Measuring Quantum Dot Interactions Using Coherent Two-Dimensional Spectroscopy

半导体量子点具有类似于原子的离散能级，因此它们通常被描述为“人造原子”。与原子不同，量子点的能级结构可以被设计，这使得它们在从高效激光到量子信息处理的广泛应用中具有吸引力。此外，量子点可以嵌入固态设备中，这是应用中使用的一个关键特征。当量子点彼此靠近时，它们的相互作用会产生新的状态，就像原子之间的相互作用会导致分子的形成一样。该项目将采用一种新的相干光学方法来观察和表征量子点之间的相互作用，其空间分辨率足以隔离单个点或一小组相互作用的点。这种新的光学方法的灵感来自于用于确定分子结构的已建立的磁共振技术。此外，本项目开发的光谱技术将有更广泛的应用。例如，它可用于研究光伏太阳能电池中的激发过程。该项目将联合收割机的研究和教育相结合，培养学生在多学科领域的相干光学光谱的固态系统。该培训将包括凝聚态系统与光的相互作用的基础科学以及光子学和微加工的实践方面。技术摘要通过分子束外延生长的半导体量子点有可能的应用范围从低阈值激光二极管到实现量子信息科学中的量子比特。外延生长的量子点已经使用单点技术进行了广泛的研究，然而单点技术在探测量子点之间的耦合方面没有那么好。相互作用可以极大地改变量子点的光学和电子特性。此外，量子信息方案要求量子比特之间存在可控的相互作用，因此量子点之间也存在可控的相互作用。该项目使用基于光电流读出的光学多维相干光谱的新实现来探测量子点之间的耦合，并将其应用于InGaAs量子点的小集合。二维相干光谱学最初是在核磁共振中发展起来的。在过去的十年中，在光谱的红外和光学区域中实现二维光谱学已经取得了广泛的进展。首席研究员的团队是开发使用近红外光研究半导体电子跃迁的方法的领导者。最近，这些研究包括GaAs天然量子点和InAs自组装量子点的大合奏。然而，迄今为止使用的基于检测由样品的三阶非线性响应产生的光学信号的方法不适合用于量子发射器的小集合，这将不会产生良好形成的信号光束。因此，该项目使用了一种新的二维光谱方法，通过测量光电流来检测四阶粒子数。这种方法将被应用到InAs量子点和量子点分子嵌入在二极管结构。目标是理解和测量量子点之间的相互作用，以推进对这些纳米级系统中量子现象的基本理解。这种理解可以改进具有目标量子力学状态和动力学的系统的设计。

Non-technical abstractSemiconductor quantum dots have discrete energy levels similar to those in an atom, thus they are often described as as "artificial atoms". Unlike atoms, the energy level structure of a quantum dot can be engineered, making them attractive for a wide range of applications ranging from highly efficient lasers to quantum information processing. In addition, quantum dots can be embedded in solid-state devices, a key feature for use in applications. When quantum dots are brought into proximity to one another, their interaction results in new states, just as the interaction between atoms results in the formation of molecules. This project will employ a new coherent optical method to observe and characterize the interactions between quantum dots with sufficient spatial resolution to isolate a single dot or a small set of interacting dots. This new optical method is inspired by established magnetic resonance tachniques that are used to determine molecular structure. Furthermore, the spectroscopic technique developed by this project will have broader applications. For example, it could be used to study excitation processes in photovoltaic solar cells. This project will combine research and education by training students in the multidisciplinary field of coherent optical spectroscopy of solid state systems. This training will include the basic science of the interaction of condensed matter systems with light as well as practical aspects of photonics and microfabrication.Technical AbstractSemiconductor quantum dots grown by molecular beam epitaxy have possible applications ranging from low-threshold laser diodes to implementing qubits in quantum information science. Epitaxially grown quantum dots have been studied extensively using single dot techniques, however single dot techniques are not as good at probing the coupling between quantum dots. Interactions can dramatically alter the optical and electronic properties of quantum dots. Furthermore, quantum information schemes require that there be controllable interactions between qubits, and hence between quantum dots. This project probes the coupling between quantum dots using a new implementation of optical multi-dimensional coherent spectroscopy based on photocurrent readout and applying it to small ensembles of InGaAs quantum dots. Two-dimensional coherent spectroscopy was originally developed in nuclear magnetic resonance. Over the last decade, there has been extensive progress in implementing two-dimensional spectroscopy in the infrared and optical regions of the spectrum. The principal investigator's group is a leader in developing methods for using it to study electronic transitions in semiconductors using near-infrared light. Recently these studies have included large ensembles of GaAs natural quantum dots and InAs self-assembled quantum dots. However, the methods used so far, based on detecting an optical signal produced by the third-order nonlinear response of the sample, are not appropriate for use on small ensembles of quantum emitters, which will not generate a well formed signal beam. Thus this project uses a new approach to two-dimensional spectroscopy that detects a fourth-order population through measurement of photocurrent. This approach will be applied to InAs quantum dots and quantum dot molecules embedded in a diode structure. The goal is to understand and measure the interactions between quantum dots to advance the fundamental understanding of quantum phenomena in these nanoscale systems. This understanding can improve the design of systems with target quantum mechanical states and dynamics.