20, 2006 --- Houston, TX--- Researchers at Rice University's Laboratory
for Nanophotonics (LANP) have unveiled
the "nanoegg," the latest addition to their family
ultrasmall, light-focusing particles. A cousin
of the versatile nanoshell, nanoeggs are asymmetric
specks of matter whose striking optical properties
can be harnessed for molecular imaging, medical
diagnostics, chemical sensing and more.
Nanoeggs are described in the July 18 issue of the Proceedings of the National
Academy of Sciences.
Like nanoshells, nanoeggs are about 20 times smaller than a red blood cell,
and they can be tuned to focus light on small regions of space. But each nanoegg
interacts with more light about five times the number of wavelengths than
their nanoshell cousins, and their asymmetric structure also allows them to
focus more energy on a particular spot.
"The field of nanophotonics is undergoing explosive growth, as researchers gain
greater and greater sophistication in the design and manipulation of light-active
nanostructures," said LANP Director Naomi Halas, the Stanley C. Moore Professor
of Electrical and Computer Engineering and professor of chemistry. "The addition
of nanoeggs and, earlier this year, nanorice to LANP's family of optical nanoparticles
is a direct result of our increased understanding of the interaction between
light and matter in this critical size regime."
Like nanoshells, nanoeggs have a spherical, non-conducting core that's covered
with a thin metal shell. But where the casing on a nanoshell has a uniform
thickness like the peel covering an orange the nanoegg's covering
is thicker on one side than the other in much the same way that a hard-boiled
egg white is thick in some places and thin in others. The off-center core in
the nanoegg radically changes its electrical properties, said co-author and
theoretical physicist Peter Nordlander, professor of physics and astronomy.
The reasons for this have to do with the odd and often counterintuitive rules
that govern how light interacts with electrons at the nanoscale.
"All metal particles have a sea of free electrons flowing continuously over their
surface called plasmons," Nordlander said. "These plasmons slosh around constantly,
just like waves in the ocean. Light also travels in waves, and when the wavelength
of incoming light matches the wavelength of the plasmon, the amplitude of their
sloshing gets bigger and bigger, much like the waves in a bathtub when a child
rhythmically sloshes bathwater until it spills out of the tub."
In order for plasmons to be excited by light, the electrons on a particle's
surface must behave in such a way as to create a 'dipole moment,' a state marked
by two equal but opposite poles, one positive and the other negative much
like a magnet that attracts on one end and repels on the other. "Without a
dipole moment, there is no 'handle' for light to grab hold of," Nordlander
said. "In symmetric nanoshells, most of the light energy is lost to these 'dark
modes.' With symmetry breaking, we are able to make these dark modes bright
by providing dipole moments for more of the incoming light."
Co-authors on the paper include Jason Hafner, assistant professor of physics
and astronomy and of chemistry, and graduate students Hui Wang, Yanpeng Wu,
Britt Lassiter and Colleen Nehl. The research was supported by the U.S. Army
Research Office, the National Science Foundation and the Welch Foundation.
About Rice University:
Rice University is consistently ranked one of America's best teaching and research
universities. It is distinguished by its: size: 2,850 undergraduates and
1,950 graduate students; selectivity: 10 applicants for each place in the
freshman class; resources: an undergraduate student-to-faculty ratio of 6-to-1,
and the fifth largest endowment per student among American universities;
residential college system, which builds communities that are both close-knit
and diverse; and collaborative culture, which crosses disciplines, integrates
teaching and research, and intermingles undergraduate and graduate work.
Rice's wooded campus is located in the nation's fourth largest city and on
America's South Coast.