IEEE Signal Processing - May 2018 - 12

louiSE Connor/univErSity of SydnEy

emits light at 1.1 nm. "We are trying other
layered semiconductor materials and heterostructures to explore other frequency
ranges for optical communications," Bie
says. "We are also trying to improve the
light source and coupling efficiency by
integrating with nanocavities."
"A great deal of work still needs to
be done before the technology can be
developed into a commercial product,"
Bie notes. The researchers are also continuing to investigate numerous other
materials that could be used for on-chip
optical communication.

Light stored as sound
In an effort directed at developing a new
generation of photonic integrated circuits,
Figure 2. University of Sydney research fellow Birgit Stiller (left) and Ph.D. student Moritz Merklein
University of Sydney researchers say they
in their University of Sydney Nanoscience Hub laboratory. The duo are part of a team investigating
have discovered a method of storing lightthe storage of light-based information as sound waves on a computer chip.
based information as sound waves on a
computer chip (Figure 2).
To
verify
the
photodetector's
high"The transfer of light to acoustic inforpotentially less thermal quenching at
speed response, the researchers used an
mation on a chip, and using an acoustic
room temperatures," Bie explains.
electro-optical modulator to modulate the
buffer to park photonic information in a
The proof-of-concept device marks the
incident light signal as a way to observe
sound wave for later retrieval, is a necesfirst time that an electrical-powered light
the device's response limit. "Although,
sary step to enable a shift from convensource made of two-dimensional mateeventually, we were limited by the circuit
tional electronic computers to light-based
rial has integrated onto a passive silicon
response speed-not the MoTe2 response
systems capable of moving data at the
photonic crystal waveguide. Compared
speed of light," states Birgit Stiller, a
to traditional light sources, such as III-V
speed itself-this information has demresearch fellow at the University of Sydsemiconductor lasers that are usually waferonstrated a high-speed detector that has
ney and the project's supervisor. "The
bonded on the photonic circuits, the transfer
not been [previously] achieved in this
achievement of our research is that we
of MoTe2 enables smaller footprint devices.
kind of device," Bie says. The team also
were able to transfer the information carused a current amplifier to improve the
To use the material as a light emitter,
ried by light waves coherently to sound
photocurrent signal level in the experithe researchers first had to convert it into
waves on this chip and back again while
ment. "This is crucial in small photocura p-n junction diode, a device in which
preserving the properties of the optical
rent detection," she notes.
one side, the p side, is positively charged,
waves, such as frequency and bandwidth,"
"High-speed photocurrent measurewhile the other, the n side, is negatively
she says. "This leads to short-term storage
ment is the biggest problem that we've
charged. Because the emission waveof the information as
had to overcome so
length from MoTe2 falls outside of the silfar," Bie says. "We
icon absorption range, silicon absorption
To verify the photodetector's sound waves, which
t r ave l a t a s p e e d
need to look for the
loss is dramatically reduced. "We stacked
high-speed response,
100,000 times slowcorrect amplifier that
the bi-layer MoTe2 p-n junction and a silithe researchers used an
er than light waves."
matches the device
con photonic crystal waveguide together,"
electro-optical modulator to
While light is an
impedance, does not
Bie says. The light emitted from the lightmodulate the incident light
excellent information
introduce delay in
emitting diode can go through the wavesignal as a way to observe
carrier, highly usef ul
the detection, and, at
guide to another place; and vice versa, the
for transporting data
the same time, the
p-n junction can detect the light coming
the device's response limit.
over long distances,
gain should be large
from the waveguide.
its speed advantage can actually become
enough," she remarks.
Once the diode was created, the rea hindrance when information is being
In its current form, MoTe2 is suitsearchers were able to run a current
processed within computers and telethrough the device, making it emit light.
able for use in the silicon chips used by
communication systems. "Data centers,
By simply reversing the voltage polarcomputers. It is not yet compatible with
such as the ones from Google, Microsoft,
ity, the device may also be operated as a
telecommunications systems, though.
Amazon, and Facebook, are currently
photodetector, enabling it to both transMost telecom networks require light with
suffering from high energy consumption
mit and receive optical signals.
a wavelength of 1.3 or 1.5 nm. MoTe2

IEEE Signal Processing Magazine

May 2018

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