Generation of electrical signals in THz band from coherent charge dipoles in microcavities

在本项目中，我们的目标是通过亚皮秒激光脉冲激发半导体量子阱（qw）中的相干电荷偶极子产生太赫兹电信号，并将太赫兹电信号应用于半导体微腔，在半导体微腔中激发极化模振荡。为此，在搭建共平面微带线测量装置的准备实验之后，我们设计了一种微带线结构，其中直流和太赫兹电场可以垂直施加于量子阱平面。基于时域有限差分法（FDTD）的数值分析，我们发现可以采用直线间隙结构来实现这一目的。利用自制的器件，利用量子阱作为探测器，我们成功地观测到了上升时间约为400fs的太赫兹电信号。然后，我们搜索了导电线材料，这使我们能够通过线进行光学访问。我们决定使用ITO薄膜作为这种材料的候选材料。在建立了制造工艺后，我们成功地形成了一个基于ITO的微带线器件。虽然性能不尽如人意，但我们可以观察到垂直施加在量子阱平面上的电信号。在研究过程中，我们发现即使使用单根微带线，如果传播距离不太长，太赫兹信号也可以被引导。这种引导模式在某些应用中可能是有用的。我们还研究了半导体微腔在静电场作用下的极化子模式振荡，发现振荡的相位随电场的变化而变化，并且相位的变化强烈地表明激子-激子相互作用的变化，这是激子非线性的关键因素。据我们所知，这是第一次观察到激子-激子相互作用的变化。

In this project, we aimed at the generation of THz-electrical signals from coherent charge dipoles in semiconductor quantum wells (QWs) by exciting them with sub-picosecond laser pulses, and also at applying THz electrical signals to the semiconductor microcavities, where polariton mode oscillation is excited. For this purpose, after preparatory experiments for the building a measurement setup using co-planer microstrip lines, we designed a microstrip line structure, where dc and THz electric fields can be applied perpendicularly to the QW plane. From the numerical analysis based on finite difference time domain (FDTD) method, we found that an in-line gap structure can be used for this purpose. Using the fabricated such kind of devices, we successfully observed THz electrical signals, whose rise-time is about 400 fs, using a QW as a detector. Then, we searched the conducting line materials, which allow us a optical access through the line. We decided to use ITO films as a candidate of such a material. After the establishment of fabrication process, we successfully formed a microstrip line device based on ITO. Although the properties are not satisfactory, we could observe an electrical signal perpendicularly applied to the QW plane. During the investigation, we found that even with a single microstrip line, THz signals can be guided if the propagation distance is not too long. Such a guiding mode may be useful in some kind of applications. We also investigated the polariton mode oscillation in semiconductor microcavities under static electric field, and found that the phase of the oscillations vary with the change of the electric field and that the phase change suggests strongly that the variation of exciton-exciton interactions, which is a key factor in excitonic nonlinearity. To the best of our knowledge, this is the first observation of the alteration of exciton-exciton interactions.