Silicon Laser Relies on Quantum Wells

Breck Hitz

Silicon-based lasers have been much in the news recently, as some researchers have explored Raman lasers in silicon, while others have pursued doping or the introduction of a lasing element into silicon. These developments hold the promise of integrating electronics and photonics into monolithic, superefficient chips.

Now scientists at the University of California, Santa Barbara, have demonstrated a novel approach to silicon-based lasers by bonding AlGaInAs quantum wells to a silicon waveguide on a silicon substrate. The electromagnetic fields generated in the quantum wells are evanescently coupled into the waveguide, where lasing occurs. Although the laser was optically pumped in its initial demonstration, the researchers believe that it could be electrically pumped with additional development.

Figure 1. Demonstrating what they believe is the first silicon-based evanescently coupled laser, scientists fabricated a device integrating a silicon waveguide and AlGaInAs quantum wells. Inset: H = 0.97 µm, W = 1.3 µm, D = 0.78 µm. Images ©OSA.

The integrated device comprises a silicon-on-insulator waveguide and an active region that includes five 70-nm-thick AlGaInAs quantum wells (Figure 1).

Conceptually, although the population inversion is physically outside the silicon waveguide, its energy is coupled by evanescent fields into the waveguide. This effective gain from evanescent coupling, together with feedback provided by the facets at either end of the 600-µm-long waveguide, provides the necessary conditions for lasing.

In a demonstration of the device, the scientists pumped the laser with up to 80 mW of 980-nm pulsed power, which they focused to a 10 × 916-µm rectangle on the top surface of the device. They observed up to 1.4 mW of output at 1538 nm; the laser threshold was 30 mW. The calculated mode profile closely resembled the mode profile photographed with an infrared camera (Figure 2).

Figure 2. The calculated beam profile, using BeamProp from RSoft Design Group Inc. of Ossining, N.Y., shows the intense lasing field in the silicon waveguide (a). The observed beam profile, photographed with an infrared camera, closely resembles the calculated profile (b).

Although energy also was coupled by evanescent fields into the slabs of III-V material between the quantum wells and the silicon waveguide, there was insufficient feedback to support lasing in these regions. By translating the pump beam across the top surface of the laser, the researchers saw that lasing occurred only when the quantum-well area directly above the waveguide was illuminated.

The laser’s maximum output of 1.4 mW was limited by the available pump power and not by thermal effects in the laser itself, so the investigators believe that the device is capable of higher outputs. They observed a differential quantum effi-ciency of 3.2 percent, taking into account the output from only one facet of the laser, and they estimate that the total efficiency, taking into account the outputs from both facets and ignoring coupling losses, would be ~20 percent.

For the ultimate design flexibility in integrating silicon photonics with silicon electronics, an electrically pumped — rather than optically pumped — laser is highly desirable. The group believes that its concept can be readily extended to demonstrate an electrically pumped laser by providing the well-understood semiconductor structures to facilitate flow of electrical current through the III-V section of the device.