub.
With the growing importance
of these resonators,
scientists need a reliable and
efficient way to make sure the
devices are working properly.
That's best accomplished by
carefully studying the acoustic
waves that the resonators
generate.
Now, researchers at the National
Institute of Standards
and Technology (NIST) and
their colleagues have developed
an instrument to image these
acoustic waves over a wide range of frequencies
and produce " movies " of them
with unprecedented detail.
The researchers measured acoustic
vibrations as rapid as 12 gigahertz
(GHz) and may be able to extend those
measurements to 25 GHz, providing
the necessary frequency coverage for
5G communications as well as for potentially
powerful future applications in
quantum information.
The challenge of measuring these
acoustic vibrations is likely to increase as
5G networks dominate wireless communications,
generating even tinier acoustic
waves.
The new NIST instrument captures
these waves in action by relying on a
device known as an optical interferometer.
The illumination source for this interferometer,
ordinarily a steady beam
of laser light, is in this case a laser that
pulses 50 million times a second, which
is significantly slower than the vibrations
being measured.
Photonics & Imaging Technology, May 2022
Measured vibrational mode shape on the surface of a 2.35 GHz bulk
acoustic resonator. (Photo: Shao, J. Gorman / NIST)
The laser interferometer compares
two pulses of laser light that travel
along different paths. One pulse travels
through a microscope that focuses the
laser light on a vibrating micromechanical
resonator and is then reflected back.
The other pulse acts as a reference,
traveling along a path that is continually
adjusted so that its length is within a
micrometer (one millionth of a meter)
of the distance traveled by the first pulse.
When the two pulses meet, the light
waves from each pulse overlap, creating
an interference pattern - a set of dark
and light fringes where the waves cancel
or reinforce one another. As subsequent
laser pulses enter the interferometer,
the interference pattern changes as the
microresonator vibrates up and down.
From the changing pattern of the fringes,
researchers can measure the height
(amplitude) and phase of the vibrations
at the location of the laser spot on the
micromechanical resonator.
NIST researcher Jason Gorman and
his colleagues chose a reference laser
The slowdown, which converts
acoustic vibrations that
oscillate at GHz frequencies
to megahertz (MHz), is important
because the light detectors
operate much more
precisely, with less noise, at
these lower frequencies.
" Moving to lower frequencies
removes interference from
communication signals typically
found at microwave frequencies
and allows us to use photodetectors
with lower electrical
noise, " said Gorman.
Each pulse lasts only 120 femtoseconds
(quadrillionths of a second), providing
highly precise moment-to-moment
information on the vibrations.
The laser scans across the micromechanical
resonator so that the amplitude
and phase of the vibrations can be
sampled across the entire surface of the
vibrating device, producing high-resolution
images over a wide range of microwave
frequencies.
By combining these measurements, averaged
over many samples, the researchers
can create three-dimensional movies
of a microresonator's vibrational modes.
Two types of microresonators were used
in the study; one had dimensions of 12
micrometers by 65 micrometers; the other
measured 75 micrometers on a side -
about the width of a human hair.
Not only can the images and movies
reveal whether a micromechanical resonator
is operating as expected, they can
also indicate problem areas, such as places
where acoustic energy is leaking out of
the resonator. The leaks make resonators
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