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Asymmetric Particles Focus Light in Unique Way
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.
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