Research paper of the month
April 2015 - Dislocations in microdisks
Microdisk cavities trap light in "whispering gallery modes" which circulate around the periphery of the disk. By confining light in this way, it is possible to study the interactions between light and matter. However, this can only be achieved if the cavity is of extremely high quality, which can be very difficult in materials which contain defects. In this study, the impact of dislocations on the "quality factor" of microdisk cavities is studied by individually measuring the optical performance of multiple devices and then using cathodoluminescence to count the number of dislocations in each device and to measure their location. The results show that the quality factor of the cavity is reduced by each dislocation in the periphery of the device, where the whispering gallery modes reside, whilst dislocations far from the periphery have no impact on device performance. Surprisingly, modelling studies show that it is not the impact of the dislocation on the optical properties which limits performance but the formation of a whisker around the dislocation on the underside of the cavity. The whisker allows light to be coupled out of the cavity into the substrate. Hence, by developing fabrication methods which avoid whisker formation, better cavities can be formed.
Figure: (top) scanning electron microscope (SEM) image and (bottom) cathodoluminescent image of a completed microdisk from the side.
Tim J. Puchtler, Alexander Woolf, Tongtong Zhu, David Gachet, Evelyn L. Hu, and Rachel A. Oliver, "Effect of Threading Dislocations on the Quality Factor of InGaN/GaN Microdisk Cavities", ACS Photonics, 2 (1) (2015) 137–143