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  • Spontaneous Emission Enhanced by Nanoantennas

Photonics.com
Feb 2015
BERKELEY, Calif., Feb. 5, 2015 — LEDs enabled by nanoscale antennas to spontaneously emit more light could be a better solution than lasers for short-range optical communications.

Researchers at Lawrence Berkeley National Laboratory have taken a first step in this direction by developing a nanoantenna that increased the spontaneous emission rate of an InGaAsP nanorod by 115 times.

“Since the invention of the laser, spontaneous light emission has been looked down upon in favor of stimulated light emission,” said Dr. Eli Yablonovitch, an electrical engineer at Berkeley Lab and a professor at the University of California, Berkeley. “However, with the proper optical antenna, spontaneous emission can actually be faster than stimulated emission.”

Gold antenna coupled to InGaAsP nanorod
Coupling a gold antenna to a InGaAsP nanorod, isolated by TiO2 and embedded in epoxy, greatly enhanced the nanorod's spontaneous light emission. Courtesy of Berkeley Lab.


InGaAsP is already in wide use for IR laser communications and photodetectors. Here, the nanorod emitted at a frequency of 200 THz. The nanorod was coated with TiO2 to isolate it from the antenna.

An enhancement factor of about 200 would be necessary for the rate of spontaneous emission to exceed that of stimulated emission.

“With optical antennas, we believe that spontaneous emission rate enhancements of better than 2,500 times are possible while still maintaining light emission efficiency greater than 50 percent,” Yablonovitch said. “Replacing wires on microchips with antenna-enhanced LEDs would allow for faster interconnectivity and greater computational power.”

Lasers have a downside for communications over a meter or less: they consume too much power and typically take up too much space. LEDs would be a much more efficient alternative but have been limited by their slow spontaneous emission rates.

In addition to short distance communication applications, LEDs equipped with optical antennas could also find important use in photodetectors. Optical antennas could also be applied to imaging, biosensing and data storage.

Funding for this research came from the U.S. Air Force Office of Scientific Research and the Department of Energy’s Office of Science.

The research was published in the Proceedings of the National Academy of Sciences (doi: 10.1073/pnas.1423294112).

For more information, visit www.lbl.gov.


GLOSSARY
spontaneous emission
Radiation emitted when a quantum mechanical system drops spontaneously from an excited level to a lower level. This radiation is emitted according to the laws of probability without regard to the simultaneous presence of similar radiation. The rate of spontaneous emission is proportional to the Einstein "A" coefficient and is inversely proportional to the radiative lifetime.
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