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威尔斯（QW）中的相干电荷偶极子产生太赫兹电信号，并将太赫兹电信号施加到半导体微腔中，在那里激发极化激元模式振荡。为此，在为使用共面微带线构建测量装置进行预备实验后，我们设计了一种微带线结构，其中DC和THz电场可以垂直于QW平面施加。从基于时域有限差分（FDTD）方法的数值分析，我们发现，一个线间隙结构可以用于此目的。利用这种器件，我们成功地用量子阱作为探测器观测到了上升时间约为400 fs的太赫兹电信号。然后，我们寻找导电线材料，这使我们能够通过线的光学访问。我们决定使用ITO膜作为这样的材料的候选者。在确定了制作工艺之后，我们成功地制作了一个基于ITO的微带线器件。虽然性质不令人满意，我们可以观察到一个电信号垂直施加到量子阱平面。在研究过程中，我们发现，即使是一个单一的微带线，太赫兹信号可以引导，如果传播距离不是太长。这种引导模式在某些类型的应用中可能是有用的。我们还研究了静电场作用下半导体微腔中极化激元模的振荡，发现振荡的相位随电场的变化而变化，这种相位变化强烈地反映了激子-激子相互作用的变化，激子-激子相互作用是激子非线性的一个关键因素。据我们所知，这是第一次观察到激子-激子相互作用的改变。

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.