Collective effects in the emission from semiconductor microcavities

本方案旨在研究半导体微腔光发射中的集体效应。除了将现代光谱技术应用于半导体系统外，还将特别强调开发新的光谱工具。多特蒙德开发的使用条纹相机的光子相关光谱是这里最合适的方法。这项技术使我们能够以两皮秒的时间分辨率测量光子发射过程的统计相关性。作为补充技术，应使用使用零差检测的相关测量设置。这项技术提供了大约100飞秒的改进的时间分辨率和相位灵敏度，但也需要复杂的数据分析。要研究的系统是量子点微腔激光器和胶体纳米结构，即所谓的纳米小片。关于量子点激光，我们将研究在脉冲期间如何建立相干性的问题。在脉冲开始时，许多量子点仍处于激发态，因此与腔内光场的集体相互作用似乎是可能的。在脉冲形成的初始阶段，这应该会导致光子数波动的显著短暂增加。结合腔模和量子点系综之间失谐量的系统变化，这一结果将使我们能够更深入地理解量子点激光器的动力学。胶体纳米血小板以其各向异性和向巨型振子强度的转变而闻名，即使在没有空腔的情况下也是如此。这种转变的微观起源尚不清楚，但集体效应似乎是一个合理的解释。对辐射相干性的时间分辨研究应该澄清，是否正在发生受激辐射、超辐射或其他效应。另一项需要实现的实验技术是量子光学激发光谱。这项技术背后的主要思想在于，任何非线性系统的响应不仅取决于该激发的平均强度，还取决于其在平均值附近起伏的量子光学性质。利用光激发强度涨落的系统变化，我们将实现这一技术，并验证一些预测。例如，当受到强度强烈波动的光场激励时，来自非线性系统的响应预计会显示出响应的噪声显著增加。这一增长应该已经远远低于非线性门槛。因此，它应该为识别和表征非线性提供了一种有趣和方便的可能性。

The present proposal aims at investigations of collective effects in the optical emission from semiconductor microcavities. Besides applying modern spectroscopic techniques to semiconductor systems, special emphasis will also be placed on the development of novel spectroscopic tools. Photon correlation spectroscopy using a streak camera - as developed in Dortmund - is the most adequate method here. The technique enables us to perform measurements of statistical dependences of photon emission processes with a temporal resolution of two picoseconds. As a complementary technique, a setup for correlation measurements using homodyne detection shall be used. This technique offers improved temporal resolution of about 100 femtoseconds and phase sensitivity, but also requires sophisticated data analysis. The systems to be investigated are quantum dot microcavity lasers and colloidal nanostructures, so called nanoplatelets. Concerning quantum dot lasers, we will investigate the question how coherence builds up during a pulse. At the beginning of a pulse, many quantum dots are still in the excited state, so a collective interaction with the light field inside the cavity seems probable. This should result in a significant short-lived increase of photon number fluctuations during the initial phase of pulse formation. In combination with systematic variation of the detuning between the cavity mode and the quantum dot ensemble, the results will allow for a much deeper understanding of the dynamics of quantum dot lasers. Colloidal nanoplatelets are well known for their anisotropy and the presence of a transition towards giant oscillator strength even without the presence of a cavity. The microscopic origin of this transition is not known yet, but collective effects seem like a reasonable explanation. Time-resolved investigations of the coherence of the emission should clarify, whether stimulated emission, superradiance or other effects are taking place. Another experimental technique that shall be realized is quantum optical excitation spectroscopy. The main idea behind this technique lies in the fact that the response of any non-linear system does not only depend on the mean intensity of that excitation, but also on its quantum optical properties in terms of fluctuations around the mean value. Using systematic variations of intensity fluctuations of the optical excitation, we will realize this technique and test some predictions. For example, the response from a non-linear system is expected to show a significant increase in the noise of the response when excited with a light field of strongly fluctuating intensity. This increase should already take place way below the non-linear threshold. Therefore it should open up an interesting and convenient possibility to identify and characterize non-linearities.