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

如果强光照射固体，一种被称为玻色-爱因斯坦凝聚物（BEC）的奇特物质相是可能存在的。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，称为激子极化子，其中许多可能占据相同的量子力学状态，因此它们都像一个整体一样运动，类似于一大群成对的花样滑冰运动员，当聚光灯照射在它们身上时，它们都在表演精美的舞蹈。当激光照射到整个固体时，单个粒子失去了它们的身份，就像“量子液体”一样在整个固体上合作。这种宏观的自发相干性（集体量子行为极性，很像大量花样滑冰运动员）是许多重要但人们知之甚少的凝聚态现象（如超流体）的起源。这些状态只在被称为半导体量子阱的非常特殊的固体中被观察到，这种固体是由排列在晶体中的原子构成的，其中电子被限制在二维空间中运动，并且在非常低的温度下（~ 20k，或-253°C）。我将解决以下问题：我们能在室温下在分子构成的固体中找到这样的冷凝物吗？这是一个大问题，因为它解决了在我们熟悉的温度环境下，由我们周围和我们体内的有机分子构成的“简单”固体以外的材料中这些状态的物理学问题。理解这种基本行为将通过推广这种物质新状态的形成和耗散的物理学，为量子力学带来新的突破。这可能会导致比传统激光器消耗更少功率的新型相干光源，以及可能用于量子计算机的设备。**分子在结构上是“柔软蓬松的”，导致结构混乱。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，可能阻碍冷凝。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远远大于量子阱中的数量级，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体，这在激子结合能低一个数量级以上的无机量子阱中通常是不可能的。我将制造以塑料为基础的新设备，这将允许对这些基础物理进行复杂细节的研究，因为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，并可能用于光电子器件，从分子组成的晶体到导电塑料。我强调拟议的工作方案可能产生变革性影响。我们将使用新的制造工艺来制造器件，然后用复杂的实验技术来研究它们，产生短激光脉冲（短于百万分之一秒），实时研究极化子凝聚过程。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，以及从长远来看，激光等现实生活中的应用可能会出现。这项资助将使人们对塑料半导体的光物理学有更全面的了解，并将经典聚合物科学、凝聚态物理和化学物理的概念联系起来。

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.