Tiny lasers that enable chips to run faster

Scientists have fabricated tiny high-performance lasers directly on silicon wafers, a breakthrough which can enable next-generation microprocessors to run faster and consume less power.
For more than 30 years, the crystal lattice of silicon and of typical laser materials could not match up, making it impossible to integrate the two materials.
Now, a group of scientists, including those from University of California, Santa Barbara and Harvard University have been able to fabricate tiny lasers directly on silicon - a breakthrough for the semiconductor industry and well beyond.
Integrating sub-wavelength cavities - the essential building blocks of tiny lasers - onto silicon enabled researchers to create and demonstrate high-density on-chip light-emitting elements.

They first had to resolve silicon crystal lattice defects to a point where the cavities were essentially equivalent to those grown on lattice-matched gallium arsenide (GaAs) substrates.
Nano-patterns created on silicon to confine the defects made the GaAs-on-silicon template nearly defect free and quantum confinement of electrons within quantum dots grown on this template made lasing possible.

The group was then able to use optical pumping, a process in which light, rather than electrical current, "pumps" electrons from a lower energy level in an atom or molecule to a higher level, to show that the devices work as lasers.
"Putting lasers on microprocessors boosts their capabilities and allows them to run at much lower powers, which is a big step towards photonics and electronics integration on the silicon platform," said professor Kei May Lau from Hong Kong University of Science and Technology.

Traditionally, the lasers used for commercial applications are quite large - typically 1 mm x 1 mm. Smaller lasers tend to suffer from large mirror loss.
However, the scientists were able to overcome this issue with "tiny whispering gallery mode lasers - only 1 micron in diameter - that are 1,000 times shorter in length, and 1 million times smaller in area than those currently used.
Whispering gallery mode lasers are considered an extremely attractive light source for on-chip optical communications, data processing and chemical sensing applications.
"Our lasers have very low threshold and match the sizes needed to integrate them onto a microprocessor," Lau said.

"And these tiny high-performance lasers can be grown directly on silicon wafers, which is what most integrated circuits (semiconductor chips) are fabricated with," he said.
The group's tiny lasers on silicon are ideally suited for high-speed data communications.
The research was published in the journal Applied Physics Letters.