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
An all-optical switching device has been proposed by using self-assembled InAs/GaAs quantum dots (QDs) within a
vertical cavity structure for ultrafast optical communications. This device has several desirable properties, such as the
ultra-low power consumption, the micrometre size, and the polarization insensitive operation. Due to the threedimensional
confined carrier state and the broad size distribution of self-assembled InAs/GaAs QDs, it is crucial to
enhance the interaction between QDs and the cavity with appropriately designed 1D periodic structure. Significant
QD/cavity nonlinearity is theoretically observed by increasing the GaAs/AlAs pair number of the bottom mirror. By this
consideration, we have fabricated vertical-reflection type QD switches with 12 periods of GaAs/Al0.8Ga0.2As for the top mirror and 25 periods for the bottom mirror to give an asymmetric vertical cavity. Optical switching via the QD excited
state exhibits a fast switching process with a time constant down to 23 ps, confirming that the fast intersubband relaxation of carriers inside QDs is an effective means to speed up the switching process. A technique by changing the light incident angle realizes wavelength tunability over 30 nm for the QD/cavity switch.
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