When the nanopillar is pumped with light from another laser, the light spirals around inside the pillar, as if running up and down a spiral staircase. The difference in materials between the core and the shell encourages this effect, trapping the photons in this spiral until they reach a high enough energy threshold and are emitted. This spiraling effect is something that hasn’t been seen in other types of lasers before. These results are described in the journal Nature Photonics. The next step is to demonstrate that the laser can be pumped with electrical energy, key to making a compact laser. Chang-Hasnain is confident the Berkeley researchers will make an electrically pumped laser. In another publication in Nano Letters her group demonstrated exotic semiconductor diodes on silicon, which they’re now adapting to pump the nanolasers.
Another key to making lasers on silicon chips is not to let the temperature get too high. Chang-Hasnain says that her process could eventually be used to grow high-quality lasers on otherwise finished silicon chips patterned with transistors and optical components, giving them the capability of encoding data into pulses of light. Depositing high-quality III-V semiconductor crystals usually requires higher temperatures—instead of 400 °C, these materials are usually grown at 700 °C, a temperature that would destroy a microprocessor. Chang-Hasnain says it’s the nanostructure of the lasers that makes this possible: high-quality nanostructures can generally be grown at lower temperatures than large films made from the same materials.
“A lot of progress has been made on silicon optical components,” says Intel’s Paniccia. However, progress on lasers that are compatible with silicon chips has lagged behind. Researchers have made various optical components from silicon using materials and processes already present in chip-manufacturing lines. But they have to add on the lasers afterward. Intel, IBM, and other companies have been developing such workarounds.
Chang-Hasnain acknowledges that the group has many more things to prove, from electrical pumping of the lasers to proving they provide enough light of the right wavelengths and can couple it to other optical components. But Intel’s Paniccia says the demonstration that these laser materials can be made compatible with silicon is “a big step.”
Katherine Bourzac is the materials science editor for Technology Review.