Silicon Chips May Go 'Green'

Harvesting optical energy currently lost through heat dissipation in optoelectronic devices could allow them to actually generate more power than they use and help make silicon chips and compound semiconductors more "green."

In a well-attended presentation Wednesday afternoon at Frontiers in Optics 2009, Sasan Fathpour, assistant professor of integrated photonics & energy solutions at the University of Central Florida's CREOL (College of Optics and Photonics), outlined the benefits and challenges facing engineers attempting to make integrated photonic components more environmentally friendly.

Computing-related power usage represented 15 percent of the total power consumption in the US in 2007, he said, and most of the power used by large data centers is consumed by its communications equipment. The annual power requirements of such massive centers, including Internet giant Google, represents seven times the power generation of the Hoover Dam.

Integrated circuits have become faster and faster in recent years by adding more and more transistors to chips – some have nearly 2 billion.

In a related FiO talk on the road to building an exascale supercomputer, Jeffrey Kash of IBM Research said progress in chip clock speeds has recently slowed because engineers have essentially reached the upper limits of on-chip electronics. To make more powerful computing systems now, they are not making more powerful microprocessors, just using more of them (dual core, quad-core etc.). Also, electrical packaging is running out of pin space. Increasing the role of optics could help with both problems, he said.

Under the high optical intensities required by today's chips, silicon begins "soaking up" photons in a process called two-photon absorption, or TPA. That process leads to more electrons being freed to gobble up even more photons (free-carrier absorption). The loss of photons is the main problem to be overcome in silicon devices that use nonlinear optics to perform optical amplification, lasing, wavelength conversion and switching.

Fathpour and his CREOL colleagues have demonstrated that the optical energy lost to TPA can be converted into electrical power used to drive the chip. They came to that conclusion by taking a concept used in photovoltaic (PV) processes – optical absorption – and applying it to silicon.

Their process, called the two-photon photovoltaic effect (TPPV), collects 40 percent of the carriers lost through TPA, Fathpour said, harvesting their energy. TPPV is the nonlinear equivalent of the single-photon PV effect exploited in solar cells.

"This is the first observation of the two-photon PV effect in any material," Fathpour said.

TPPV can also be used to harvest energy in Raman amplifiers, in optically powered sensors for fiber optic networks, and in compound semiconductors, as the free-carrier absorption effect in III-V materials is typically the same as in silicon. It could also lead to all-silicon optically powered sensors, which could be beneficial for applications where sparks generated by electrical wiring would be dangerous, such as in coal mines and near airliner fuel tanks, Fathpour said.

For more information, visit: www.frontiersinoptics.com or see the article on green silicon photonics cover article by Fathpour and colleagues in the June 2009 issue of Optics and Photonics News.

Melinda Rose
Senior Editor

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