Development of Polarization-Independent Absorption Edge of Shape-Controlled Quantum Dots

高质量自组装量子点的最新进展有助于开发新的光学器件，如量子点半导体光放大器（SOA）。在实际应用中，偏振特性的控制是开发偏振无关SOA的前提。为了实现偏振不敏感的带边，我们提出了一种新的方法，使用形状控制的量子点。在紧密堆叠的Stranski-Krastanov（S-K）模式InAs QD（即，柱状QD）中，通过堆叠可以改变量子限制方向的S-K模式生长层，已经成功地人工控制了高度。采用分子束外延技术生长了具有柱状量子点的SOA结构。观察到各层上的岛在垂直方向上彼此接触，使得堆叠岛结构整体上变成柱状。柱状量子点的直径约为17 nm。高度由堆叠层数（SLN）控制。SLN为8的典型高度约为13 nm。线性光致发光（PL）偏振光谱测量从一个分裂的，未涂覆的边缘表面的样品。通过沿着面内（生长）方向设置检偏器来测量TE（TM）模式PL。半导体光放大器的偏振不灵敏度在InAs/GaAs自组装量子点中得到了精确的控制。偏振不敏感带宽大于40 nm。这些结果表明，宽带偏振不灵敏度已完成通过控制量子点的形状。

Recent advance in high-quality self-assembled QDs contributes to the development of new optical devices such as QD semiconductor optical amplifiers (SOAs). For practical applications, control of the polarization property is prerequisite for developing polarization-independent SOAs. To accomplish the polarization insensitive bandedge, we propose a new approach using shape-controlled QDs. In closely stacked Stranski-Krastanov (S-K)-mode InAs QDs, namely, columnar QDs, the height has been successfully controlled, artificially, by stacking the S-K-mode growth layer, which can change the quantum-confinement direction. SOA structures with columnar QDs were grown by molecular beam epitaxy. The islands on each layer were observed to be in contact with each other in the perpendicular direction, so that the stacked island structure, as a whole, becomes columnar in shape. The diameter of the columnar QDs is about 17 nm. The height was controlled by the stacking layer number (SLN). The typical height for the SLN of 8 is about 13 nm. Linear-photoluminescence (PL) polarization spectra were measured from a cleaved, uncoated edge surface of the sample. The TE (TM)-mode PL was measured by setting an analyzer along the in-plane (growth) direction. Polarization insensitivity of semiconductor optical amplifier has been shown to be precisely controlled in closely stacked InAs/GaAs self-assembled quantum dots. The polarization insensitive bandwidth achieved is larger than 40nm. These results demonstrate that wideband polarization insensitivity has been accomplished by controlling QD shape.