Vertically stacked InAs quantum dots for polarization-independent semiconductor optical amplifiers

Abstract

This paper describes a technique to control the polarization property in quantum dot (QD)-semiconductor optical amplifiers (SOAs) using vertical stacking of self-assembled InAs QDs. QD-SOAs have been expected to realize high saturation power, multi-channel processing, and high-speed response. However, in conventional QDs, the significant polarization dependence in the optical gain caused by the flattened QD shape has been a serious problem. One of the well-known approaches to realize the polarization-independent gain relies on columnar QDs, in which InAs QDs layers are closely stacked with very thin (several monolayers) intermediate layers. The isotropic shape of columnar QDs realizes a polarization-independent gain. On the other hand, in this paper, we propose a different approach, where QDs are vertically stacked with moderately thick intermediate layers. Therefore each QDs layer is well separated geometrically and high precision control of overall QD shape is expected. Vertically aligned InAs QDs are known to create the electronically coupled states, where we expect the enhancement of the optical transition probability along the vertical direction. We have achieved such vertical stacking of QDs up to 9 layers by optimizing the amount of GaAs and InAs deposition. The 9-stacked QDs have shown transverse-magnetic-mode dominant emission in edge photoluminescence in the 1.3 μm telecommunication wavelength region. Our results have suggested that the electronically coupled QDs can be a powerful tool to realize the polarization-independent QD-SOAs