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A scanning electron microscope image, left, shows a 15-um line of 50-nm spherical gold nanoparticles. At right is a fluorescence image of the same chain, coated with a thin film of Cardiogreen dye using 785-nm laser excitation. Image: Link Laboratory/Rice University
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Microscopic channels of gold nanoparticles have the ability to transmit
electromagnetic energy that starts as light and propagates from "dark
plasmons," according to researchers at Rice University.
A new paper in Nano Letters shows
how even disordered collections of nanoparticles in arrays as thin as 150 nm
can be turned into waveguides and transmit signals an order of magnitude better
than previous experiments were able to achieve. Efficient energy transfer on
the micrometer scale may greatly improve optoelectronic devices.
The Rice laboratory of Stephan Link, an assistant professor of chemistry and
electrical and computer engineering, has developed a way to "print"
fine lines of gold nanoparticles on glass. These lines of nanoparticles can
transmit a signal from one nanoparticle to the next over many microns, much
farther than previous attempts and roughly equivalent to results seen using gold
nanowires.
Complex waveguide geometries are far easier to manufacture with nanoparticle
chains, Link said. He and his team used an electron beam to cut tiny channels
into a polymer on a glass substrate to give the nanoparticle lines their shape.
The gold nanoparticles were deposited into the channels via capillary forces.
When the rest of the polymer and stray nanoparticles were washed away, the
lines remained, with the particles only a few nanometers apart.
Plasmons are waves of electrons that move across the surface of a metal like
water in a pond when disturbed. The disturbance can be caused by an outside
electromagnetic source, such as light. Adjacent nanoparticles couple with each
other where their electromagnetic fields interact and allow a signal to pass
from one to the next.
Link said dark plasmons may be defined as those that have no net dipole
moment, which makes them unable to couple to light. "But these modes are
not totally dark, especially in the presence of disorder," he said.
"Even for the subradiant modes, there is a small dipole oscillation.
"Our argument is that if you can couple to these subradiant modes, the
scattering loss is smaller and plasmon propagation is sustained over longer
distances," Link said. "Therefore, we enhance energy transport over
much longer distances than what has been done before with metal-particle
chains."
To see how far, Link and his team coated the 15-um-long lines with a
fluorescent dye and used a photobleaching method developed in his laboratory to
measure how far the plasmons, excited by a laser at one end, propagate.
"The damping of the plasmon propagation is exponential," he said.
"At four microns, you have a third of the initial intensity value.
"While this propagation distance is short compared to traditional
optical waveguides, in miniaturized circuits one only needs to cover small
length scales. It might be possible to eventually apply an amplifier to the
system that would lengthen the propagation distance," Link said. "In
terms of what people thought was possible with nanoparticle chains, what we've
done is already a significant improvement."
Link said silver nanowires have been shown to carry a plasmon wave better
than gold, as far as 15 um. "We know that if we try silver nanoparticles,
we may propagate a lot longer and hopefully do that in more complex
structures," he said. "We may be able to use these nanoparticle
waveguides to link to other components such as nanowires in configurations that
would not be possible otherwise."
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