Research Projects

Recent advances in light–matter ‘strong coupling’ have ushered in a new paradigm in physical chemistry, wherein hybrid quantum states, known as polaritons, emerge when molecular vibrational modes or electronic resonances interact coherently with confined electromagnetic fields. Such states have been shown to modify chemical reaction landscapes, alter transition state energetics, modulate transfer pathways, etc. However, all these pioneering phenomena have been demonstrated inside Fabry–Pérot (FP) microcavities, which is of great academic importance but has limited real-life applicability. In this backdrop, nanoscale plexcitons, which are ‘half-light half-matter’ hybrid states created by coupling between two quasiparticles, exciton and localized surface plasmon resonance (LSPR) in plasmonic nanostructures under ambient conditions, present an alluring new opportunity. Strong coupling occurs when Rabi splitting surpasses losses. In contrast to ‘closed’ FP cavity, metallic nanoparticles are ‘open cavities’ that support widely tunable LSPR modes, giant oscillator strength, sub-diffraction mode volumes, enormously enhanced local electromagnetic field, are easy to synthesize and scalable, and most pertinent to this proposal, LSPR excitation efficiently catalyses chemical transformations (as demonstrated by the PI and others). Consequently, nanoscale plexcitons ‘inherit’ all these plasmonic advantages and also enjoy all the beneficial properties of ‘strong coupling’. Clearly, nanoscale plexcitons provide a foundation for the advancement from limited proof-of-concept research to the practical exploration of the true translational potential of strongly coupled systems, particularly in the domain of polariton chemistry. And this is the hypothesis on which this proposal is based on. The idea of exploring and employing strongly coupled plexcitons for controlling and directing chemistry is barely at its nascent stage. Also, the differences in mechanism of action between the FP cavity-based and nanoparticle open cavity-based ‘strongly coupled’ systems in modulating chemical processes is yet to be understood. However, to develop plexciton-based robust technologies or to achieve optimal device performance, a thorough and predictive understanding of corresponding mechanistic pathways operative in plexciton-driven processes (such as photocatalysis, energy- and charge transfer, etc.) and the factors that control them is necessary. And that is the central objective of this proposal. This proposal stems from a confluence of the proposers’ relevant expertise, infrastructure and preliminary findings indicating that plexcitons may offer a fundamentally new lever to control photocatalytic reactions, enabling non-trivial relaxation dynamics, and influencing energy and charge transfer processes. Our specific aims are to determine: (a) how different plexciton types, coupling strength, and excitation parameters influence photocatalysis? (b) how ultrafast plexciton-dynamics impacts reaction rate and selectivity? (c) what causes plexciton's superior photocatalytic efficiency: superior hot carrier generation, longer lifetime, or efficient charge separation and transfer? (d) what roles plexciton dark states play in controlling chemical processes? And last but not the least, (e) to probe and compare the effect of ‘multi-plexciton’ in photocatalytic transformation. To achieve these objectives, we will implement a multipronged research program interfacing nanoscale design, photocatalysis, ultrafast spectroscopy, and advanced quantum dynamical modelling, which will not only aid in the development of a detailed physical and predictive understanding of plexciton-assisted chemistry but also drive this advanced field of research to the true translational regime. This project will be a first-of-its-kind investigation in India into plexciton-assisted processes, advancing beyond conventional polaritonic chemistry confined to closed Fabry-Pérot cavities.