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

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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.