PORTLAND, Ore. — Continuous-wave silicon lasers were demonstrated for the first time last year by Intel Corp. But the new lasers generated too much heat to be practical for CMOS devices.
Researchers at the University of California at Los Angeles now claim to have the solution: Harness the excess energy with a photovoltaic effect that converts heat back into electricity to power the chip.
"We are basically using the same architecture that Intel showed last year when it demonstrated the world's first continuous-wave silicon laser," said Bahram Jalali, a professor in UCLA's electrical engineering department. "Our discovery shows how to make that sort of laser practical for CMOS, by recycling the power that would otherwise cause excessive heat."
The researchers also beat Intel in the race for the world's first silicon laser in 2004, when they reported a pulsed silicon laser. The laser could not operate continuously because it produced excess hot electrons. In 2005, Intel followed up with a technique for sinking hot electrons, clearing the way for the first continuous-wave silicon laser. The first version did not use excess energy.
The UCLA group has again topped Intel by showing how a Raman scattering architecture can be used in CMOS devices to harness excess energy from hot electrons.
"Intel's solution uses a diode to remove the excess electrons, but that adds an additional watt or so of heat to the chip," said Jalali. "Our discovery shows how to use the photovoltaic effect to convert that excess energy into usable on-chip electricity."
The mechanism, stimulated Raman scattering, sidesteps silicon's indirect bandgap that prevents normal lasing. However, Raman scattering works through double-photon absorption, which creates hot electrons as a byproduct and generates excess heat.
The researchers claimed that a photovoltaic sink not only enables continuous operation of silicon laser chips, optical amplifiers and similar photonic devices, but it can now be economically incorporated into CMOS. Moreover, the excess energy can now be harvested to drive other CMOS circuitry.
"We now believe that not only can optical amplification in silicon be achieved with zero power consumption, but that any excess power can be converted and used on-chip by other CMOS circuits," said Jalali.
The team is next attempting to design a CMOS laser chip that would power its circuitry by harvesting hot electrons using the photovoltaic effect. The technique was announced Wednesday (June 28) at the 2006 International Optical Amplifiers and Applications Conference in Vancouver, B.C. The research was funded by the Defense Department, Defense Advanced Research Project Agency and Northrop Grumman Corp.
