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

如果强光照射固体，就有可能形成一种叫做玻色-爱因斯坦凝聚体(BEC)的奇异物质相。通过与材料强烈相互作用，光与电子耦合，形成“半光、半电子”准粒子，称为激子-极化子，其中许多可能具有相同的量子力学状态，因此它们都作为一个整体移动，类似于一大对花样滑冰运动员，当聚光灯照射在他们身上时，所有人都以精致的编排表演。单个粒子失去了身份，当激光照射到整个固体上时，它们协同作用，成为整个固体的“量子液体”。这种宏观的自发相干性(集体量子行为极化子，很像大量的花样滑冰运动员)是许多重要但鲜为人知的凝聚态现象的根源，例如超流。这些状态只在被称为半导体量子阱的非常特定的固体中观察到--由排列在晶体中的原子建造，在晶体中，电子被限制在二维中运动，并且在非常低的温度(~20K，或-253°C)下。我将回答以下问题：在由分子组成的固体中，我们能在室温下找到这种凝聚态吗？这是一个很大的问题，因为它解决了物质中这些状态的物理问题，这些物质不是用我们熟悉的温度环境中的有机分子建立的，而是由我们周围和我们体内的有机分子构成的。理解这一基本行为将通过概括这种新的物质状态的形成和耗散的物理学，在量子力学方面带来新的突破。这可能会导致比传统激光消耗更少能量的新相干光源，以及可能用于量子计算机的设备。 分子在构型上“柔软而蓬松”，导致结构无序。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，因为它们对光的吸收非常强，使得光子(光)和电子之间的耦合强度比量子阱中的大一个数量级以上，而且在良好的光学设备中可以比能量无序更强。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，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。我将制造基于塑料的新设备，这将允许以复杂的细节研究这些基础物理，因为它将可能更容易地将分子材料融入其中。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。 我强调拟议工作方案有可能产生变革性的影响。我们将使用新的制造协议来制造器件，然后用产生短激光脉冲(不到百万分之一秒)的复杂实验技术来研究它们，以实时研究极化子凝聚过程。我工作的影响将是开发一个严格的框架，以了解和控制光与这些材料的相互作用，并从长远来看，激光等现实生活中的应用可以出现。这笔赠款将使人们能够从宏观上理解塑料半导体的光物理，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.