Investigation of dynamical conductivity in quantum nanostructures by time-domain terahertz spectroscopy

在本研究项目中，我们利用太赫兹(THz)辐射作为超快载流子运动的探针，研究了量子纳米结构中的动力学电导。主要结果如下：1.半导体超晶格中载流子动力学与Bloch振荡电子的THz增益：通过分析飞秒激光脉冲泵浦半导体超晶格的THz波形，首次证明了Bloch振荡电子具有色散的THz增益。此外，Bloch振荡的主要退相机制是界面粗糙度散射。2.超快晶体管中的非平衡输运：为了了解超快晶体管的性能，我们研究了体相GaAs中非平衡输运产生的太赫兹辐射。结果表明，在50kV/cm以上，光激发电子的初始加速度开始随着电场的增加而减小，这说明了能带混频效应的重要性。此外，我们提出了一种新的方法来表征超快晶体管的截止频率，超出了传统网络分析仪所能达到的频率范围。3.单分子/点结的导电动力学：我们计划研究单分子或量子点被纳米隙电极探测的纳米结的导电动力学。虽然我们可以测量直流特性，但由于其稳定性问题，无法表征THz动力学。这个子项目将在我们的新项目“量子纳米系统的太赫兹动力学的物理和控制”中继续进行(资助编号18201027)。

In this research project, we investigated dynamical conductivities in quantum nanostructures by using terahertz (THz) radiation as a probe for ultrafast carrier motion. The main results are ;1.Carrier dynamics in semiconductor superlattices and THz gain of Bloch oscillating electrons : By analyzing the THz waveforms emitted from semiconductor superlattices pumped by femtosecond laser pulses, we have shown that Bloch oscillating electrons have dispersive THz gain for the first time. Furthermore, the dominant dephasing mechanism of Bloch oscillation has been identified to be interface roughness scattering.2.Non-equilibrium transport in ultrafast transistors: To understand the performance of ultrafast transistors, we have investigated the THz radiation from non-equilibrium transport in bulk GaAs. It is found that the initial acceleration of photoexcited electrons starts to decrease with increasing field above 50 kV/cm, suggesting the importance of band mixing effect. Furthermore, we proposed a novel method to characterize the cutoff frequencies of ultrafast transistors beyond the frequency region accessible by conventional network analyzers.3.Conduction dynamics of single molecule/dot junctions : We planned to investigate the conduction dynamics of nanojunctions in which single molecules or quantum dots are probed by nanogap electrodes. Although we could measure dc characteristics, THz dynamics could not be characterized mainly due to their stability problem. This sub-project is being continued in our new project "Physics and control of terahertz dynamics of quantum nanosystems" (Grant in Aid No.18201027).