Nanowire wafer tunes over 200 nm

06 Feb 2009

Precise fabrication is the key to achieving a nanowire-based laser with a tuning range of 200 nm.

A single semiconductor laser chip with a tuning range of 200 nm has been unveiled by researchers at Arizona State University, US. The device tunes between 500 and 700 nm, a record-breaking performance according to research leader Cun-Zheng Ning. (Nano Letters doi: 10.1021/nl803456k)

Tuning over 200 nm

Widely tunable semiconductor lasers could be deployed in applications as diverse as spectroscopy and telecommunications to on-chip biological and chemical detection. Achieving such a widely tunable laser has been difficult however due to the limited lattice mismatch permitted by standard epitaxial-based approaches.

The emission wavelength of a semiconductor source depends on the material's bandgap. Widely tunable lasers require alloy semiconductors with similarly tunable bandgaps, which are determined by the alloy composition. In this case, lattice mismatch with the chosen substrate is the limiting factor.

With nanowire technology, Ning and colleagues say that the lattice mismatch limitation can be relaxed or removed completely. This means it is possible, in principle, to grow alloy semiconductors of widely changing composition on a single substrate.

By carefully controlling parameters such as the growth temperature, the team successfully fabricated ternary alloy CdS_xSe_1-x nanowires where the composition x changed from 0 to 1 continuously from one end of the substrate to the other. As a result, the wafer had a continuous spatial grading of bandgaps between 1.75 and 2.5 eV, or in terms of the wavelength, between 500 and 700 nm.

Since the wafer is covered by nanowires that individually act as nanolasers, numerous such nanolasers of continuously tuning wavelength are formed by a single growth on a single substrate. Under optical pumping, the team was able to show lasing behaviour at each spot along the wafer's length.

Ning's research group is currently working to extend the tunability to an even wider wavelength range and is developing several applications of its unique alloy materials in solar cells, detection, lighting and displays. Many more applications will open up once such lasers are made electrically driven.