Silicon Sees the Light

SEM image of the fabricated micro-disk (Source: AIP)

SEM image of the fabricated micro-disk (Source: AIP)

A group of scientists from Hong Kong University of Science and Techno­logy, the University of California, Santa Barbara, Sandia National Laboratories and Harvard University were able to fabricate tiny lasers directly on silicon. 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 until now.

As the group reports, integrating sub wavelength cavities – the essential building blocks of their tiny lasers – onto silicon enabled them to create and demonstrate high-density on-chip light-emitting elements. To do this, 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 toward photonics and elec­tronics inte­gration on the silicon platform,” said Kei May Lau, Depart­ment of Electronic and Computer Engineering, Hong Kong University of Science and Techno­logy.

Traditionally, the lasers used for commercial appli­cations are quite large. Smaller lasers tend to suffer from large mirror loss. But 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,” said Lau.

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 pointed out. “And these tiny high-performance lasers can be grown directly on silicon wafers, which is what most integrated circuits are fabricated with.”

In terms of applications, the group’s tiny lasers on silicon are ideally suited for high-speed data communi­cations. “Photonics is the most energy-ef­ficient and cost-effective method to transmit large volumes of data over long distances. Until now, laser light sources for such applications were missing from the component,” Lau explained. “Our work enables on-chip inte­gration of lasers, an indis­pensable component, with other silicon photonics and micro­processors.”

The researchers expect to see this techno­logy emerge in the market within 10 years. Next, the group is “working on elec­trically pumped lasers using standard micro­electronics techno­logy,” Lau said. (Source: AIP)

Reference: Y. Wan et al.: Sub-wavelength InAs quantum dot micro-disk lasers epitaxially grown on exact Si (001) substrates, Appl. Phys. Let. 108, 221101 (2016), DOI: 10.1063/1.4952600

Link: Photonics Technology Center, Dept. of Electronic and Computer Engineering (K. M. Lau), Hong Kong University of Science and Technology, Kowloon, Hong Kong


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