18 February 2005--Scientists at Intel Corp., in Santa Clara, Calif., disclosed on 16 February that they have built a silicon laser that goes beyond their effort of a month ago by operating continuously, a prerequisite for carrying digital information. The discovery could allow the integration of electronics and optics in silicon chips rather than in exotic-semiconductor chips, which are much more expensive to make. That way, PCs and even the chips inside them could converse over fiber-optic connections boasting bandwidths now seen only in long-haul telecommunications networks.
Intel found a way to overcome silicon's uncooperative nature. Many other materials respond to an intense light by emitting photons, which is the first step in the lasing process. Silicon, however, also produces stray electrons, which absorb the photons, quenching the laser. The problem is called two-photon absorption, because it takes collisions from two separate photons to knock an electron loose.
The laser described yesterday in the magazine Nature, as well as the noncontinuous version described 5 January on Nature's Web site, exploited what is known as the Raman effect, in which light scatters in certain materials in such a way as to produce another, longer wavelength. In a Raman laser, a separate laser, called a pump laser, is fired into a cavity, in this case a long narrow channel with a mirror at one end. There the beam scatters and reflects, stimulating the emission of laser light with a longer wavelength.
The Intel researchers formed the laser cavity by building an S-shaped ridge on a 15-mm-by-15-mm silicon chip. The ridge sits between two chemically altered tracks of silicon, forming a type of diode. Voltage across the diode sweeps the unwanted electrons away and keeps the light flowing through the chip.
The laser described last month could operate only in 130-nanosecond pulses because the applied voltage couldn't sweep away enough electrons to keep the laser from eventually fizzling out. The newer laser shines steadily thanks to the addition of a more reflective coating on the waveguide and the optimization of the diode. More light from the pump laser got into the chip, and more of the Raman light was captured in the waveguide.
The silicon laser is just one part of Intel's recent drive to develop an integrated suite of silicon analogues to optoelectronic devices made of other materials. The company is developing photodetectors, modulators, and other optical components in silicon in the hope that one day it will be able to put them together with existing chip-making processes and infrastructure.
To Probe Further
For more about silicon lasers and Intel's effort, see "Intel Makes a Silicon Laser," by Samuel K. Moore
"Intel Makes Experimental Components for Linking Computers with Light" by Linda Geppert, Web-Only News, Spectrum Online, February 2004.
"Light from Silicon," by Neil Savage, Web-Only News, Spectrum Online, January 2004.