November 15, 2011

Stanford LED Nanophotonics uses two thousands of times less energy at 0.25 femto-joules per bit of data

A team at Stanford's School of Engineering has demonstrated an ultrafast nanoscale light-emitting diode (LED) that is orders of magnitude lower in power consumption than today's laser-based systems and is able to transmit data at the very rapid rate of 10 billion bits per second. The new nanoscale device transmits data at ultrafast rates while using thousands of times less energy than current technologies. The nanophotonics device is a major step forward for on-chip data transmission.

Nature Communications - Ultrafast direct modulation of a single-mode photonic crystal nanocavity light-emitting diode

Compact, low power onchip photonics is key to developing zettaflop supercomputers.

The LED in question is a "single-mode LED," a special type of diode that emits light more or less at a single wavelength, similarly 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 light-emitting material indium arsenide, which, when pulsed with electricity, produce light. These "quantum dots" 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 center of the device, confining it inside the LED and forcing it to resonate at a single frequency.

"In other words, it becomes single-mode," 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.

Engineering ingenuity

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 transmission and modulation functions into one device, drastically reducing energy consumption.

In tech-speak, the new LED device transmits data, on average, 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 the same bit.

"Our device is some 2,000 times more energy efficient than best devices in use today," said Vuckovic.

Low-power and electrically controlled optical sources are vital for next generation optical interconnect systems to meet strict energy demands. Current optical transmitters consisting of high-threshold lasers plus external modulators consume far too much power to be competitive with future electrical interconnects. Here we demonstrate a directly modulated photonic crystal nanocavity light-emitting diode (LED) with 10 GHz modulation speed and less than 1 fJ per bit energy of operation, which is orders of magnitude lower than previous solutions. The device is electrically controlled and operates at room temperature, while the high modulation speed results from the fast relaxation of the quantum dots used as the active material. By virtue of possessing a small mode volume, our LED is intrinsically single mode and, therefore, useful for communicating information over a single narrowband channel. The demonstrated device is a major step forward in providing practical low-power and integrable sources for on-chip photonics.

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