Strobe light for 5G: Imaging system spotlights the tiny mechanical
hearts at the core of every cellphone
Movies of minuscule vibrations reveal how well 5G and other mobile
networks are operating

Date:
February 4, 2022
Source:
National Institute of Standards and Technology (NIST)
Summary:
Researchers have developed an instrument to image the acoustic
waves generated by micromechanical resonators over a wide range of
frequencies and produce 'movies' of them with unprecedented detail.



FULL STORY ========================================================================== Inside 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.


==========================================================================
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, or billions of cycles per second) 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.



==========================================================================
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 deliberately chose a
reference laser 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.

The slowdown, which converts acoustic vibrations that oscillate at
GHz frequencies to megahertz (MHz, millions of cycles per second),
is important because the light detectors used by the NIST team 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 (millionths of a meter) 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 less efficient and lead to loss of information in
quantum acoustic systems. By pinpointing problematic areas, the technique
gives scientists the information they need to improve resonator design.

In the Feb. 4, 2022, edition of Nature Communications, the researchers
reported that they could image acoustic vibrations that have an amplitude (height) as small as 55 femtometers (quadrillionths of a meter), about one-five-hundredth the diameter of a hydrogen atom.

Over the past decade, physicists have suggested that micromechanical
resonators in this frequency range may also serve to store fragile quantum information and to transfer the data from one part of a quantum computer
to another.

Establishing an imaging system that can routinely measure micromechanical resonators for these applications will require further research. But
the current study is already a milestone in assessing the ability of micromechanical resonators to accurately perform at the high frequencies
that will be required for effective communication and for quantum
computing in the near future, Gorman said.

========================================================================== Story Source: Materials provided by National_Institute_of_Standards_and_Technology_(NIST).

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Lei Shao, Vikrant J. Gokhale, Bo Peng, Penghui Song, Jingjie Cheng,
Justin Kuo, Amit Lal, Wen-Ming Zhang, Jason J. Gorman. Femtometer-
amplitude imaging of coherent super high frequency vibrations in
micromechanical resonators. Nature Communications, 2022; 13 (1)
DOI: 10.1038/s41467-022-28223-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220204161713.htm

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