July 25, 2006, Houston, TX--Researchers at Rice University's Laboratory for Nanophotonics (LANP) unveiled the "nanoegg," the latest addition to their family of plasmonic nanoparticles (see
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 regions of the optical spectrum (about five times as many as nanoshells), and their asymmetric structure also allows them to focus more energy on a particular spot.
The resonances of nanoparticles depend on their geometry. The most-prevalent plasmonic biosensor nanoparticle is the nanoshell--a dielectric sphere coated with a noble metal. But the Rice researchers have been experimenting with other shapes such as nanorods or even nanocages, because each geometry has its own set of optical properties arising from plasmonic effects. Nanoeggs have a conducting shell shaped like that of a hard-boiled egg--thicker on one end than the other. 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.
In order for plasmons to be excited by light, the electrons on a conducting nanoparticle'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.
"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."
"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."
The researchers published their results in the July 18 issue of the Proceedings of the National Academy of Sciences. Co-authors of 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.