Nanoscale Lasers Come In Out of the Cold

Ultratiny light emitters no longer need high power and low temperatures

Photo: UC San Diego

24 February 2012—Researchers at the University of California, San Diego, reported in the journal Nature earlier this month that they have invented a new kind of nanometer-size laser that requires much less power to generate a coherent beam than previous designs. This type of laser could finally make it practical to use light instead of electricity to send terabits of data between different parts of a computer processor, the researchers suggest.

Because of the nanolaser’s negligible power needs, it can be modulated much more quickly than existing lasers, allowing information to be encoded directly onto the beam, says Mercedeh Khajavikhan, a postdoctoral researcher who is a member of UCSD’s Ultrafast and Nanoscale Optics Group, which made the breakthrough. “[It] can also become a backbone for future communication devices,” she says. The UCSD group, which is working on integrating nanophotonics with CMOS electronics, envisions other applications for the nanolaser design: It could be used in ultrahigh-resolution imaging and for figuring out the chemical makeup of a material at a distance.

The new design addresses some of the problems inherent in today’s nanoscale lasers. The main issue is that as the size of the laser cavity decreases, the amount of light amplification, or “gain,” shrinks along with it. (The amount of energy lost as photons strike the metal parts of the cavity’s walls also diminishes with size, but it does so at a slower rate than gain does.) A laser can get only so small before its input energy requirement is impossibly large.

The UCSD team got the photons moving in the right direction by designing their device as a coaxial waveguide—in this case, a metallic rod surrounded by an indium gallium arsenide phosphide semiconductor ring that was then coated in metal. To form the lasing cavity, the waveguide was capped at both ends. One end cap, made of silicon dioxide coated in silver, formed a totally reflecting mirror. The other plug, which allowed the pump beam—the laser’s power source—to enter and the laser light to escape, was filled with air.

This setup helped to solve the gain problem, because the laser cavity was restricted to emitting energy only in a mode wherein the amount of light amplification was high enough to overcome energy losses from the cavity itself. The result: a device efficient enough to generate coherent beams even when powered by just 720 picowatts of light. What’s more, its performance at room temperature was comparable to that achieved when it was put through its paces at 4.5 kelvins. Nanolaser efficiency usually drops off dramatically without cryogenic cooling.

The nanolaser’s room-temperature abilities are in part due to the metallic coating around the semiconductor ring, which serves as a heat sink. “The direct contact of metal [coating] and semiconductor is a rather bold approach and required special fabrication recipes,” says Khajavikhan.

Still, the researchers were determined to make a working nanolaser using standard nanofabrication tools, and they wanted to ensure that the devices could be fabricated in batches. “We can claim that, despite many challenges, we did just that,” says Khajavikhan.

Making the semiconductor’s surfaces smoother will further increase the laser’s efficiency and allow fabrication at even smaller sizes. “We feel this is just the beginning for a new family of light emitters with superior characteristics,” adds Yeshaiahu (Shaya) Fainman, director of the Ultrafast and Nanoscale Optics Group. “Many advances in this new area are yet to come.”

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