Ultralow-power optical information processing is based on graphene on silicon photonic crystal nanomembranes. Credit: Nicoletta Barolini

New
research by Columbia Engineering demonstrates remarkable optical
nonlinear behavior of graphene that may lead to broad applications in
optical interconnects and low-power photonic integrated circuits. With
the placement of a sheet of graphene just one-carbon-atom-thick, the
researchers transformed the originally passive device into an active one
that generated microwave photonic signals and performed parametric
wavelength conversion at telecommunication wavelengths.

   

“We
have been able to demonstrate and explain the strong nonlinear response
from graphene, which is the key component in this new hybrid device,”
says Tingyi Gu, the study’s lead author and a Ph.D. candidate in
electrical engineering. “Showing the power-efficiency of this
graphene-silicon hybrid photonic chip is an important step forward in
building all-optical processing elements that are essential to faster,
more efficient, modern telecommunications. And it was really exciting to
explore the ‘magic’ of graphene’s amazingly conductive properties and
see how graphene can boost optical nonlinearity, a property required for
the digital on/off two-state switching and memory.”

   

The
study, led by Chee Wei Wong, professor of mechanical engineering,
director of the Center for Integrated Science and Engineering, and
Solid-State Science and Engineering, will be published online in the
Advance Online Publication on Nature Photonics’s
website on July 15 and in print in the August issue. The team of
researchers from Columbia Engineering and the Institute of
Microelectronics in Singapore are working together to investigate
optical physics, material science, and device physics to develop
next-generation optoelectronic elements.

   

They
have engineered a graphene-silicon device whose optical nonlinearity
enables the system parameters (such as transmittance and wavelength
conversion) to change with the input power level. The researchers also
were able to observe that, by optically driving the electronic and
thermal response in the silicon chip, they could generate a radio
frequency carrier on top of the transmitted laser beam and control its
modulation with the laser intensity and color. Using different optical
frequencies to tune the radio frequency, they found that the
graphene-silicon hybrid chip achieved radio frequency generation with a
resonant quality factor more than 50 times lower than what other
scientists have achieved in silicon.

   

“We
are excited to have observed four-wave mixing in these graphene-silicon
photonic crystal nanocavities,” says Wong. “We generated new optical
frequencies through nonlinear mixing of two electromagnetic fields at
low operating energies, allowing reduced energy per information bit.
This allows the hybrid silicon structure to serve as a platform for
all-optical data processing with a compact footprint in dense photonic
circuits.”

   

Wong
credits his outstanding students for the exceptional work they’ve done
on the study, and adds, “We are fortunate to have the expertise right
here at Columbia Engineering to combine the optical nonlinearity in
graphene with chip-scale photonic circuits to generate microwave
photonic signals in new and different ways.”

   

Until
recently, researchers could only isolate graphene as single crystals
with micron-scale dimensions, essentially limiting the material to
studies confined within laboratories. “The ability to synthesize
large-area films of graphene has the obvious implication of enabling
commercial production of these proven graphene-based technologies,”
explains James Hone, associate professor of mechanical engineering,
whose team provided the high quality graphene for this study. “But
large-area films of graphene can also enable the development of novel
devices and fundamental scientific studies requiring graphene samples
with large dimensions. This work is an exciting example of
both—large-area films of graphene enable the fabrication of novel
opto-electronic devices, which in turn allow for the study of scientific
phenomena.”

   

Commenting
on the study, Xiang Zhang, director of the National Science Foundation
Nanoscale Science and Engineering Center at the University of California
at Berkeley, says, “this new study in integrating graphene with silicon
photonic crystals is very exciting. Using the large nonlinear response
of graphene in silicon photonics demonstrated in this work will be a
promising approach for ultra-low power on-chip optical communications.”

   

“Graphene
has been considered a wonderful electronic material where electron
moves like an effectively massless particle in the atomically thin
layer,” notes Philip Kim, professor of physics and applied physics at
Columbia, one of the early pioneers in graphene research and who
discovered its low-temperature high electronic conductivity. “And now,
the recent excellent work done by this group of Columbia researchers
demonstrates that graphene is also unique electro-optical material for
ultrafast nonlinear optical modulation when it is combined with silicon
photonic crystal structures. This opens an important doorway for many
novel optoelectronic device applications, such as ultrafast chip-scale
high-speed optical communications.”

This
research is supported by the Columbia Energy Frontier Research Center
program, which is funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, and the Columbia Optics and
Quantum Electronics IGERT (Integrative Graduate Education and Research
Traineeship) program, which is funded by the National Science
Foundation.

Source: Columbia University