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Strong exciton–photon coupling results in the formation of hybrid light–matter states whose relaxation dynamics often experience a bottleneck due to a large manifold of dark states. This bottleneck results in dynamics like those of the uncoupled material, hampering the observation and utilization of polaritonic properties. A possible solution to this problem is to use cavities with fewer dark states, suggesting a need for an increased understanding of the influence of the cavity platform on the polariton photophysics. Here, we use pump-probe spectroscopy to compare Fabry–Perot cavities and plasmonic nanorods coupled under identical conditions and analyze how the cavity influences relaxation kinetics. Fabry–Perot cavities host a large number of dark states, leading to dynamics fully dominated by the bottleneck. However, the relaxation from polaritonic states in nanorod systems is distinctly different compared to the uncoupled materials, with a sub-100 fs decay that dominates the kinetics. This fast relaxation emerges only when selectively exciting within the polaritonic region: off-resonance excitation yields plasmonic hot-carrier dynamics. These findings show that the cavity critically defines polariton relaxation. Moreover, this work suggests that plasmonic cavities can operate as either polaritons or plasmons, offering tunable functionality for applications in, for instance, polaritonic chemistry.

Phonon engineering has improved charge transport in semiconducting organic molecules by introducing high mass side chains, which ameliorate harmful transient localization. The influence of these side chains on thermal energy transfer and dynamic disorder has not been fully explored. In this work, we first use low temperature X-ray diffraction to probe thermally induced structural changes in functionalized acene molecular semiconductors, combined with low temperature IR spectroscopy to track changes to their vibrational energy landscape. Furthermore, time-resolved IR pump–IR probe spectroscopy is employed to measure IR absorption kinetics associated with the high mass side chains of molecules, revealing intramolecular vibrational energy redistribution pathways. Alkyne groups in the side chains are shown to act as vibrational energy traps, remaining hot for time scales of >2 ns. The results reveal nonequilibrium vibrational pathways associated with side chains that may influence the phonon manifold relevant to dynamic disorder.

The incorporation of Ruddlesden–Popper (RP)/3D perovskite heterostructures into photovoltaic cells has been shown to increase both the efficiency and stability of the devices. Here, a series of methylammonium lead triiodide (MAPbI3) thin films treated with varying 2-phenylethylammonium (PEA) concentrations are investigated with static and ultrafast spectroscopic techniques to reveal the mechanisms of the observed performance benefits. Transient absorption spectroscopy is employed to elucidate the effect of a surface RP layer on the excited state of the MAPbI3 films and reveal that several different RP structures are formed and participate in the charge-carrier dynamics. The passivation effects of PEA are investigated with optical pump–terahertz probe (OPTP) experiments using a variety of excitation conditions to simultaneously probe the surface and bulk recombination dynamics. Fitting models to the OPTP data for each excitation scheme allows the material parameters that govern the ultrafast dynamics to be quantified. It is found that as the PEA concentration increases, the surface recombination velocity exhibits a monotonic decrease, suggesting the RP layer is effective at passivating surface traps. Furthermore, the bulk monomolecular recombination rate is also found to decrease with the addition of PEA, indicating that the benefits of this passivation approach are not limited to the upper surface of the MAPbI3 films.

We investigated ultrafast defect-lattice dynamics in diamond using the N_s:贬鈭扖⁰ defect, an analog of bond-centered hydrogen in semiconductors. Combining synthesis, ultrafast vibrational spectroscopy, and ab initio calculations, we show that excitation of the defect鈥檚 stretch mode leads to the generation of localized phonons and the formation of a hot ground state, where the interatomic potential is transiently modified. Our results reveal unexpected nonequilibrium phonon effects despite diamond鈥檚 exceptionally high thermal conductivity, with implications for quantum defect engineering.