The difference is a function of one of nanotechnology's principle phenomena: the traits of a bulk material are different than structures of the same material on the nanoscale.
"When you're working at bigger sizes, the surface is not as important. The surface to volume ratio the number of atoms on the surface divided by the number of atoms in the whole material is a very small number," Agarwal said. "But when you make a very small structure, say 100 nanometers, this number is dramatically increased. Then what is happening on the surface critically determines the device's properties."
Other researchers have tried to make polariton cavities on this small a scale, but the chemical etching method used to fabricate the devices damages the semiconductor surface. The defects on the surface trap the excitons and render them useless.
"Our cadmium sulfide nanowires are self-assembled; we don't etch them. But the surface quality was still a limiting factor, so we developed techniques of surface passivation. We grew a silicon oxide shell on the surface of the wires and greatly improved their optical properties," Agarwal said.
The oxide shell fills the electrical gaps in the nanowire surface, preventing the excitons from getting trapped.
"We also developed tools and techniques for measuring this light-matter coupling strength," Piccione said. "We've quantified the light-matter coupling strength, so we can show that it's enhanced in the smaller structures,"
Being able to quantify this increased coupling strength opens the door for designing nanophotonic circuit elements and devices.
"The stronger you can make light-matter coupling, the better you can make photonic switches," Agarwal said. "Electrical transistors work because
|Contact: Evan Lerner|
University of Pennsylvania