news digest ♦ LEDs


impractical for commercial use. The new device operates at room temperature and could, therefore, represent an important step toward next-generation computer processors.


“Low-power, electrically controlled light sources are vital for next generation optical systems to meet the growing energy demands of the computer industry,” said Vuckovic. “This moves us in that direction significantly.”


said Shambat.


“Without these nanophotonic ingredients – the ‘quantum dots’ and the photonic crystal – it is impossible to make an LED efficient, single-mode and fast all at the same time,” said Vuckovic.


Single-mode Nanophotonic LED Chip Carrier which holds hundreds of the Stanford low-power LEDs at its centre (Credit:Jan Petykiewicz, Stanford School of Engineering)


The LED in question is a “single-mode LED,” a special type of diode that emits light more or less at a single wavelength, very similar to a laser.


“Traditionally, engineers have thought only lasers can communicate at high data rates and ultralow power,” said Shambat. “Our nanophotonic, single- mode LED can perform all the same tasks as lasers, but at much lower power.”


Nanophotonics is key to the technology. In the heart of their device, the engineers have inserted little islands of the material indium arsenide, which, when pulsed with electricity, produce light. These islands are surrounded by photonic crystal – an array of tiny holes etched in a semiconductor. The photonic crystal serves as a mirror that bounces the light toward the centre of the device, confining it inside the LED and forcing it to resonate at a single frequency.


“In other words, the light becomes single-mode,” 76 www.compoundsemiconductor.net November/December 2011


Members of the Vuckovic team in the lab from left to right: Arka Majumdar, Tomas Sarmiento, Jan Petykiewicz, Jelena Vuckovic, and Gary Shambat (holding the chip carrier). (Credit:Michal Bajcsy, Stanford School of Engineering)


The new device includes a bit of engineering ingenuity, too. Existing devices are actually two devices, a laser coupled with an external modulator. Both devices require electricity. Vuckovic’s diode combines light emission and modulation functions into one device that drastically reduces energy consumption.


On average, the scientists say the new LED device transmits data at 0.25 femto-Joules per bit of data. By comparison, today’s typical ‘low’ power laser device requires about 500 femto-Joules to transmit a single bit. Some technologies consume as much as one pico-Joule per bit.


“Our device is 2000 to 4000 times more energy efficient than best devices in use today” said Vuckovic.


Further details of this work have been published in the paper “Ultrafast direct modulation of a single- mode photonic crystal nanocavity light-emitting diode” by Shambat et al, Nature Communications, 2,


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