Unlocking organic polariton lasers with systematic molecular design

Non-technical description: This research seeks to alter the properties of carbon-based organic semiconductors by combining light with materials engineering to open a route for new technological applications from tunable, low-power lasers to quantum information devices. Such carbon-based materials are ubiquitous in our daily lives. For instance, they enable plants to harvest light through photosynthesis. They feature in lightweight, flexible solar cells and light-emitting diode (LED) displays of top-end phones and televisions. Significant research goes into ways to control their properties – emitting light of the right color or transporting current efficiently – by changing their molecular structure. However, it is also possible to tune many materials properties with light, which is the key concept behind this project. When two mirrors are placed very close together with high precision, they act as tiny boxes, trapping light. If molecular semiconductors are placed between the mirrors, they can strongly interact with this light and begin to behave in entirely new surprising ways, forming new states called ‘polaritons’. These polaritons can emit light through new physical processes – potentially improving LED devices – and can undergo chemical reactions through new pathways. In this research, the principal investigator aims to exploit polaritons for a new generation of organic semiconductor lasers. Crucially, the project uses systematic control of the organic material to develop the first rational design rules to improve laser efficiency and performance. This approach lays the foundation for versatile, ‘plug-and-play’ organic lasers for communications, sensing, and new quantum technologies. Beyond these technological impacts, the project develops a portable mechanical demonstration of the physical concept behind polariton lasers. The research team aims to run the demonstration at local community outreach centers. Through hands-on exploration of how the collective behavior is affected by small structural changes, the demonstration engages the audience in the scientific method and in cross-cutting ideas of physics and chemistry like coherence. Technical description: Strong light-matter coupling to form exciton-polaritons holds immense promise for materials engineering. When applied to organic semiconductors, it offers a way to non-synthetically manipulate the molecular wavefunction and energy structure. This approach can alter fundamental behaviors like charge and energy transport, allowing functional properties of molecules to be rewritten at will. These effects range from redirecting chemical reactions to enabling the formation of Bose-Einstein condensates at room temperature. The latter have the potential to provide a general platform for low-threshold, electrically injected lasers. However, critical questions about the nature of polaritons hold back their rational application: How does the complex electronic structure of molecular materials impact the polariton energy landscape? What molecular levers can be identified to control the dynamical behavior of polaritons? How should devices be structured to maximize unique properties of polaritons? Focusing on polariton condensation, the research team uses a suite of ultrafast (fs to ps) time- and angle-resolved spectroscopies to reveal the molecular basis for the dynamical processes leading to condensation. These methods are combined with systematic optical microcavity variation, including control over critical properties of the semiconductor active layer and the overall device structure. By correlating polariton dynamics and condensation thresholds across these structures, the project aims to identify the crucial properties that govern polariton condensation and highlight the structural features that can be optimized. The ultimate goal of the research is to develop a roadmap to systematically reduce organic polariton condensation thresholds to achieve a platform for electrically injected lasing. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.