By creating semiconductor nanowires surrounded by multiple concentric quantum wells, researchers at Harvard University and the Georgia Institute of Technology have succeeded in fabricating tunable miniature lasers [Qian et al, Nature Materials (2008) doi:10.1038/nmat2253].
These nanowires are long, thin rods, with diameters of just a few hundred nanometers but lengths of several micrometers. They can be synthesized through self-assembly, and they demonstrate electronic properties that are not present in the bulk material.
To date, most nanowires have consisted of a single material, leaving little possibility to vary the properties directly. When applied to lasing, for example, the material choice determines both the optical gain of the medium and the dimensions of the cavity, making it difficult to tune the emission wavelength.
In their study the researchers addressed this limitation by starting out with a single gallium-nitride core, but surrounding it with alternating layers of indium-doped material. While these layers (quantum wells), have thicknesses of just a few nanometers and alter the structure very little, they strongly impact its electronic properties. For the miniature lasers, it means that the properties of the gain medium can now be adjusted nearly independently from the optical cavity itself.
This approach is not limited to lasing, however, as Charles Lieber of Harvard University says. “These Multi-Quantum Well (MQW) nanowires go far beyond the compositional variation demonstrated in previous experiments, and enable new or enhanced properties associated with quantum confinement”.
By changing the composition of the shell layers, the researchers were able to tune the lasing wavelength from 365 to 494 nm. “However, these lasers are only an example of what is possible with this new approach,” Lieber continues. “The ultimate goal of this research is to develop electrically-driven, multicolor, low-threshold nanolaser arrays, and to apply them to integrated optical-electronic chips. We are actively pursuing this goal in two directions, including optimization of the MQW structure to achieve lower laser thresholds (based on optical pumping studies), and electrical excitation of MQW lasers through complementary doped core/shells.”