UCLA announces breakthrough in silicon photonics

July 3, 2006, Los Angeles, CA--Building on a series of recent breakthroughs in silicon photonics, researchers at the UCLA Henry Samueli School of Engineering and Applied Science have developed a novel approach to silicon devices that combines light amplification with a photovoltaic or solar panel effect.

In a study presented last month at the 2006 International Optical Amplifiers and Applications Conference in Vancouver, Canada, UCLA Engineering researchers reported that not only can optical amplification in silicon be achieved with zero power consumption, but power can now be generated in the process. The team's research shows that silicon Raman amplifiers possess nonlinear photovoltaic properties, a phenomenon related to power generation in solar cells. In 2004, the same group at UCLA Engineering demonstrated the first silicon laser, a device that took advantage of Raman amplification.

The fundamental challenge in silicon photonics is the material stops being transparent at high optical intensities, making light unable to pass through.

"As light intensifies in silicon, it generates electrons through a process called two-photon-absorption," said Bahram Jalali, the UCLA Engineering professor who led researcher Sasan Fathpour and graduate student Kevin Tsia in making the recent discovery. "Excess electrons absorb the light and turn it into heat. Not only is the light and the data-carrying capacity lost, the phenomenon exacerbates one of the main obstacles in the semiconductor industry, which is excessive heating of chips. The optical loss also makes it all but impossible to create optical amplifiers and lasers that operate continuously."

In previous attempts to deal with this challenge, a diode attached to the chip has been used to "vacuum" out the electrons which block light. This approach presents further problems, however, because the vacuum adds an additional watt of heat onto the chip nearly a million times the power that a single transistor consumes in a digital circuit.

"In the past, two-photon absorption in silicon has resulted in significant loss for high power Raman amplifiers and lasers, reducing efficiency and necessitating complex mitigation schemes. UCLA Engineering's new development will enable recycling power that would otherwise be lost. In space and military laser systems, the impact of device efficiency on electrical power and thermal management is a prime consideration," said Robert Rice, senior scientist at Northrop Grumman Space Technology's Laser and Sensor Product Center.

The challenge of power dissipation in traditional silicon semiconductors already is so severe that it threatens to halt the continued advance of the technology described by Moore's law. Because the UCLA Engineering team's discovery creates an advantage in heat dissipation, it represents a new perspective.

Jalali's research has been funded by the U.S. Department of Defense through the Defense Advanced Research Project Agency. The research was also co-sponsored by Northrop Grumman.

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