†Department of Physics, Chemistry and Biology, Linköping University, Linköping S-581 83, Sweden.
‡Department of Electrical and Computer Engineering, University of California, La Jolla, California 92093, United States.
ACS Nano. 2015 Jun 23;9(6):5741-9. doi: 10.1021/acsnano.5b01387. Epub 2015 May 15.
Fine-structure splitting (FSS) of excitons in semiconductor nanostructures is a key parameter that has significant implications in photon entanglement and polarization conversion between electron spins and photons, relevant to quantum information technology and spintronics. Here, we investigate exciton FSS in self-organized lateral InAs/GaAs quantum-dot molecular structures (QMSs) including laterally aligned double quantum dots (DQDs), quantum-dot clusters (QCs), and quantum rings (QRs), by employing polarization-resolved microphotoluminescence (μPL) spectroscopy. We find a clear trend in FSS between the studied QMSs depending on their geometric arrangements, from a large FSS in the DQDs to a smaller FSS in the QCs and QRs. This trend is accompanied by a corresponding difference in the optical polarization directions of the excitons between these QMSs, namely, the bright-exciton lines are linearly polarized preferably along or perpendicular to the [11̅0] crystallographic axis in the DQDs that also defines the alignment direction of the two constituting QDs, whereas in the QCs and QRs, the polarization directions are randomly oriented. We attribute the observed trend in the FSS to a significant reduction of the asymmetry in the lateral confinement potential of the excitons in the QRs and QCs as compared with the DQDs, as a result of a compensation between the effects of lateral shape anisotropy and piezoelectric field. Our work demonstrates that FSS strongly depends on the geometric arrangements of the QMSs, which effectively tune the degree of the compensation effects and are capable of reducing FSS even in a strained QD system to a limit similar to strain-free QDs. This approach provides a pathway in obtaining high-symmetry quantum emitters desirable for realizing photon entanglement and spintronic devices based on such nanostructures, utilizing an uninterrupted epitaxial growth procedure without special requirements for lattice-matched materials combinations, specific substrate orientations, and nanolithography.
半导体纳米结构中激子的精细结构分裂（FSS）是一个关键参数，它在光子纠缠和电子自旋与光子之间的极化转换方面具有重要意义，与量子信息技术和自旋电子学有关。在这里，我们通过偏振分辨微光致发光（μPL）光谱研究了自组织横向 InAs/GaAs 量子点分子结构（QMS）中的激子 FSS，包括横向对准的双量子点（DQD）、量子点团簇（QC）和量子环（QR）。我们发现，在所研究的 QMS 中，FSS 随其几何排列而呈现明显的趋势，从 DQD 中的大 FSS 到 QC 和 QR 中的小 FSS。这种趋势伴随着这些 QMS 中激子的光学偏振方向的相应差异，即明亮激子线优选地沿或垂直于 DQD 中的[11̅0]晶体学轴线性偏振，该轴也定义了两个构成 QD 的对准方向，而在 QC 和 QR 中，偏振方向是随机的。我们将观察到的 FSS 趋势归因于 QR 和 QC 中激子的横向限制势的非对称性显著降低，这是由于横向形状各向异性和压电场的补偿效应的结果。我们的工作表明，FSS 强烈依赖于 QMS 的几何排列，这有效地调节了补偿效应的程度，并能够将 FSS 甚至在应变 QD 系统中降低到类似于无应变 QD 的极限。这种方法提供了一种途径，可以获得高对称的量子发射器，这对于基于这种纳米结构实现光子纠缠和自旋电子器件是理想的，它利用了无需特殊要求的晶格匹配材料组合、特定衬底取向和纳米光刻的连续外延生长过程。
