Tailoring exciton-photon interactions in organic semiconductor microcavities: From resonance-controlled photophysics to spontaneous coherence

如果强光照射固体，物质的奇异相称为玻色-爱因斯坦凝聚（BEC）是可能的。通过与材料的强烈相互作用，光与电子耦合，形成“半光半电子”的准粒子，称为激子-极化激元，其中许多可能占据相同的量子力学状态，因此它们都作为一个整体运动，类似于一个大型的花样滑冰运动员组合，当聚光灯照射在他们身上时，所有人都在进行精致的舞蹈。当激光照射到粒子上时，单个粒子失去了它们的身份，在整个固体上协同作用，就像一种“量子液体”。这种宏观自发的相干性（集体量子行为极化激元，很像大量的花样滑冰运动员）是许多重要但知之甚少的凝聚态现象的起源，比如超流。这些状态只在称为半导体量子威尔斯的非常特殊的固体中观察到-由排列在晶体中的原子构成，其中电子被限制在二维中移动，并且温度非常低（约20 K或-253 °C）。我将讨论以下问题：我们能在室温下在由分子组成的固体中找到这样的凝聚物吗？这是一个大问题，因为它解决了这些状态在材料中的物理学，而不仅仅是“简单”的固体，这些固体是由我们周围和我们体内的有机分子组成的，在我们熟悉的温度环境中。理解这种基本行为将通过概括这种新物质状态的形成和耗散的物理学来带来量子力学的新突破。这可能会导致新的相干光源消耗比传统激光器更少的功率，以及可能用于量子计算机的设备。 分子在构型上是“柔软和蓬松的”，导致结构紊乱。因此，分子之间的电子相互作用是复杂的，这可能是相干耗散的重要原因，可能会阻碍凝聚。尽管如此，有机材料是BEC的理想候选者，因为它们非常强烈地吸收光，使得光子（光）和电子之间的耦合强度比量子威尔斯中的耦合强度大一个数量级以上，并且可以比良好光学器件中的能量无序更强。此外，在有机半导体中，理论上可以在室温下在这些材料中形成量子凝聚物，这对于具有超过一个数量级的较低激子结合能的无机量子威尔斯通常是不可能的。我将制造基于塑料的新设备，这将允许研究这些具有复杂细节的基础物理学，因为它将有可能更容易地将分子材料纳入其中。我将研究一系列导电材料，并可能用于光电器件，从分子组成的晶体到导电塑料。 我强调拟议工作方案具有变革性影响的潜力。我们将使用新的制造协议来制造设备，然后用产生短激光脉冲（短于百万分之一秒）的复杂实验技术来研究它们，以真实的时间研究极化激元凝聚过程。我的工作的影响将是开发一个严格的框架来理解和控制光如何与这些材料相互作用，从长远来看，激光等现实应用可能会出现。该补助金将使塑料半导体物理学的宏观理解，连接经典聚合物科学，凝聚态物理学和化学物理学的概念。

If intense light irradiates solids, an exotic phase of matter called a Bose-Einstein condensate (BEC) is possible. By interacting strongly with materials, light couples with electrons to make 'half-light, half-electron' quasiparticles, termed exciton-polaritons, many of which may occupy the same quantum-mechanical state, so they all move as one, akin to a large ensemble of pairs of figure skaters, all performing in exquisite choreography when the spotlight is upon them. Individual particles lose their identity, acting cooperatively as a 'quantum liquid' over the entire solid when the laser shines on it. Such macroscopic spontaneous coherence (the collective quantum behaviour polaritons, much like the large number of figure skaters) of is at the origin of many important but poorly understood condensed-matter phenomena such as superfluidity. These states have only been observed in very specific solids called semiconductor quantum wells – built by atoms arranged in a crystal, in which electrons are confined to move in two dimensions, and at very low temperatures (~20 K, or -253 °C). I will address the following question: can we find such condensates at room temperature in solids made with molecules? This is a big question because it addresses the physics of these states in materials beyond 'simple' solids, built with organic molecules like those that surround us and are in us, in a temperature environment familiar to us. Understanding this fundamental behaviour will bring new breakthroughs in quantum mechanics by generalising the physics of formation and dissipation of this new state of matter. This may lead to new coherent light sources that consume less power than conventional lasers, and to devices that may be used in quantum computers. Molecules are configurationally 'soft and fluffy', resulting in structural disorder. Electronic interactions between molecules are therefore complex, which can be an important cause of coherence dissipation, potentially hindering condensation. Nonetheless, organic materials are ideal candidates for BEC because they absorb light very strongly, rendering the strength of the coupling between photons (light) and electrons well over an order of magnitude larger than in quantum wells, and can be stronger than energetic disorder in good optical devices. Furthermore, in organic semiconductors it is theoretically possible to form quantum condensates in these materials at room temperature, which is not generally possible with inorganic quantum wells with over an order of magnitude lower exciton binding energies. I will fabricate new devices based on plastics, which will permit the study of these fundamental physics with intricate detail because it will be possible to more easily incorporate molecular materials in them. I will study a range of materials that conduct electricity and may be used in optoelectronic devices, ranging from crystals composed of molecules to conducting plastics. I underline the potential for transformative impact of the proposed programme of work. We will use new fabrication protocols to make devices, and then study them with sophisticated experimental techniques producing short laser pulses (shorter than a millionth of a millionth of a second) to study polariton condensation processes in real time. The impact of my work will be to develop a rigorous framework to understand and control how light interacts with these materials, and real-life applications such as lasers can emerge in the long term. This grant will enable big-picture understanding of photophysics of plastic semiconductors, connecting concepts from classical polymer science, condensed-matter physics, and chemical physics.