2025 Theses Doctoral

Trapping light to direct energy flow in photoswitches and unlock few-molecule polaritons

Lee, Inki

Strong coupling between light and matter forms part-light part-matter states called polaritons, which offer a promising avenue for controlling chemical reactions through wavefunction delocalization and renormalized free energy landscapes. However, experimental demonstrations of polariton-modified chemical reactions remain inconclusive, with the molecular polariton community remaining heavily divided in its interpretation of several key studies.

My thesis attempts to resolve some outstanding issues in the field by designing methods and systems that allow: (i) Careful interrogation of Purcell enhancement, reaction rates, and intermolecular energy transfer in molecular photoswitches strongly coupled to Fabry-Perot cavities; (ii) Realizing macroscopically-addressable few-molecule strong-coupling regimes that bridge the gap between theoretical models and experimental realizations of strong light-matter coupling.

In my first project, I developed a broadband Fourier-plane optical microscope to monitor the kinetics of molecular photoisomerization within Fabry-Perot microcavities. This microscope allows non-invasively and rapidly tracking chemical kinetics occurring inside a microcavity. We investigate various coupling and photoexcitation conditions to demonstrate modified photoisomerization kinetics of two photoswitch molecules, merocyanine and diarylethene.

We show three key results: 1) enhanced absorption and isomerization rates when exciting the polariton bands resonantly due to a Purcell-like electromagnetic field enhancement; 2) suppression of photoisomerization yield due to kinetic competition between polariton localization into reactive molecular states and cavity losses; and 3) cavity-mediated energy funneling between molecular isomers with distinct isomerization pathways based on spectral overlap. A key realization from this study is that polariton modifications to photochemistry can arise from cavity-mediated energy transfer between different molecular sub-ensembles in an inhomogeneously-broadened system, providing a simple and plausible alternative to polariton-modified molecular free-energy landscapes.

Indeed, reaching dramatic modifications to molecular free-energy landscapes likely requires large per-dipole light-matter coupling, which is often not reached in the light-matter coupling regime pertinent to most molecular polaritons. For example, in the Fabry-Perot cavities explored in my first project, an estimated ~106 molecular dipoles collectively couple to a single cavity mode. A central challenge of the field is to realize scalable single- or few-molecule polaritons, such that the per-molecule field-dipole coupling strength is on par with other relevant energy scales in the system. In pursuit of this goal, we investigate precisely engineered, three-dimensional superlattices of gold nanocubes, which form an ordered array of uniformly defined 4 nm plasmonic nanogaps. The nanogap plasmons enhance electromagnetic fields by 2-3 orders of magnitude through extreme confinement.

By submerging these structures in dye solutions and performing in-situ spectroscopy, we demonstrate strong polaritonic coupling between molecular excitons and nanogap plasmons with <100 molecules per gap, realized simultaneously over 105 gaps within a single 3D superlattice. We show that the plasmon energies and light-matter coupling strength are homogeneous across ~100 µm3 superlattices, allowing macroscopic addressability of few-molecule polaritons. We additionally reveal that the strong plasmonic enhancement allows for high-contrast imaging down to the single molecule level via label-free interferometric scattering microcopy. These platforms will allow us to bridge the gap between experimental and theoretical studies, which predict dramatic polaritonic effects on chemistry only for few-dipole polaritons. We anticipate that the next steps with this platform will involve tracking how photochemical and thermal reactions are influenced by the plasmonic nanogap environment.