Toss a rock into a quiet pond, and watch the ripples spread out across its sur-face. This is pretty much what happens when a photon hits the surface of a
metal — except that in this case, the ‘ripples’
consist of electrons oscillating en masse and
have wavelengths measured in nanometres.
Once they are set in motion, these ‘surface
plasmons’, as the oscillations are known, can
pick up more light and carry it along the metal
surface for comparatively vast distances. “A
river of light” is how Satoshi Kawata, a physi-
cist at Osaka University in Japan, describes the
phenomenon to his students.
Plasmons can also focus light into the
tiniest of spots, direct it along complex circuits
or manipulate it many other ways. And they
can do all of this at the nanoscale — several
orders of magnitude smaller than the light’s
own wavelength, and therefore far below the
resolution limits of conventional optics.
The result is that plasmonics has become one
of the hottest fields in photonics today, with
researchers exploring potential applications in
solar cells, biochemical sensing, optical com-
puting and even cancer treatments (see ‘Plas-
mons at work’).
Their efforts, in turn, have benefited greatly
from the flowering of nanotechnology in gen-
eral over the past decade, which brought with
it a proliferation of techniques for fabricating
structures at the nanoscale — exactly what
plasmonics needed to progress from laboratory
curiosity to practical applications. “The late
1990s was kind of the turn-
ing point” for plasmonics,
says Harry Atwater, a
physicist at the California
Institute of Technology in
Pasadena.
One suprising example
of the light-carrying phe-
nomenon was witnessed
in 1989 by Norwegian-
born physical chemist
Thomas Ebbesen, now at
the Louis Pasteur Univer-
sity in Strasbourg, France. As he held to the
light a thin film of metal containing millions
of nanometre-sized holes, he found that it was
more transparent than he expected. The holes
were much smaller than the wavelength of vis-
ible light, which should have made it almost
impossible for the light to get through at all. “I
first thought, ‘Here was some kind of mistake’,”
says Ebbesen.
But it wasn’t a mistake, although it took
Ebbesen and his colleagues the better part of a
decade to work out what was happening. When
the incoming photons struck the metal film,
they excited surface plasmons, which picked
up the photons’ electromagnetic energy and
carried it through the holes, re-radiating it on
the other side and giving the film its transpar-
ency1.
Hole arrays are increasingly finding their
way into applications, for example as selec-
tive filters for colour sensors. It turns out that
the increased transmission through the sheet
works only for light around the plasmons’
natural oscillation frequency. But this fre-
quency, which is typically in the visible or
near-infrared part of the spectrum, can be
adjusted by changing the geometry of the
holes and their spacing. So hole arrays can be
made into highly selective filters for sensors
that depend on detecting specific colours, or
for efficiently extracting monochromatic light
from light-emitting diodes (LEDs) and lasers.
Indeed, a number of commercial research labs,
such as the Panasonic laboratory in Kyoto,
Japan, and NEC in Tsukuba, Japan are working
on prototypes of plasmon-enhanced devices
for displays and telecommunications.
Hole arrays can also be used to channel
light into optical devices. In imaging chips for
digital cameras, for example, researchers are
studying how hole arrays placed on top of indi-
vidual pixels might help capture incoming light
Light manipulation: surface plasmons could be generated (above) to help
direct light using nanoantennas in devices such as solar cells (left).
Small oscillations of surface electrons that manipulate
light on the nanoscale could be the route to applications
as disparate as faster computer chips and cures for
cancer. Joerg Heber reports.
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125 nm
400 nm
more efficiently, and thus reduce
pixel noise and improve camera
sensitivity.
Another plasmonic technique for channelling
light into a device is to sprinkle its surface
with nanoscale particles made of a metal such
as gold. These nanoparticles function like an
array of tiny antennas: incoming light is taken
up by plasmons and then redirected into the
device’s interior.
Slimming down
From a commercial perspective, perhaps the
most promising application of such nano-
antennas — or indeed, of hole arrays — is in
the improvement of solar cells. Present-day
solar cells are made from semiconductors such
as silicon. But to catch as much light as pos-
sible from the broadest range of wavelengths,
particularly in the red and infrared part of the
spectrum, the semiconductor layer has to be
relatively thick. “Right now a silicon solar cell
is up to 300 micrometres thick,” says Albert
Polman, a photonics researcher who directs
the AMOLF institute in Amsterdam, where
he works on improving solar-cell designs. And
when cells are being deployed in arrays that
cover a rooftop or more, he says, that adds up
to a lot of expensive silicon. The price would
come down a long way if the silicon was only
1 micrometre thick. “But then you don’t catch
the red light because it goes straight
through the chip,” he says, thus wasting
much of the sunlight’s available energy. Other
solar-cell materials have the same problem.
With plasmonics, however, the problem
goes away. In one approach that researchers
are exploring, gold nanoparticles on the sur-
face would act as reflectors that focus light into
the semiconductor, where absorption effi-
ciency increases with the light concentration.
