DIỄN ÐÀN CHUYÊN MÔN

MIT researchers have demonstrated the first laser built from germanium that can emit wavelengths of light useful for optical communications - (Christine Daniloff for MIT)

Các nhà nghiên cứu của MIT lần đầu tiên đã biểu diễn việc bắn ra tia laser từ germanium, có thể ứng dụng trong kỹ thuật truyền thông quang học, trong đó có việc làm ra những con chip điện tử vận hành với tốc độ ánh sáng, trong điều kiện nhiệt độ bình thường, mà không cần phải dùng đến hệ thống giảm nhiệt phức tạp rất tốn kém để hạ nhiệt xuống dưới 123 Kelvin ( −150 °C, −238 °F ). Tuy nhiên từ nay cho đến ngày các dụng cụ laser quang học làm bằng germanium được bán ra thị trường có thể phải cần thêm nhiều năm nữa.

Lướt web với tốc độ ánh sáng
Surf the Web at the Speed of Light

By ERIC BLAND - ABC NEWS/Technology
March 6, 2010
Lee Rainie, of the Pew Internet Project, talks about his study's latest findings

Light-speed computing is one step closer to reality.

Scientists from the Massachusetts Institute of Technology have created a new infrared laser made from germanium that operates at room temperature.

The research removes the cryogenic cooling systems previously needed for infrared lasers and could lead to powerful computer chips that operate at the speed of light.

"Using a germanium laser as a light source, you could communicate at very high data rates at very low power," said Jurgen Michel of the Massachusetts Institute of Technology, who developed the new germanium laser. "Eventually you could have the computing power of today's supercomputers inside a laptop."

The creation of a new laser, even one based on germanium, is not newsworthy; more than 15,000 different lasers, some of which use germanium, have been created since the 1950s. What makes this particular germanium laser unique is that it creates an infrared beam at room temperature.

Until now infrared germanium lasers required expensive cryogenic cooling systems to operate. The new germanium laser operates at room temperature.

Laser Could Lead to Computer Chips That Operate at Speed of Light

Light-Speed Laser Is Still Years Away

To create the germanium laser, the scientists take a six-inch, silvery-gray disk of silicon and spray it with a thin film of germanium. These same disks are actually used to produce chips in today's computers.

An electrically powered, room-temperature, infrared laser for laptop computers is still years away, however, cautioned Michel. If and when those laptops do arrive, they will be powerful -- more powerful in fact than even today's supercomputers.

The battery that powers the laptop won't necessarily last any longer, but the power it does hold will make calculations orders of magnitude faster than today.

"We can't keep doing what we are currently doing," said Tom Koch, a scientist at Lehigh University who was familiar with the work but not directly involved with it.

"We need high-density, low-power solutions," said Koch. Computer chips are constantly getting smaller and smaller, but they are approaching the fundamental limits of electron-based computing. Light-based computing is one option to improve the speed and power of computers.

Germanium-Based Optical Computing Is Especially Attractive

Germanium-based optical computing is an especially attractive material for optical computing because it wouldn't require any change to the existing computer chip industry, Koch said. The same machines that use silicon could also use germanium to make future chips.

Despite germanium's advantages, Koch agreed with Michel that years will pass before any consumer devices are made with germanium. Scientifically, however, "this is a really nice result," said Koch.

People have tried to use germanium for an infrared laser for decades without success. The fact that the MIT scientists got the laser working at all is quite impressive, said Koch.

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Lasers made of silicon-compatible material could bring optical data transmission onto computer chips - Image: David Freund/iStockphoto

FIRST GERMANIUM LASER

BY Neil Savage // February 2010
http://spectrum.ieee.org/semiconductors/optoelectronics/first-germanium-laser

10 February 2010—A laser made of germanium may open the door to optical interconnects on computer chips that have multiple processing cores, say researchers at MIT, who recently demonstrated such a laser.

The group, led by Jurgen Michel, a research scientist at MIT’s Microphotonics Center, achieved low-level lasing at room temperature with a laser built from layers of germanium grown on a silicon wafer and powered by pulses from a separate laser. The paper is set to be published soon in Optics Letters.

Michel envisions using such a laser to make an optical bus, which would carry data around a multicore microprocessor. On a chip with 64 cores, such a system could have 40 times the power efficiency of traditional wire connections, he estimates. And in the next few years, chips with hundreds of cores are expected to make the concept even more useful. Another approach to such a system involves bonding lasers that are made of compound semiconductors and built separately onto silicon chips, but that could prove too costly and error prone for mass production. Germanium is already used in silicon manufacturing processes, and growing a laser directly on a chip should be vastly more efficient, Michel says.

Germanium, like silicon, is an elemental semiconductor that doesn’t easily emit light, because it has an indirect band gap. So when electrons within the germanium are excited by an outside energy source, such as a laser, and then drop back to a lower-energy state, they emit the excess energy as heat rather than light. To get the material to produce photons instead, the researchers had to alter the germanium to achieve a direct band gap.

“Luckily with germanium, the difference between the direct and the indirect band gap in terms of energy is not that great,” Michel says.

The researchers used two tricks to make up that difference. First they doped the germanium with phosphorus. The electrons in the phosphorus occupy the lower-energy state so that it’s no longer available to excited electrons, which instead drop to the direct band-gap energy. ”We filled the indirect band gap so much that the electrons would spill into the direct band gap,” Michel says.

The team doped the germanium with 1019 atoms of phosphorus per cubic centimeter. Michel says they need to increase that to at least 5 x 1019 to get about 10 times as much light out of the laser and thus make it practical. At the high temperatures used to make the laser, phosphorus tends to leak out faster than the researchers can put it in, so such an increase will require changes to their doping methods.

Increasing the doping will also produce a wider range of laser wavelengths. In the experiment, the team saw light from 1590 to 1610 nanometers, but with more doping that could become 1500 to 1620 nm. Silicon is transparent at those wavelengths, so the same material used to build circuits could also build waveguides to route the light around a chip.

The team also reduced the difference between the two energy states by putting mechanical strain on the germanium. They built the device by depositing a silicon oxide on a silicon chip, with a 1.6-micrometer-wide trench in the oxide; then they grew the germanium in the trench at about 600 °C. They annealed the device at 850 °C. Because silicon and germanium shrink at different rates as they cool from the annealing temperature, the process stretched the germanium so that its atoms were farther apart.

“The key issue for applications is whether an electrically pumped laser can be made,” says Philippe Fauchet, a professor at the University of Rochester’s Institute of Optics, who calls the work “really important.” Using electricity instead of another laser to power the device is necessary to make the laser practical for on-chip use.

It will also need to emit light in a continuous wave instead of in pulses, as it does now. Michel says there appears to be a clear path to reaching those goals, though he expects it to take at least a couple of years.

About the Author

Neil Savage writes about optoelectronics and other technology from Lowell, Mass. In January 2010 he reported on a new way to get silicon to emit light by using plasmonic waveguides.