Simultaneous Realization of Ultrafast and High-Nonlinearity Optical Response Characteristics by Shape-Controlled Quantum Dots

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Quantum dots are expected to play a key role in future optical communications due to their advantages in enhancing linear optical gain and nonlinear optical response, as well as ultrafast response speed, which result from the three dimensionally confined electron states. However, quantum dots formed by SK-mode growth have flattened shape much different from the ideal sphere, and this causes polarization sensitiveness, usually being heavily TE-polarized, and also there are no precise understanding of the effect of dot shape on the optical response speed and nonlinearity.In this study, it has been shown from polarization discriminating photoluminescence measurements that the polarization property can be controlled by optimizing the dot shape using columnar dot structure, in which SK-dot layers are stacked. Also it has been shown that the polarization properties are controlled by changing the composition of InGaAs capping layer on InAs dots primarily due to the effect of dot aspect ratio, which is enhanced by the introduction of InGaAs capping layer resulting in TM dominant polarization. This indicates that the polarization insensitiveness, which is one of the most important device characteristics in practical application, can now be controlled by designing the dot shape.Our previous results have demonstrated that quntum dot optical amplifier exhibits ultrafast absorption recovery time in the range of 1-3 ps. Also it has been shown from degenerate four-wave mixing (DFWM) experiments that large size dots show phase relaxation time larger that that for small size dots, indicating the possibility of enhanceing nonlinearity in larger dots.Combining all these results obtained, it is concluded that polarization insensitive, ultarafast and high-nonlinearity optical response can be simultaneously achieved by adopting isotropically shaped, large size dots.