In another scheme, tiny gold nanoantennas
could redirect sunlight by 90°, so that it prop-
agates along the semiconductor rather than
passing straight through. Either way, the cell
Although plasmonic effects have
been known for more than a
century, the history of plasmon-
based applications began in
the early 1970s, when Martin
Fleischmann, a chemist at the
University of Southampton, UK,
and others began to study how
light scatters from molecules
stuck to a silver surface7.
Richard Van Duyne, a chemist
at Northwestern University in
Evanston, Illinois, then discovered
this scattering to be enhanced by a
seemingly incredible six orders of
magnitude8.
In today’s optimized devices,
this enhancement, known
as surface-enhanced Raman
spectroscopy (SERS), can be
several orders of magnitude larger
still — strong enough to detect
a single molecule9. Moreover,
SERS has proved very useful in
the biochemical and materials
sciences by providing information
on the chemical composition
of molecules at very small
concentrations.
SERS is a plasmonic effect:
silver nanoparticles act as
antennas that take the incoming
laser light and, through their
surface plasmons, concentrate
it. The concentrated light is then
scattered by nearby molecules
and amplified again by the silver
nanoparticles on the way back
out. This dual amplification
results in a huge overall signal
enhancement.
Some applications have reached
the market. For example, in
specifically prepared colloids of
gold nanoparticles, a clustering of
these nanoparticles is triggered
by the presence of pregnancy
hormones. This leads to a colour
change induced by plasmonic
effects that has been widely
commercialized in pregnancy
tests.
The commercialization of SERS
has been hampered in many areas
by difficulties in achieving highly
accurate control over the surface
nanostructures. For this reason,
researchers are also looking at
other sensing techniques such
as localized surface plasmon
resonance (LSPR). The idea is
that, when a surface is covered
with nanostructures in the
shape of rods or triangles, their
plasmonic properties depend
strongly on the properties of
medium that surrounds them. For
example, a solution containing
a certain type of molecule has
a refractive index that varies
with the concentration of those
molecules10. “These changes
to the refractive index lead to
measurable changes to the
surface plasmon resonance
wavelength, which can be
observed experimentally,” says
Stefan Maier from Imperial
College London, who studies
plasmonic nanostructures and
their applications. “The effects
can be dramatic.” Devices based
on LSPR are becoming so sensitive
that Van Duyne thinks that they,
too, are about to reach the limit of
single-molecule detection.
And at Rice University in
Houston, Texas, biomedical
engineer Naomi Halas is pursuing
an optical technique to destroy
cancer cells. She hopes to
inject cancer patients with gold
nanoparticles that will be guided
to the tumour by antibodies bound
to the particles’ surface. Once the
nanoparticles are in place, she can
illuminate the area with a low dose
of infrared laser light that leaves
healthy tissue undamaged, but
gets absorbed to create plasmons
in the gold. The energy heats up
the nanoparticles and kills the
cancer cells11.
So far, Halas’s cancer therapy
has been successful in trials
with mice, where she achieved
seemingly complete elimination
of the tumours. The technology is
now in human clinical trials with
patients who have head and neck
cancers. Halas says the results
have been very encouraging
so far. “There is no reason one
would expect complications
from something like this in
humans relative to animal trials,
because you are using physical
mechanisms, heat and light, to
induce cell death.” Halas is also
optimistic that the treatment will
be approved for use more quickly
than a drug, which can involve
difficult and expensive trials
and many years to reach the
clinic. She says the technique is
being considered as a ‘device’
by the US Food and Drug
Administration rather than a
drug, which could also accelerate
the approval process. J.H.
Plasmons at work
Naomi Halas (centre, above) wants to use plasmons to fight cancer;
others use them as sensors (inset) to detect single molecules.
J.
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could get by with a much thinner
semiconductor layer.
Even as plasmonic techniques are
decreasing the cost of the cells, they
could also greatly improve the cells’
efficiency at extracting the available
energy from sunlight — in a field in
which even a few percentage points
in efficiency improvement are cele-
brated. Overall, the use of plasmon-
ics could increase the absorption
two to five times, says Atwater, who
has co-founded Alta Devices in Santa
Clara, California, to commercialize
such solar cells. For cells made from
amorphous silicon, which today
have efficiencies of around 10–12%,
the predicted enhancements could
translate into efficiencies of about
17%. For crystalline silicon cells,
which currently have efficiencies
around 20%, the new figure could
approach the theoretical maximum
of 29%. For commercial applications,
the remaining challenges include
developing workable device designs
and fabrication techniques for mass
production.
Guiding light
Plasmonics researchers are also grap-
pling with a longer-term challenge:
the integration of optics and electronics on
a single microchip. The decades-old idea is
that, just as a fibre-optic cable can carry much
more information than a copper wire, a light
beam could, in principle, relay information
through the chip on more channels and at a
higher speed than conventional integrated cir-
cuitry can handle. But the experimental opti-
cal devices produced to date have been too
large, and have showed rather
high losses in the optical signal
strength.
