Metal-wrapped nanowires slash semiconductor emission lifetimes

July 25, 2011 — University of Pennsylvania researchers have applied the amplification concept of Renaissance cathedral "whispering galleries" to semiconducting nanowires, reducing semiconductor emission lifetime for ultrafast photonic devices like LEDs.  

Excited semiconductors require a few nanoseconds to return to "the ground state accompanied by emission of light," said associate professor Ritesh Agarwal, explaining emission lifetime. Modulators are "limited by this time constant," Agarwal noted, so the researchers reduced it to less than a picosecond, which Agarwal says is "more than a thousand times faster" than current technologies. The Penn researchers’ nanowires can jump directly from excited to ground states, without the cool-down period semiconductors typically require.

The nanowires are cadmium sulfide wrapped in a buffer layer of silicon dioxide and an outer layer of silver. The silver coating supports surface plasmons, which combine oscillating metal electrons and light on the surface where the silicon dioxide and silver layers meet. The materials wrapped around the nanowire create a "nanoscale plasmonic cavity" to acheive the whispering gallery amplification effect, said Agarwal (see above figure).

Depending on the nanowire size, the silver coating creates pockets of resonance, generating highly confined electromagnetic fields within the nanostructure. The researchers were able to tune emission lifetime by manipulating the high-intensity electromagnetic fields inside the light-emitting cadmium sulfide core. Further tuning balanced the inverse relationship of EM field to the cavity’s energy storage capability, or quality factor. This results in emission lifetimes of femtoseconds, and could be applied in LEDs, and other nano-photonic devices, such as plasmonic computers. The physics research could also be applied to solar cell improvements.

The research was conducted by associate professor Ritesh Agarwal, postdoctoral fellows Chang-Hee Cho and Sung-Wook Nam and graduate student Carlos O. Aspetti, all of the Department of Materials Science and Engineering in Penn’s School of Engineering and Applied Science.  Michael E. Turk and James M. Kikkawa of the Department of Physics and Astronomy in the School of Arts and Sciences also contributed to the study.

Their research was published in the journal Nature Materials. Access it here: http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat3067.html

The research was supported by the U.S. Army Research Office, the National Institutes of Health, the National Science Foundation, Penn’s Nano/Bio Interface Center and the U.S. Department of Energy.

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