From silicon seeds, laser might sprout

Silicon microchips could perform faster and better if light rather than electrons was used to ferry signals between transistors. Packing silicon into tiny nuggets known as quantum dots (SN: 6/17/00, p. 392) may make such light-accelerated silicon circuitry possible, new research shows.

Despite being the mainstay material of electronics, silicon generally is ill-suited for light-producing devices such as lasers and light-emitting diodes. A research team led by Lorenzo Pavesi of the University of Trento in Povo and Italy’s National Institute for the Physics of Matter nevertheless reports amplifying a laser beam’s intensity by passing it through a layer of silicon quantum dots. The researchers first excited the dots, each just 3 nanometers in diameter, with another laser beam.

Pavesi and his colleagues report their work in the Nov. 23 Nature. “Before this paper, people said that building a laser with silicon is impossible. Now . . . people say that it’s worth a try,” Pavesi notes.

At the heart of the light amplifier are quantum dots, each of only 300 to 500 silicon atoms. The dots form a dense layer inside a tiny rail of silicon dioxide, a widely used electric insulator.

A laser beam passing lengthwise through the rail brightened by 10 percent, Pavesi says. To make a silicon laser, the researchers still need to put the quantum-dot layer in a mirror-lined cavity. As light waves emitted by the dots bounce within the cavity, they’ll become amplified and coherent–perfectly aligned crest-to-crest and trough-to-trough.

Even if the researchers succeed in that task, the resulting device would be considered an optically pumped laser, which works only when an external light source stimulates it. For silicon lasers to become commercially appealing, they must generate light from electric input. Pavesi says that would be tough since the dots are shielded from electricity by an insulator.

Still, the work may spur researchers to pursue making lasers of deeply etched, or porous, silicon, which has been shown to emit light in response to electric signals but not to amplify light, he says.

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