Polarization independent and ultra-fast optical response in quantum dot nanostructures for optical amplifier

光放大器是未来光通信系统不可缺少的组成部分。半导体光放大器（SOA）具有成本低、体积小等优点，但其存在严重的偏振各向异性问题。为了在真实的系统中使用，我们必须实现偏振无关特性。为了克服光学偏振各向异性的问题，我们专注于柱状量子点，可以通过紧密堆叠Sttranski-Krastanow模式InAs-QD层生长。量子点的纵横比可以通过改变堆叠层数（SLN）来控制。该技术有望实现偏振无关的半导体光放大器。已经在柱状量子点中研究了SLN对边缘发光偏振特性的影响。从这项研究中发现，TM模式的强度是占主导地位的SLN增加。该TE模式和TM模式的强度比在7层和9层之间翻转，因此在该区域中实现了偏振无关特性。偏振无关特性不仅在发光方面必须实现，而且在增益特性方面也必须实现，以便在真实的系统中实际使用。因此，我们采用可变条纹长度（VSL）方法研究了柱状量子点的光学增益，其中光学激发长度随着监测样品边缘放大的发光而变化。已经发现光增益敏感地依赖于SLN。随着SLN的增加，TM敏感的光学增益已被证明。此外，我们还获得了具有七个堆叠层的几乎偏振不敏感的光学增益样品。

Optical amplifier is indispensable for future optical communication systems. Semiconductor optical amplifier (SOA) is desirable in low cost and compactness, but it has a serious problem of optical polarization anisotropy. We must realize polarization-independent characteristics for their use in real systems. To overcome the optical polarization anisotropy problem, we focus on the columnar QDs that can be grown by closely stacking Stranski-Krastanow-mode InAs-QD layers. The aspect ratio of the QDs can be controlled by changing the stacking layer number (SLN). It is expected to enable polarization-independent SOA by using this technique.SLN dependence of the polarization characteristics in edge-luminescence has already been investigated in columnar QDs. From this research it was found that TM mode intensity is being dominated with increasing SLN. This TE mode and TM mode intensity ratio is turned over between 7 and 9 layers, so polarization-independent characteristics is realized in this region. The polarization-independent characteristics must be achieved not only in luminescence but also in gain characteristics for practical use in real systems. So we have investigated optical gain of columnar QDs by using variable stripe length (VSL) method in which optical excitation length is varied with monitoring the amplified luminescence from the sample edge. The optical gain has been found to sensitively depend on the SLN. With increasing the SLN, the TM-sensitive optical gain has been demonstrated. Furthermore, we have obtained almost polarization insensitive optical gain sample with seven staking layers.