One
of the great challenges in the field of nanotechnology
is optical imaging—specifically, how to design
a microscope that produces high-resolution images
of the nano-sized objects that researchers are
trying to study. For example, a typical DNA molecule
is only about three nanometers wide—so tiny that
the contours of its surface are obscured by light
waves, which are hundreds of nanometers long.
Now,
researchers from Stanford University have greatly
improved the optical mismatch between nanoscale
objects and light by creating the "bowtie nanoantenna," a
device 400 times smaller than the width of a human
hair that can compress ordinary light waves into
an intense optical spot only 20 nanometers wide.
These miniature spotlights may one day allow researchers
to produce the first detailed images of proteins,
DNA molecules and synthetic nano-objects, such carbon
nanotube bundles.
"One of our goals is to build a microscope with
bowtie antennas that we can scan over a single molecule," says
W.E. Moerner, the Harry S. Mosher Professor of Chemistry
at Stanford.
He and his Stanford colleagues introduced the bowtie
nanoantenna earlier this year in a study published
in the journal Physical Review Letters that
was co-authored by postdoctoral fellow P. James Schuck
and graduate student David Fromm in the Department
of Chemistry, and Professor Emeritus Gordon Kino
and graduate student Arvind Sundaramurthy in the
Department of Electrical Engineering.
Golden bowties
The bowtie nanoantenna consists of two triangular
pieces of gold, each about 75 nanometers long, whose
tips face each other in the shape of a miniature
bowtie. The device operates like an antenna for a
radio receiver, but instead of amplifying radio waves,
the bowtie takes energy from an 830-nanometer beam
of near-infrared light and squeezes it into a 20-nanometer
gap that separates the two gold triangles. The result
is a concentrated speck of light that is a thousand
times more intense than the incoming near-infrared
beam.
"What you end up with is a very small optical spot
that you could scan to make detailed images of molecules
and other nano-particles," says Kino, the W.M. Keck
Foundation Professor of Electrical Engineering, Emeritus. "Normally
we use lenses to focus, but it's not possible to
resolve detail in objects smaller than one-half the
wavelength of light."
Because
the shortest wavelength of visible light is 400
nanometers, a conventional microscope cannot resolve
objects 200 nanometers or smaller. "But the
bowtie antenna produces an optical spot that's 20-nanometers
wide, so we're improving the resolution by a factor
of 10," Kino says.
Polymers and sensors
In
addition to nano-scale optical imaging, Moerner
says that bowties may be useful in photopolymerization,
a process that uses light to create synthetic compounds
(polymers), which researchers can use to trap nano-particles
and place them in specific locations. "It's difficult
to put molecules and crystals exactly where you want
them when you're working at a nano-scale," Schuck
explains.
Bowties
also may have applications in Raman spectroscopy,
a technique that allows scientists to identify individual
molecules by measuring the vibrational energy the
molecule emits when exposed to light. "It's analogous
to fingerprinting," Schuck explains. "Each molecule
has a unique vibrational energy, and bowties have
a potential use as biological or chemical sensors
that can differentiate molecules."
The Stanford team plans to explore these and other
practical applications of bowtie nanoantennas in
future experiments. On Aug. 30, Moerner discussed
bowties and other developments in the field of nanophotonics
at the annual meeting of the American Chemical Society
in Washington, D.C.
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