Nanowire Lasers Emit at Useful Wavelengths

Threadlike semiconductors can be made to emit light at useful wavelengths, demonstrating potential as lasers for applications in computing, communications, and environmental and biological sensing.

Scientists at Technical University Munich (TUM) demonstrated laser action in the semiconductor nanowires at room temperature, which they say could work with silicon chips, optical fibers and even living cells.

Nanowire laser researchers in the laboratory are (l-r) doctoral candidates Benedikt Mayer and Daniel Rudolph; Dr. Gregor Koblmüller; and professors Jonathan Finley and Gerhard Abstreiter. Courtesy of A. Heddergott/TUM.

"Nanowire lasers could represent the next step in the development of smaller, faster, more energy-efficient sources of light," said professor Jonathan Finley, director of TUM's Walter Schottky Institute. "But nanowires are also a bit special in that they are very sensitive to their surroundings, have a large surface-to-volume ratio, and are small enough, for example, to poke into a biological cell."

The experimental nanowire lasers emit light in the near-IR, approaching the "sweet spot" for fiber optics communications. They can be grown directly on silicon, presenting opportunities for integrated photonics and optoelectronics, and they operate at room temperature, a prerequisite for real-world applications.

The nanowires’ tailored core-shell structure, which is a complex profile of differing semiconductor materials tailored virtually atom by atom, enables them to act both as lasers, generating coherent pulses of light, and as waveguides, similar to optical fibers.

As with conventional communication lasers, the nanowires are made of III-V semiconductors, materials with the right bandgap to emit light in the near-IR. A unique advantage, Finley said, is that the nanowire geometry is "more forgiving than bulk crystals or films, allowing you to combine materials that you normally can't combine."

Because the nanowires arise from a base only tens to hundreds of nanometers in diameter, they can be grown directly on silicon chips in a way that alleviates restrictions due to crystal lattice mismatch – yielding high-quality material with the potential for high performance.

Semiconductor nanowires. Courtesy of WSI/TUM.

A number of significant challenges remain for the laser to move from applied research to future applications, they said. For example, laser emission from the nanowires was stimulated by light – as were the nanowire lasers reported almost simultaneously by a team at the Australian National University – yet practical applications are likely to require electrically injected devices.

The new results are largely due to scientists who are beginning their careers, under the guidance of Dr. Gregor Koblmüller and other senior researchers, and at the frontier of a new field. Doctoral candidates, including Benedikt Mayer, Daniel Rudolph, Stefanie Morkötter and Julian Treu, combined their efforts, working together on photonic design, material growth, and characterization using electron microscopy with atomic resolution.

Ongoing research is directed toward better understanding the physical phenomena at work in such devices as well as toward creating electrically injected nanowire lasers, optimizing their performance and integrating them with platforms for silicon photonics.

"At present, very few labs in the world have the capability to grow nanowire materials and devices with the precision required," said professor Gerhard Abstreiter, founder of the Walter Schottky Institute and director of the TUM Institute for Advanced Study. "And yet our processes and designs are compatible with industrial production methods for computing and communications. Experience shows that today's hero experiment can become tomorrow's commercial technology, and often does."

The work appears in Nature Communications doi: 10.1038/ncomms3931. In Nano Letters, (doi: dx.doi.org/10.1021/nl403341x) the team discloses additional results showing the nanowires' enhanced optical and electronic performance.

For more information, visit: www.wsi.tum.de or www.tum-ias.de