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

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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.