Assistant professor of physics Swastik Kar and associate professor of
mechanical and industrial engineering Yung Joon Jung are the first to
create a device that integrates both optical and electronic signals to
perform the most elementary computational operations. Credit: Image courtesy of Northeastern University
Every second, your computer must process billions of computational
steps to produce even the simplest outputs. Imagine if every one of
those steps could be made just a tiny bit more efficient. "It would
save precious nanoseconds," explained Northeastern University
assistant professor of physics Swastik Kar. Kar and his colleague Yung Joon Jung, an associate professor in the
Department of Mechanical and Industrial Engineering, have developed
a series of novel devices that do just that. Their work was published
Sunday in the journal Nature Photonics. Last year, the interdisciplinary duo combined their expertise --
Kar's in graphene, a carbon-based material known for its strength and
conductivity, and Jung's in the mechanics of carbon nanotubes, which
are nanometer-sized rolled up sheets of graphene -- to unearth a
physical phenomenon that could usher in a new wave of highly
efficient electronics.
They discovered that light-induced electrical currents rise
much more sharply at the intersection of carbon nanotubes and
silicon, compared to the intersection of silicon and a metal, as in
traditional photodiode devices. "That sharp rise helps us design
devices that can be turned on and off using light," Kar said.
This
finding has major implications for performing computations,
which, in simple terms, also rely on a series of on-off switches. But
in order to access the valuable information that can be stored on
these switches, it must also be transferred to and processed by other
switches. "People believe that the best computer would be one in which
the processing is done using electrical signals and the signal
transfer is done by optics," Kar said.
This isn't too surprising
since light is extremely fast. Kar and Jung's devices -- which are the
first to integrate electronic and optical properties on a single
electronic chip -- represent a critical breakthrough in making this
dream computer a reality.
The computational modeling of
these junctions were performed in close collaboration with the
group of Young-Kyun Kwon, a professor at Kyung Hee University, in
Seoul, Korea.
In the new paper, the team presents three such new
devices. The first is a so-called AND-gate, which requires both an
electronic and an optical input to generate an output. This switch
only triggers if both elements are engaged.
The second device,
an OR-gate, can generate an output if either of two optical sensors
is engaged. This same configuration can also be used to convert
digital signals into analog ones, an important capability for
actions such as turning the digital content of an MP3 file into actual
music.
Finally, Kar and Jung also built a device that works like
the front-end of a camera sensor. It consists of 250,000 miniature
devices assembled over a centimeter-by-centimeter surface. While
this device would require more integration to be fully viable, it
allowed the team to test the reproducibility of their assembly process.
"Jung's
method is a world-class technique," Kar said. "It has really enabled
us to design a lot of devices that are much more scalable."
While
computers process billions of computational steps each second,
improving their capability of performing those steps, Kar said, begins
with the "demonstration of improving just one." Which is exactly what
they've done.
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