A team at Max Planck Institut für Festkörperforschung in Stuttgart, Germany, has produced InGaAs/GaAs tubes with InAs quantum dots embedded in their walls. The structures suggest the feasibility of using self-assembly techniques to produce high-quality-factor (Q) resonant cavities coupled with quantum emitters for use in basic research into quantum electrodynamics and in the development of devices — such as low-threshold ring lasers — and of technologies for quantum information processing. The starting material, grown by molecular beam epitaxy, incorporates InAs quantum dots in an InGaAs/GaAs strained bilayer (a). The AlAs acts as a sacrificial layer that can be etched away to release the bilayer from the GaAs substrate, enabling it to roll into a tube. Photoluminescence experiments were performed by illuminating the cryogenically cooled tubes and bulk material with 532-nm laser light (b). A false-color photoluminescence image reveals whispering-gallery modes at the laser excitation in the middle of a tube (c). The broad, blue spots at both ends of the tube correspond to waveguided light. Courtesy of Stefan Mendach. In the experiments, the scientists employed solid-source molecular beam epitaxy to grow a stack of semiconductor materials incorporating InAs quantum dots embedded in an InGaAs/GaAs strained bilayer. Using lithography and wet chemical etching, they selectively freed the bilayer, which rolled upon itself under the strain of the lattice mismatch. The number of rotations, and the length and location of the suspended tubes can be controlled with the process, and their diameter is determined by the thickness and composition of the strained bilayer.Greater signalTo investigate the optical properties of the structures, the researchers cooled 8-μm-long, 10-μm-diameter tubes to 7 K and illuminated them with a 2-μm-diameter spot of 14-μW, 532-nm light from a Coherent Inc. Verdi V10 Nd:YVO4 laser, collecting the resulting photoluminescence with a Princeton Instruments SpectraPro 2500i 500-mm-focal-length spectrometer and an InGaAs array detector. For high-resolution photoluminescence imaging, they employed a Spiricon Inc. LBA-FW-SCOR20 laser beam analysis system.They found that the signal from dots in the tubes increased by 3.5 times over that from those in the stack, with an accompanying redshift of approximately 10 meV attributable to changes in the strain distribution before and after the self-assembly process. The difference in the photoluminescence from the bulk material and from a 50-μm-long, 10-μm-diameter tube also indicated that the tubes act as waveguides.Other methods have yielded whispering-gallery-mode resonators with extremely high Q’s, but at present, resonators that incorporate quantum emitters (e.g., semiconductor microdisks) suffer from losses caused by surface roughness. Stefan Mendach of the institute noted that the rolled-up tubes could be optimized to have atomically flat outer surfaces, avoiding this source of loss. Tube resonators incorporating dots promise two other advantages over alternative approaches, he said. For one, they confine the quantum dots in the maximum of the mode field of the structures, which is particularly important for quantum electrodynamics experiments. For another, they are mechanically flexible, so it should be possible to tune the mode peaks into resonance with the dots as desired by slightly deforming the structures.Mendach said the team plans to investigate the experimentally accessible limits of the Q of such structures and whether they are suitable for strong-coupling experiments.Applied Physics Letters, March 13, 2006, 111120.