Optical anisotropy in vertically coupled quantum dots

We have studied the polarization of surface and edge-emitted photoluminescence (PL) from structures with vertically coupled ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum dots (QD’s) grown by molecular beam epitaxy. The PL polarization is found to be strongly dependent on the number of stacked layers. While single-layer and 3-layer structures show only a weak TE polarization, it is enhanced for 10-layer stacks. The 20-layer stacks additionally show a low-energy side-band of high TE polarization, which is attributed to laterally coupled QD’s forming after the growth of many layers by lateral coalescence of QD’s in the upper layers. While in the single, 3- and 10-layer stacks, both TE polarized PL components are stronger than the TM component, the [110] TE component is weaker than the TM component in the 20-layer stack. This polarization reversal is attributed to an increasing vertical coupling with increasing layer number due to increasing dot size.