Theoretical studies of coupled quantum well excitons and microcavity dipolaritons, their transport dynamics and applications in optical devices

在过去的100年里，量子力学的出现催生了快速的技术进步，并重新促进了我们对周围世界的理解。几十年前，量子井--只有几十个原子厚度的材料层--使研究一个新的二维(2D)世界成为可能，在这个世界中，量子效应格外突出。拟议中的工作需要研究被捕获在量子井中的激子粒子。激子由一个带负电荷的电子和一个带正电荷的空穴组成，它们之间受到引力的约束。激光激发一个电子，使它自由地在2D量子井面上漫游。这就留下了一个空位--一个空穴--电子最终可以重新占据。这个空穴也穿过量子井，可以被视为一个具有质量和电荷的真正粒子。一种被称为间接激子的令人着迷的种类是主要的焦点。在这种情况下，电子和空穴被分隔成紧密间隔的势垒，以便激子获得偶极取向。特别是，研究了它们的二维传输和控制它们运动的方法。激子是短暂的，会衰变而发光。在它们短暂的存在中，它们展示了各种戏剧性的物理现象。一种这样的现象是宏观有序激子态。在这种状态下，激子自发地将自己组织成团簇，间隔相等，大小均匀。自十多年前发现这一现象以来，人们一直在激烈争论这一现象的确切原因。用电荷之间错综复杂的作用力相互作用来解释这种效应将是可能的。激子也会产生被称为极化子的粒子。光可以被吸收以产生激子，激子随后衰变而发光。然而，这种光被重新吸收，产生另一种激子。永恒的循环以如此快的速度继续，以至于我们不再认为是激子和光，而是一种新的混合态，称为极化子。当量子井被放置在两个反射镜之间以捕获光时，就实现了微腔极化激元。它们有自己独特的性质，既不像激子，也不像光。它们显示出惊人的特征，如玻色-爱因斯坦凝聚--近一个世纪前玻色和爱因斯坦预测的一种奇异物质状态。激子和极化子为以一种全新的方式研究量子力学之美提供了一种手段，是创造人类理解的外部界限的最佳工具之一。在这项工作中，将研究一种新型的极化子，其中极化子的激子部分是偶极间接激子。这些偶极化子的运动可以通过电学和光学来控制。这项工作的最终目标是在发展新的光学技术时利用激子和偏振子的非凡性质。光学晶体管等设备有可能给通信时代带来革命性变化。目前，光纤使用光高速传输信息，而信息处理则使用电子晶体管完成。光信号和电信号之间的转换导致了通信网络的瓶颈。光晶体管将解决这一问题，并将成为未来通信系统不可或缺的一部分。其目标是找到新的方法来制造由激子和极化子介导的光学晶体管。这项工作的成功将为一个全球新兴行业做出贡献。

In the last 100 years, the advent of quantum mechanics has spawned rapid technological advances and renewed growth in our understanding of the world around us. A few decades ago, quantum wells - layers of material just tens of atoms thick - made possible the study of a new two-dimensional (2D) world where quantum effects are exceptionally prominent.The proposed work entails the study of particles known as excitons trapped inside quantum wells. An exciton is comprised of a negatively charged electron and a positively charged hole that are bound by the attractive forces between them. Laser light excites an electron so that it freely roams the 2D quantum well plane. This leaves an empty space - a hole - that the electron can eventually re-occupy. The hole also traverses the quantum well and can be treated as a real particle with mass and charge. A fascinating variety, known as indirect excitons, is the main focus. These are where electrons and holes are separated into closely spaced wells so that excitons acquire a dipole orientation. In particular, their 2D transport and ways to control their motion are studied.Excitons are short lived and decay to emit light. In their brief existence, they display a dramatic variety of physical phenomena. One such phenomenon is the macroscopically ordered exciton state. In this state, excitons spontaneously organise themselves into clusters, equally spaced and uniform in size. The exact cause of this has been heavily debated since its discovery more than a decade ago. An explanation of this effect in terms of the intricate interplay of forces between charges will be sought.Excitons also give rise to particles known as polaritons. Light can be absorbed to make an exciton which later decays to emit light. However, that light gets reabsorbed to make another exciton. The perpetual cycle continues at such a rapid pace that we no longer think in terms of an exciton and light but rather a new mixed state called a polariton. When quantum wells are placed between two mirrors to trap the light, microcavity polaritons are realised. These have their own unique properties and are neither like excitons or light. They display striking features such as Bose-Einstein condensation - an exotic state of matter predicted by Bose and Einstein almost a century ago. Excitons and polaritons provide a means to study the beauty of quantum mechanics in a whole new way and are among the best tools to craft the outer limits of human understanding. In this work, a new breed of polariton will be studied where the polariton's exciton part is a dipolar indirect exciton. The motion of these dipolaritons can be controlled both electrically and optically. They enable new types of experiment and new ways to manipulate light.The ultimate goal of the work is to employ the remarkable nature of excitons and polaritons in the development of new optical technology. Devices such as optical transistors have the potential to revolutionise the communication era. Currently, optical fibres transfer information at high speed using light whilst information processing is done using electronic transistors. The conversion between optical and electronic signals leads to bottlenecks in communication networks. Optical transistors will solve this problem and will become an integral part of future communication systems. The goal is to identify new ways to create optical transistors mediated by excitons and polaritons. The success of this work will contribute to a globally emerging industry.