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

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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.