Power Play: Shift from loss to gain may boost silicon devices

Slender, light-transmitting channels might someday replace many electrical wires as connectors between microchips. However, silicon components that could render such light-based connections affordable and easy to fabricate use excessive power and so produce excessive heat.

SHINING PATH. A matchstick-length silicon optical amplifier, touched by two copper probes, undergoes tests. Researchers have found a new way to run such amplifiers without wasting power. Fathpour

Now, researchers in California have found a way to hook up a silicon light amplifier so that it harvests power—much the way a solar cell does—rather than wastes it as heat. This new development could propel the computing industry toward optical connections needed to support increasingly fast data processing, says Bahram Jalali of the University of California, Los Angeles (UCLA).

In addition, should the new configuration also harvest power now wasted by lasers made of silicon—which seems likely—the development might yield improved lasers for chemical sensing, confounding enemy missiles, and laser-assisted mapping and surveillance, he adds.

Jalali heads a UCLA team, including Sasan Fathpour and Kevin K. Tsia, that orchestrated the power harvest. The team accomplished the effect in a light amplifier composed of a layer of silicon sitting on an electrically insulating material. In this amplifier, light from two lasers passes through a ridge of silicon on the top layer, causing the signal from one of the lasers to be amplified by the other.

In 2004, Jalali and some other colleagues were the first researchers to make silicon—a substance long regarded as a poor-quality material for optical components—act as a laser (SN: 10/30/04, p. 275: Available to subscribers at Laser Landmark: Silicon device spans technology gap). The perception that optical signals and silicon don’t go together has been changing as the UCLA team and other researchers have made increasingly capable light-manipulating devices from silicon (SN: 3/6/04, p. 157: Available to subscribers at Silicon goes optical).

However, a flaw has plagued the prototype silicon lasers and light amplifiers, all of which derive their energy from laser light. The intense light knocks loose electrons from silicon atoms in the devices. Those electrons then get in the way, absorbing light that the silicon components are striving to produce.

To solve that problem, researchers at Intel Corp in Santa Clara, Calif., added an electronic component—a diode—to a silicon laser and applied a large voltage to it to sweep the troublesome electrons out of the way (SN: 3/19/05, p. 189: Available to subscribers at Silicon chips land a lasting laser). The method worked, but at the cost of a large amount of power needed to run the diode.

In many solar cells, a diode also sweeps along electrons—in that case, those freed by solar radiation. However, the solar cell’s diode needs no applied voltage to do the job because of an intrinsic voltage naturally generated by the device itself.

Acting on a hunch, the UCLA team looked for that same behavior in their silicon light amplifier and found it. Without an applied voltage, the device amplified the laser light and generated a current from the laser-liberated electrons. “Not only do we not consume any power or generate any heat to achieve [amplification], we actually generate power from it,” Jalali says. In fact, current from the diode could power electronic devices used in tandem with optical ones, he adds.

“What’s novel here is that they’ve actually made [the electron flow] useful, whereas everyone else was getting rid of it,” comments Alexander L. Gaeta of Cornell University. “It’s a really clever scheme.”

Tsia presented the UCLA findings on June 28 at an optical-amplifier conference in Whistler, British Columbia. A report on the work will also appear in the Aug. 7 Applied Physics Letters.

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