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Synopsis: Optical Device is More Than 100% EfficientSource; http://physics.aps.org (American Physical Society)Thermoelectrically Pumped Light-Emitting Diodes Operating above Unity EfficiencyParthiban Santhanam, Dodd Joseph Gray, Jr., and Rajeev J. RamPhys. Rev. Lett. 108, 097403 (2012)Published February 27, 2012

Physicists have known for decades that, in principle, a semiconductor device can emit more light powerthan it consumes electrically. Experiments published in Physical Review Lettersfinally demonstrate this inpractice, though at a small scale.

The energy absorbed by an electron as it traverses a light-emitting diode is equal to its charge times theapplied voltage. But if the electron produces light, the emitted photon energy, which is determined by thesemiconductor band gap, can be much larger. Usually, however, most electrons create no photon, so theaverage light power is less than the electrical power consumed. Researchers aiming to increase the powerefficiency have generally tried to boost the number of photons per electron. But Parthiban Santhanam andco-workers from the Massachusetts Institute of Technology in Cambridge took a gentler approach,achieving power enhancement even though less than one electron in a thousand produced a photon.

The researchers chose a light-emitting diode with a small band gap, and applied such small voltages that itacted like a normal resistor. With each halving of the voltage, they reduced the electrical power by a factorof , even though the number of electrons, and thus the light power emitted, dropped by only a factor of.Decreasing the input power to picowatts, the team detected nearly picowatts of emitted light. The extraenergy comes from lattice vibrations, so the device should be cooled slightly, as occurs in thermoelectriccoolers.These initial results provide too little light for most applications. However, heating the light emittersincreases their output power and efficiency, meaning they are like thermodynamic heat engines, exceptthey come with the fast electrical control of modern semiconductor devices. Don Monroe

Thermoelectrically Pumped Light-Emitting Diodes Operating above Unity

EfficiencyPhys. Rev. Lett. 108, 097403Published 27 February 2012Parthiban Santhanam, Dodd Joseph Gray, Jr., and Rajeev J. Ram

AbstractA heated semiconductor light-emitting diode at low forward bias voltage V

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