Growing nanowire lasers directly on silicon promises to simplify photonic chip design
For over half a century, Moore's Law, which predicts that processor performance would double roughly every 18 months, has held true. But as electronics grow smaller and smaller, fundamental physical barriers loom ahead. To help stave off that day, a team of physicists at the Technical University of Munich (TUM) is working on nanowire lasers that are a thousand times thinner than a human hair and may one day lead to economical, high-performance photonic circuits.
According to Professor Jonathan Finley, Director of the Walter Schottky Institute at TUM, conventional electronics are entering a state of diminishing returns as it becomes more difficult to create ever tinier microcircuitry. One way to prevent this is by taking a lateral step and replacing the electrons with beams of light in what's known as photonics.
So far, creating silicon-based photonics chips has shown promise, but the need for an external laser source to power them is an inelegant design that complicates fabrication and limits miniaturization. In search of an alternative, a team led by Finley and Gregor Koblmüller has taken a page from previous micro-engineers by building the lasers directly on the chips in the same way that transistors and other components are today.
Koblmüller says that one major hurdle in moving the laser onto the chip was that the gallium arsenide used to make it has different thermal expansion properties to silicon, which creates stresses that can damage the laser. So instead, Finley and Koblmüller built the lasers as nanowires standing at right angles to the chip like tiny towers.
This makes for a smaller footprint and less stress and allows the towers to be made to grow thicker so they lase properly. However, another problem was how to fabricate the mirrors the lasers depend on.
"The interface between gallium arsenide and silicon does not reflect light sufficiently," says Benedikt Mayer, a doctoral candidate in the team. "We thus built in an additional mirror – a 200 nanometer thick silicon oxide layer that we evaporated onto the silicon. Tiny holes can then be etched into the mirror layer. Using epitaxy (depositing a crystalline overlayer on a crystalline substrate), the semiconductor nanowires can then be grown atom for atom out of these holes."
According to Finley, the current nanowire lasers produce light in the infrared part of the spectrum at a predefined wavelength, but the hope is to modify the emission wavelengths for better temperature stability and light propagation. Additionally, the lasers currently rely on pulsed excitation, so the team is working on a way to power the nanowire lasers using electricity supplied to them rather than relying on external lasers.
"The work is an important prerequisite for the development of high-performance optical components in future computers," says Finley. "We were able to demonstrate that manufacturing silicon chips with integrated nanowire lasers is possible."
The latest phase of the team's research will be published in Applied Physics Letters.