A Better Silicon Laser
Just a few years ago, many researchers thought it would be impossible to use silicon–the fundamental material used in the electronics industry–to make photonic devices such as lasers. But recent advances have shown that, with clever engineering tricks, silicon can emit light. (See “Intel’s Breakthrough.”) Now, researchers at Intel have reached another milestone. They have made a silicon laser that is far more efficient than previous silicon lasers, yet it operates at just a tenth of the power. The results are good enough for researchers to start integrating the laser into chips that could be used for medical and security applications, says Mario Paniccia, Intel research fellow and director of its silicon photonics lab.
In addition to having increased efficiency, says Paniccia, the new laser emits an extremely pure color of light, meaning the wavelength of the light is centered on a tiny region of the electromagnetic spectrum. “It’s actually better than any [semiconductor] laser you can buy,” says Paniccia. He says that the research over the past 12 months has taken silicon lasers “from a nice lab measurement to performance levels that are practical.” In medical devices, for instance, the laser could find cancerous tumors because it emits near-infrared light that is absorbed by tumors. The laser could also be used to detect trace amounts of certain chemicals used in weapons, which also absorb near-infrared light.
Today’s commercial lasers are made of compound semiconductors such as indium phosphide. These compound semiconductors efficiently convert electrical current into photons. In contrast, silicon doesn’t easily turn electrons into light. However, there’s a way to get around this natural limitation, says Bahram Jalali, professor of electrical engineering at the University of California, Berkeley. Shining intense light onto a silicon device, he explains, causes silicon atoms to vibrate, and the energy from these vibrations can be converted into photons. This type of laser, called a Raman laser, is well-known, but it was only demonstrated in silicon in 2004 by Jalali’s group.
Those early silicon Raman lasers didn’t work very well, Jalali says. Initially, the devices could only emit pulses of light, and even then, many of the photons were reabsorbed within the material, generating heat-producing electrons. Later in 2004, Paniccia’s group at Intel developed a silicon laser that was more efficient and could emit a continuous beam of light. The researchers did this by etching a waveguide into silicon in which the light bounced back and forth to gain intensity. And on each side of the channel, they built a diode that, when turned on, vacuumed up the extra electrons that usually hamper the emission of light.
Intel’s most recent research, published this month in Nature Photonics, uses the same basic concept, but with the waveguide designed to maximize the intensity of light emitted. The researchers deposited a material called boron phosphorous silicon glass around the waveguide to minimize the loss of light. In addition, says Paniccia, an optimal number of extra atoms of boron and phosphorus were added to the diode so that it more effectively swept away the extra electrons, making room for more light emission.
“I think Intel’s work is another milestone,” says Jalali. The laser is useful, he says, because it’s made of silicon, which is an inexpensive material. What’s more, he says, it emits intense light, and it can achieve wavelengths that are traditionally difficult to produce but are useful to many applications.
Paniccia says that the group’s next step is to integrate the device onto a chip with a light source that can power the silicon laser. “In terms of commercialization,” he says, “[the chip design] depends on what we’re trying to do.” For instance, if the laser were used to find cancerous tumors, both the silicon laser and the powering laser would need to be optimized to operate within a certain wavelength range and at certain intensities.
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