“You want to bring the optics
closer in size to the transistor,”
says Polman. And that’s the
beauty of plasmonics, which
can offer optical pathways on
virtually the same scale as the silicon struc-
tures found in advanced microchips. “Metals
can be well integrated with the chip design,”
says Polman, “so you may be able to distribute
light over an integrated circuit by plasmons.”
Indeed, structures such as silver nanowires2 or
grooves etched into metal surfaces3 can provide
pathways that guide light across a chip in what-
ever direction the designers might need.
But there is a trade-off as the structures get
smaller. If the plasmons are forced to travel
through a channel that’s too narrow, they start
to leak out from the sides and get lost, says
Sergey Bozhevolnyi from the University of
Southern Denmark in Odense, who is leading
a European research project into integrated
plasmonic circuits. Nevertheless, researchers
can guide surface plasmons over distances of
more than 100 μm, which is roughly a thousand
times bigger than the features on a current-
generation microchip. This is enough to open
rich possibilities for plasmonic
nanocircuits, in which light
would carry information along
complex paths and through
many processing steps.
Plasmonic waveguides are
particularly promising if the
light source — typically a laser
— can be incorporated on the chip as well.
This has been done with comparatively large
lasers, on the order of the wavelength of the
laser light. But plasmonics now offers the pos-
sibility of doing so at the nanoscale, at lengths
much shorter than the wavelength. Rather than
amplifying light in a conventional laser cavity,
a plasmonic ‘spaser’ would amplify it with the
help of plasmons — the first experimental
evidence for such plasmon-based lasing was
published in August4,5. To fully integrate these
plasmon lasers into standard microcircuitry,
however, researchers will need to
find a way to trigger the spasers using
standard electrical currents.
In addition to creating light and
guiding it across a chip, optical com-
puting will require a way to turn the
flow of plasmons on and off at high
speeds, so that the flow becomes a
series of bits in a digital data stream.
Many people have been working on
such devices, and a plasmonic modu-
lator based on silicon technology has
been realized by Atwater’s group. Like
a conventional transistor, in which an
electric voltage controls a tiny elec-
trical current, the group’s device is
based on the use of an electric field
to control the propagation of surface
plasmons through the device6. Apart
from their small size, compared with
conventional optical counterparts,
the operation frequency of plasmonic
modulators can easily reach tens of
terahertz, well above the gigahertz
regime of modern computers.
Many roadblocks still remain to the
commercialization of such technolo-
gies — ranging from the integration
with silicon to device issues. “The key
thing that keeps coming back are losses
in the metals,” says Mark Brongersma,
a materials scientist at Stanford Uni-
versity in California. However, he adds, smart
design of the plasmonic structures could, in
principle, reduce losses to acceptable levels.
Plasmonics research has made remarkable
progress in the past decade, and researchers
are working on pushing our knowledge of plas-
mons even further, for example to understand
the physics very close to the metal surface.
Nonetheless, says Atwater, “what has hap-
pened in the past seven or eight years is that
plasmonics has given to photonics the ability to
go to the nanoscale and properly take its place
among the nanosciences.” ■
Joerg Heber is a senior editor at Nature
Materials.
1. Ebbesen, T. W., Lezec, H. J., Ghaemi, H. F., Thio, T. &
Wolff, P. A. Nature 391, 667–669 (1998).
2. Verhagen, E., Spasenović, M., Polman, A. & Kuipers, L.
Phys. Rev. Lett. 102, 203904 (2009).
3. Bozhevolnyi, S. I., Volkov, V. S., Devaux, E., Laluet, J.-Y. &
Ebbesen, T. W. Nature 440, 508–511 (2006).
4. Noginov, M. A. et al. Nature 460, 1110–1112 (2009).
5. Oulton, R. F. et al. Nature 461, 629–632 (2009).
6. Dionne, J. A., Diest, K., Sweatlock, L. A. & Atwater, H. A.
Nano Lett. 9, 897–902 (2009).
7. Fleischmann, M., Hendra, P. J., McQuillan, A. J. Chem. Phys.
Lett. 26, 163–166 (1974).
8. Jeanmaire, D. L. & Van Duyne, R. P. J. Electroanal. Chem. 84,
1–20 (1977).
9. Nie, S. & Emory, S. R. Science 275, 1102–1106 (1997).
10. Anker, J. N. et al. Nature Mater. 7, 442–453 (2008).
11. Hirsch, L. R. et al. Proc. Natl Acad. Sci. USA 100, 13549–
13554 (2003).
”Plasmonics has
given photonics the
ability to go to the
nanoscale.”
— Harry Atwater
Plasmon resonance could be used to make very sensitive biochemical
sensors (yellow bars). The waves here represent absorption spectra.
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