data drove itself around in cars, photonics would
be a roomy minivan and electronics would be a nimble
coupe. Photonic components such as fiber optic cables
can carry a lot of data but are bulky compared to
electronic circuits. Electronic components such as
wires and transistors carry less data but can be incredibly
problem holding back the progress of computing is
that with mismatched capacities and sizes, the two
technologies are hard to combine in a circuit. Researchers
can cobble them together, but a single technology
that has the capacity of photonics and the smallness
of electronics would be the best bridge of all. A
new research group in Stanford`s School of Engineering
is pioneering just such a technology-plasmonics.
plasmons are density waves of electrons-picture bunches
of electrons passing a point regularly-along the surface
of a metal. Plasmons have the same frequencies and
electromagnetic fields as light, but their sub-wavelength
size means they take up less space. Plasmonics, then,
is the technology of transmitting these light-like
waves along nanoscale wires.
every wave you can in principle carry information,``
says Mark Brongersma, assistant professor of materials
science and engineering and head of the new plasmonics
Multidisciplinary University Research Initiative (MURI),
which earlier this month received an additional $300,000
round of funding from the Air Force Office of Sponsored
Research (AFOSR). ``Plasmon waves are interesting
because they are at optical frequencies. The higher
the frequency of the wave, the more information you
can transport.`` Optical frequencies are about 100,000
times greater than the frequency of today`s electronic
research is a prime example of work at the forefront
of two strategic initiatives of the School of Engineering:
information technology and photonics, and nanoscience
and nanotechnology. The school`s other two initiatives
are in bioengineering, and environment and energy.
by a $2.3 million grant received last October from
the AFOSR, the goal of the MURI is to demonstrate
plasmonics in action on a standard silicon chip. Brongersma
and about 20 MURI partners, including David A. B.
Miller, the W. M. Keck Foundation Professor of Electrical
Engineering, and electrical engineering Assistant
Professor Shanhui Fan, have made working plasmonic
components and have had their first journal article
accepted for publication in an upcoming issue of Optics
Letters. The next step will be to integrate the components
with an electronic chip to demonstrate plasmonic data
generation, transport and detection. A success would
be the first of its kind anywhere.
are generated when, under the right conditions, light
strikes a metal. The electric field of the light jiggles
the electrons in the metal to the light`s frequency,
setting off density waves of electrons. The process
is analogous to how the vibrations of the larynx jiggle
molecules in the air into density waves experienced
waves behave on metals much like light waves behave
in glass, meaning that plasmonic engineers can employ
all the same ingenious tricks-such as multiplexing,
or sending multiple waves-that photonic engineers
use to cram more data down a cable.
because plasmonic components can be crafted from the
same materials chipmakers use today, Stanford engineers
are hopeful they can make all the devices needed to
route light around a processor or other kind of chip.
These would include plasmon sources, detectors and
wires, which the lab already has made, as well as
splitters and even transistors.
an all-plasmonic chip might be feasible someday, Brongersma
expects that in the near term, plasmonic wires will
act as high-traffic freeways on chips with otherwise
conventional electronics. Local arrays of electronic
transistors would carry out the switching necessary
for computation, but when a lot of data needs an express
lane to travel from one section of a chip to another,
electronic bits could be converted to plasmon waves,
sent along a plasmonic wire and converted back to
electronic bits at their destination.
potential of plasmonics right now is mainly limited
by the fact that plasmons typically can travel only
several millimeters before they peter out. Chips,
meanwhile, are typically about a centimeter across,
so plasmons can`t yet go the whole distance.
distance a plasmon can travel before dying out is
a function of several aspects of the metal. But for
optimal transfer through a wire of any metal, the
surface of contact with surrounding materials must
be as smooth as possible and the metal should not
most wavelengths of visible light, aluminum allows
plasmons to travel farther than other metals such
as gold, silver and copper. It is somewhat ironic
that aluminum is the best metal to use because the
semiconductor industry recently dumped aluminum in
favor of copper-the better electrical conductor-as
its wiring of choice. Of course, it may turn out that
some kind of alloy will have even better plasmonic
properties than either aluminum or copper.
classic semiconductor industry issue that MURI researchers
will have to address is heat. Chipmakers are constantly
battling to ensure that their electronic chips don`t
run too hot. Plasmonics also will likely generate
some heat, but exactly how much is not yet known.
Even if plasmonics run as hot as electronics, Brongersma
points out, they will still have the advantage of
having a higher data capacity in the same space.
face fundamental physical barriers to their data-carrying
capacity, but the demands placed on them never seem
to stop. ``There is a great need for transporting
more information around on chips,`` Brongersma says.
Orenstein is the communications and public relations
manager for the Stanford School of Engineering.
Brongersma, Materials Science and Engineering: (650)
PLASMON MULTIDISCIPLINARY UNIVERSITY RESEARCH INITIATIVE