Energy transfer (ET) between donor and acceptor molecules in free space necessitates the fluorescence spectrum of the donor overlaps with the absorption spectrum of the acceptor. These conditions imply almost no ET when the donor and acceptor molecules possess very similar optical spectra. We propose cavity polariton formation in specially designed resonator structures represents one method to overcome this limitation. To test this proposal, we form cavity polaritons in nano-fabricated Fabry-Perot resonators using two porphyrin molecules who's intense Soret absorption peaks overlap almost completely: free base tetraphenyl porphyrin (H2TPP) and copper (II) tetraphenyl porphyrin (CuTPP). Theoretically, we use a 3-oscillator Hamiltonian to show the polaritonic states of these structures controllably mix excitations on each chromophore in three novel ways. First, comparison between our model results and experimental steady-state spectra show the presence of a 'dark' middle polariton that maximally mixes the excitons of H2TPP and CuTPP, but contains of no photonic content. Second, our experimental results indicate the standing modes sustained by our FP cavities effectively average over the energetic disorder of the embedded and lead to a lower polariton (LP) state possessing a lifetime significantly longer than expected from conventional theories of polariton dynamics. Third, we show the Hopfield coefficients characterizing each experimental polariton state indicate one can use this controllable mixing to drive ET from an upper polariton (UP) comprised mostly of CuTPP excitons to a LP state largely localized on H2TPP molecules separated by the 450 nm length of our structures. To test these interpretations of the steady-state spectra, we probe the ultrafast evolution of excited state absorption (ESA) of H2TPP at 2.1 eV following direct excitation of the UP state of our cavity polariton samples with a 3.1 eV pump pulse. We tune the angle of the probe to ensure that we investigate the LP state at a position in which the H2TPP fraction is dominant. We assess how the temporal characteristics of the H2TPP ESA signal indicate polariton-mediate ET from a CuTPP-rich donor layer. These results demonstrate that polaritons formed using quasi degenerate porphyrin excitons enable control over ET and excited state lifetimes through novel physical mechanisms inaccessible for these molecules outside of the cavity’s structured photonic environment.

Strong coupling of quasi-degenerate porphyrin excitons to cavity photons: admixed dark states and long-lived polariton

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