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  • Emission Lifetime in Semiconductors Reduced
Jul 2011
PHILADELPHIA, July 29, 2011 — The emission lifetime for photovoltaic devices has been reduced by applying the “whispering galleries” concept to semiconducting nanowires.

In architecture, a whispering gallery is a dome or a circular area where a person can hear someone whisper at the opposite end of the gallery, because the design of the building amplifies and directs the sound waves.

In semiconductors, the excited state is when energy is present in the system, and the ground state is when there is none. Normally, the semiconductor must first “cool down” in the excited state, releasing energy as heat, before “jumping” back to the ground state, releasing the remaining energy as light. The new semiconductor nanowires, however, can jump directly from a high-energy excited state to the ground, all but eliminating the cool-down period.

A rendering of the triple-layer nanowire and "whispering gallery" electromagnetic fields. (Image: University of Pennsylvania)

The advancement in emission lifetime is due to the unique construction of nanowires created by a team at the University of Pennsylvania. At their core, the nanowires are cadmium sulfide, a common nanowire material, but they are also wrapped in a buffer layer of silicon dioxide and, critically, an outer layer of silver. The silver coating supports what are known as surface plasmons — unique waves that are a combination of oscillating metal electrons and of light. These surface plasmons are highly confined to the surface met by the silicon dioxide and silver layers.

“The previous state of the art was taking a nanowire, just like ours, and laying it on a metal surface,” said Ritesh Agarwal, associate professor of materials science and engineering. “We curved the metal surface around the wire, making a complete nanoscale plasmonic cavity and the whispering gallery effect.”

For certain nanowire sizes, the silver coating creates pockets of resonance and hence highly confined electromagnetic fields within the nanostructure. Emission lifetime can then be engineered by precisely controlling high-intensity electromagnetic fields inside the light-emitting medium, which is the cadmium sulfide core.

To reach an emission lifetime measured in femtoseconds, the researchers needed to optimally balance this high-confinement electromagnetic field with an appropriate “quality factor,” the measurement of how good a cavity is at storing energy. To complicate matters, quality factor and confinement have an inverse relationship: The higher the quality factor a cavity has, the bigger it is, and the smaller its confinement. However, by opting for a reasonable quality factor, the researchers could vastly increase the confinement of the electric field inside the nanowire by using resonant surface plasmons and get the record-breaking emission lifetime.

This many-orders-of-magnitude improvement could find a home in a variety of applications such as LEDs, detectors and other nanophotonic devices with novel properties.

The research was conducted by 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 sstronomy in the School of Arts and Sciences also contributed to the study.

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cadmium sulfide
An inorganic compound, yellow to orange in color, that fluoresces strongly enough when bombarded by a high-current-density electron beam to be used as a high-intensity light source.
silicon dioxide
An abundant material found in the form of quartz and agate and as one of the major constituents of sand. The silicates of sodium, calcium, and other metals can be readily fused, and on cooling do not crystallize, but instead form the familiar transparent material glass.
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