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Laser Built on a Silicon Chip

SINGAPORE, April 2, 2012 — A laser with a novel microloop mirror (MLM) design fabricated on a silicon chip uses III-V semiconductor materials — a step forward for high-speed optical communications and interconnects on electronics chips.

Active optical fibers with silicon photonic chips carry much more information for data interconnects than copper cables. Silicon could be the material of choice for wiring lab-on-a-chip devices, but for its poor ability to emit light.

Now, scientists at the A*Star Data Storage Institute have successfully built a laser on top of a silicon chip, bonding III-V semiconductor materials to the device to provide optical gain. Compared with conventional feedback mirrors based on device facets, the laser’s unique MLM design promises enhanced operation.

Scanning electron microscope image of the silicon-based microloop mirror. Light entering the waveguide from the left is guided around the loop and redirected back into the laser structure. The inset shows the laser spot photographed with an infrared camera. (Image: A*Star)

“Integrated Si/III-V lasers can take advantage of low-loss silicon waveguides while addressing the problem of low light-emission efficiency that silicon devices typically have,” said Doris Keh-Ting Ng, an A*Star researcher.

Attaching a Si/III-V laser atop silicon requires some difficult fabrication techniques, and device performances can weaken as a result. And because lasers require mirrors to maintain lasing action, such designs typically rely on the interface between air and the semiconductor – the facets of the chip. Unfortunately, such mirrors are not perfect and further reduce operation efficiency.

To improve on the drawbacks associated with mirrors, the team designed the MLM, which guides light emitted from one end of the laser along the waveguide, around a narrow bend, and then back into the device. The mirror at the other end of the device is still formed by the interface with air so that laser radiation can exit. The scientists achieved a light reflection efficiency of 98 percent with this design.

More than 30 delicate, high-precision fabrication steps were needed to build the device. The researchers plan to further enhance the laser by miniaturizing the device.

“Further improvements, for example, at the interface between the mirror and the lasing structure itself could lead to even better performance,” Ng said. “A laser with lower threshold and higher output power can possibly be achieved, leading to a potential solution to develop high-speed and low-cost optical communications and interconnects on electronics chips.”

The work appeared in Applied Physics Letters.

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