Laser has more bang, less bulk

STANFORD (US) — In a push toward smaller, faster data transmission, researchers have produced a nanoscale laser that is much faster and vastly more energy efficient than anything available today.

“Today’s electrical data transmission circuits require a lot of energy to transmit a bit of information and are, relatively speaking, slow,” says Jelena Vuckovic, associate professor of electrical engineering at Stanford University.

Details of the research are reported in the journal Nature Photonics.

Vuckovic is working on a type of data transmitter called a photonic-crystal laser—a laser that is particularly promising, not just for its speed and size, but because it operates at low thresholds that don’t use much energy.

“We’ve produced a nanoscale optical data transmitter, a laser that uses 1,000 times less energy and is 10 times faster than the very best laser technologies in commercial use today,” she says. “Better yet, we believe we can improve upon those numbers.”

While others have created low-threshold lasers, the most promising have required a second laser to inject them with the energy they need to work, known as “pumping,” hardly an ideal solution.

“We really needed a laser pumped with electricity, not light,” Vuckovic says. The only available electrically pumped photonic-crystal laser was inefficient and difficult to fabricate, making it commercially impractical. Now, for the first time, researchers have demonstrated an electrically pumped laser that is both easy to manufacture and delivers dramatically reduced energy consumption.

Deck of cards

To create the laser, the researchers first “grow” a wafer of gallium arsenide, a semiconductor crystal, using a beam that sprays molecules to build layers one by one. At certain points in the layering process, they shuffle in three thin layers of a second crystal—indium arsenide. A cross-section reveals that the indium arsenide appears like little bumps or hills—quantum dots—within the wafer.

When done, the wafer resembles a sort of nanophotonic deck of cards a mere 220 nanometers thick. Thick, however, is a relative term. It would take over 1,000 of Vuckovic’s wafers stacked atop one another to equal the thickness of a single playing card.

Next, the engineers “dope” two discrete areas on top of the wafer with ions. On one side, the researchers seed ions of silicon, and on the other they implant ions of beryllium.

These two regions are faintly visible on the surface, widening toward each other, approaching but never quite meeting at the center of the wafer. These ion-infused regions help focus the current flow to a very precise area at the core of the wafer where light is emitted, improving the performance of the laser.

Hall of mirrors

Finally, with the basic wafer fabricated, the researchers have yet one more trick up their engineering sleeves. They finish by etching a precise honeycomb pattern of circular holes through the wafer.

The size and positioning of these holes is critical to the success of the laser. If the holes are too small or too large, spaced too closely or too far apart, the laser will not perform optimally and in some cases, won’t perform at all.

“These holes are almost perfectly round with smooth interior walls and are very important to the laser’s function. They act like a hall of mirrors to reflect photons back toward the center of the laser,” Vuckovic says.

Here, in the heart of the wafer, the photons are concentrated and amplified into a a laser which can be modulated up to 100 billion times per second, 10 times the best data transmitters now in use. Thus the light becomes binary data – light on, 1; light off, 0.

Real-world possibilities

At one end of a semiconductor circuit is a laser transmitter beaming out 1s and 0s as blasts of light. At the other end is a receiver that turns those blasts of light back into electrical impulses. All that is needed is a way to connect the two.

To do this, the researchers heat and stretch a thin fiberoptic filament, hundreds of times thinner than a human hair. The light from the laser travels along the fiber to the next junction in the circuit.

All this happens in a layer so thin hundreds of these nanophotonic transmitters could be arranged on a single layer, and many layers could then be stacked into a single chip.

Before Vuckovic’s laser interconnect becomes commonplace, however, certain questions will need to be resolved. The new laser operates at relatively cold temperatures, 150 degrees Kelvin and below – about 190 degrees below zero Fahrenheit – but Vuckovic is confident and pressing forward.

“With improvements in processing,” she says, “we can produce a laser that operates at room temperature while maintaining energy efficiency at about 1,000 times less than today’s commercial technologies. We can see a light on the horizon.”

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