Fast silicon chips that use both light and electricity to process information and communicate could be made more efficient using a new "energy harvesting" technique developed by US researchers.

Silicon-based optoelectronic devices promise to revolutionise computing and telecommunications by boosting the speed at which information can be fed through a system and the amount of data, or bandwidth, that can be processed. But existing devices require a lot of light and also give off unacceptable amounts of heat.

Now a team at the University of California, Los Angeles (UCLA), in the US, has found a way to make optoelectronic devices more efficient by having them "harvest" electrons to generate current instead of heat.

Chain reaction

Performing basic optical operations in silicon, such as amplifying and modulating light, converting one wavelength to another and lasing, require an intense beam of light. This light interacts with silicon in a non-linear way, which is necessary for useful optical effects.

However, pushing intense beams of light through silicon also results in a problem known as two-photon absorption. This occurs when part of the silicon's crystal lattice absorbs two photons at once, which frees more electrons to absorb still more photons. "It's like a chain reaction," says Bahram Jalali, head of UCLA's optoelectronics lab.

"The problem is fundamental," he adds. "You have to have non-linear processes, so you have to pump silicon hard. When you pump it hard, silicon becomes like a sponge – it absorbs all the light."

Computer chip giant Intel tried one approach to solving the problem of two-photon absorption. They surrounded a silicon-based laser with positive and negative charges that swept out light-absorbing electrons (See Silicon-based laser breakthroughs promise practicality). However, the technique was extremely inefficient – the device produced 125 times more waste heat than usable light.

Jalali and UCLA colleagues Sasan Fathpour and Kevin Tsia wondered if two-photon absorption could instead be harnessed as a way of generating electricity.

Electron harvesting

They found that adding a diode to a silicon-based laser creates an electric field, which allows the free elections to be harvested, and as a result makes it possible to generate electric power.

"It's one of the few places where nature allows you to have your cake and eat it too," says Jalali. In recent months, Jalali's team have implemented this photovoltaic effect in two other simple silicon-based devices – an optical modulator and a wavelength converter.

But they think the approach will work in the full range of optoelectronic applications, from teraflop computer chips to transcontinental communication lines. "You can apply it to any optical device that suffers from loss," says Fathpour.

"It's a very clever approach," says Philippe Fauchet, an applied physicist at the University of Rochester in New York State. "I did not expect it at all, which is always a nice surprise."

The UCLA team will present its findings at the Conference on Lasers and Electro-Optics, in Maryland, US, on 11 May.