YSM Issue 90.1

FOCUS
physics
IMAGE COURTESY OF HONG TANG
►Professor Hong Tang (right), along with graduate students Chang-Ling Zou (left) and Xu Han
(not pictured), developed a piezo-optomechanical resonator that has applications to quantum
memory storage.
move, the object responds by changing
shape physically. In this way, vibrations
in physical objects and electrical fields
can easily be connected, or, as physicists
say, coupled. Piezoelectrics in smartphones
often power tiny speakers— they
convert electrical signals into sound
waves, which arise from physical pulses.
The Yale piezo-optomechanical device
consists of a pair of tiny resonators:
a silicon wafer and a wire loop situated
above it. “It is useful to think of a resonator
like a tuning fork because it responds
most powerfully to a particular resonant
frequency,” said Han, an electrical engineering
Ph.D candidate who worked on
the project. The two ends of the wire
loop do not quite connect, so electrical
charges tend to bounce back and forth
around the circle, which functions as an
electrical resonator in the microwave region
of the electromagnetic spectrum.
The wafer, which is about as thick as five
sheets of paper, functions as an acoustic,
or mechanical, resonator. This resonator
is coated with a thin layer of aluminum
nitride, a piezoelectric material,
which facilitates the exchange of oscillations—and
energy— between mechanical
and electrical components. “If you
want to transfer information between
two systems, it is necessary to have an
efficient coupling mechanism,” Han said.
The idea of coupling between mechanical
and microwave electrical domains
is not new; the Yale team’s innovation
is achieving stronger coupling on a
smaller scale. The key is using resonators
with a higher frequency: Whereas
other designs have used frequencies on
the order of a few million oscillations
per second, the Yale design runs at ten
billion oscillations per second. As a result,
the device is solidly in the so-called
strong-coupling regime —meaning that
the rate of information transfer is greater
than the natural energy dissipation
rates of the individual systems—and
transmitted signals are clearer and longer-lasting.
Yet high frequency comes at
the cost of increased construction difficulties.
“Since the device is small, it
is more susceptible to perturbations in
the environment,” Tang said. As a result,
the design carefully balances considerations
of compactness and robustness.
The researchers believe that applications
of their breakthrough lie mostly in the far
future. “This is fundamental research, so
it’s not immediately pertinent to daily life,”
Han said. Instead, the piezo-optomechanical
resonator’s real value is as a component
of more complex systems. Because of the
strong coupling achieved, it is well suited
for quantum uses where “noise” from ambient
heat (analogous to TV static) would
otherwise be disruptive. “For high-frequency
devices, the temperature requirement is
not as low,” Han said. Chang-Ling Zou, a
postdoctoral student in Tang’s lab, hopes to
develop this strength into a basis for quantum
memory storage, which is currently
unfeasible at most temperatures. Small
vibrating crystals would serve as physical
memory, and the resonators would convert
between these crystals and the computational
part of the computer, which would
likely operate in the microwave domain.
The Yale team is also looking to incorporate
visible light into their design. “The
next step is integrating an optical resonator
and using the acoustic resonator
as an intermediary between microwave
and optics,” Han said. Accomplishing
this feat could improve computer signal
processing, radio receiving efficiency,
and information transmission across
long distances via optical fiber cables.
Given its versatility, the piezo-optomechanical
resonator may find its way into
all kinds of applications. From analyzing
the stock market to sending trans-Atlantic
messages, you can expect to hear more about
this small device in big-time situations.
ABOUT THE AUTHOR
NOAH KRAVITZ
NOAH KRAVITZ is a freshman in Calhoun College. He is interested in
studying math, music, physics and philosophy.
THE AUTHOR WOULD LIKE TO THANK Xu Han, Chang-Ling Zou, and
Professor Hong Tang for explaining their research.
FURTHER READING
Li, Mo, W.H.P. Pernice, and H.X. Tang. “Ultrahigh-Frequency Nano-
Optomechanical Resonators in Slot Waveguide Ring Cavities.” Applied
Physics Letters 07 (2010).
24 Yale Scientific Magazine December 2016 www.yalescientific.org