This is part of IEEE Spectrum's SPECIAL REPORT: WINNERS & LOSERS 2009, The Year's Best and Worst of Technology.
Japanese start-up QD Laser’s Yasuhiko Arakawa [left] and Mitsuru Sugawara oversaw the 15-year effort to commercialize a temperature-stable semiconductor laser.
Suppose you had a dog whose personality fluctuated with the weather. On cool, crisp mornings, he’s a champ, fetching, rolling over, and shaking hands at your slightest command. But as the sun climbs higher and the day warms up, he becomes less and less responsive, and you have to ply him with doggy treats to get him to obey. And during heat waves? Forget about it—he barely plays dead unless you double or triple his kibble ration.
While you could excuse such behavior in Fido, something remarkably similar goes on all the time with the semiconductor lasers used in CD and DVD players and in optical communications. These tiny devices are incredibly sensitive to heat. Even a small rise in temperature causes the electrons within to move around faster and migrate out of the laser’s active layer—the thin slice of semiconducting material where the electrons recombine with positively charged holes to make light. As a result, the laser’s light output fluctuates, and it needs stronger and stronger electrical currents to keep lasing. At 85 °C, the device might need two or three times as much current to produce the same amount of light as at 25 °C.
To get around that shortcoming, developers of semiconductor lasers must either cool them or introduce extra circuitry that maintains the device’s output even as the temperature fluctuates. But those workarounds increase both the cost of making the lasers and the power they consume. Ever since this problem came to light, researchers have been hunting for a semiconductor laser that is inherently stable.
One promising technology, first proposed 27 years ago, is the quantum-dot laser. Such a device tightly confines the electrons and holes within many nanoscale blobs, or dots, of semiconducting material. With enough dots—millions or billions, that is—lasing will occur and steady output maintained, regardless of external temperature. While researchers can now grow these devices using standard molecular-beam epitaxy equipment, mass-producing them has been very tricky.
The Japanese start-up QD Laser, of Tokyo, a joint venture of Fujitsu and Mitsui Venture Capital Corp., has finally succeeded. Its quantum-dot lasers use inexpensive substrates made from gallium arsenide (GaAs) and boast an industry-leading density of 60 billion dots per square centimeter [see images, ”Collecting the Dots”]. Compared with the conventional indium-phosphide lasers now used in optical networks, QD Laser’s devices will consume just half the power while transmitting up to 10 gigabits of data per second at a wavelength of 1.3 micrometers. Best of all, they will generate the same output at any temperature from –40 to 100 °C.
To mass-produce the GaAs laser chips, QD Laser has partnered with one of Japan’s leading consumer-electronics firms, which will use the same production lines on which it currently cranks out conventional red lasers for DVD and CD players, video-game consoles, and other products. (QD Laser says it will reveal the name of its partner later this year.) The initial shipments of laser chips are destined for an unnamed optical equipment vendor, which sometime this spring will begin offering the world’s first optical transceivers incorporating a quantum-dot laser. Fujitsu will almost certainly buy the transceivers for use in optical LANs and fiber-to-the-home networks.
The quantum-dot laser has long been envisioned as a successor to the quantum-well laser, itself an improvement on earlier laser designs because it confined the injected electrons to an extremely thin layer—no more than tens of nanometers thick—of active material. That way, it required less current to induce lasing. But like the ”bulk” semiconductor lasers it superseded, the quantum-well laser is sensitive to temperature.
In the active layer of a bulk semiconductor laser, which you can picture as a fat, rectangular slab, the electrons and holes move in three dimensions, and that makes their interactions hard to control. In a quantum-well laser, they can move in only two dimensions, but electrostatic fields tend to build up, pulling the electrons away from the holes. In both cases, an increase in temperature makes the electrons more unruly.
Researchers began looking at ways to confine the electrons even further. In 1980, Yasuhiko Arakawa, a 28â¿¿year-old associate professor at the University of Tokyo, had an epiphany. ”I thought, if we fix the position of each electron by confining it in a small box, the energy distribution will not be affected by temperature,” Arakawa recalled in a recent interview at his office at the University of Tokyo. Each ”box” would be a semiconducting nanosize crystal into which electrons and holes would be injected. The box would effectively prevent the electrons and holes from being thermally excited to higher energy states.
He presented his quantum-box laser idea at the annual meeting of the Japanese Society of Applied Physics in March 1981. Then, collaborating with another professor, Hiroyuki Sakaki, he published a paper on the topic in the 1 June 1982 issue of Applied Physics Letters . The two researchers followed up with a series of experiments in which they confined electrons using 30-tesla magnets and demonstrated that the devices worked the same over a wide temperature range. ”But I thought it would be impossible to fabricate such nanostructures until the 21st century,” Arakawa says.
The quantum-box laser concept didn’t exactly set the world on fire. Some people found it interesting but not particularly useful, while others concluded that the boxes would be structurally unstable. His early work ”attracted almost no one to the field,” says Arakawa, now an IEEE Fellow. Today, he adds, thousands of researchers worldwide are working to advance the field.
Just three years after Arakawa and Sakaki’s paper, a research group at France’s Centre National d’Etudes des Télécommunications (CNET) noticed a strange phenomenon in the ”superlattices” they were trying to build out of extremely thin alternating layers of indium arsenide and gallium arsenide. Studying their handiwork under an electron microscope, they noticed that some of the indium arsenide had formed tiny regular blobs atop the underlying layer of gallium arsenide. Each blob, it turned out, was a quantum dot. The French team didn’t actually produce lasing from their weird structure, but it was a start.