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

由于量子点具有提高线性光增益和非线性光响应的优势，以及由于三维受限的电子态而产生的超快响应速度，有望在未来的光通信中发挥关键作用。然而，sk模式生长形成的量子点的形状扁平，与理想的球体有很大的不同，这导致了偏振敏感性，通常是严重的te偏振，并且对于点形状对光学响应速度和非线性的影响也没有精确的理解。在本研究中，偏振分辨光致发光测量表明，可以通过使用SK-dot层堆叠的柱状dot结构优化dot形状来控制偏振特性。此外，由于点长宽比的影响，改变InGaAs盖层的组成可以控制InAs点的极化特性，而引入InGaAs盖层导致TM优势极化，则增强了其极化特性。这表明，作为器件在实际应用中最重要的特性之一，偏振不灵敏度现在可以通过设计点的形状来控制。我们之前的研究结果表明，量子点光放大器在1-3 ps范围内具有超快的吸收恢复时间。并且从简并四波混频（DFWM）实验中表明，大尺寸点的相位弛豫时间比小尺寸点的要大，这表明在大尺寸点中增强非线性的可能性。综上所述，采用各向同性形状的大尺寸点可以同时实现偏振不敏感、超快和高非线性的光学响应。

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.