We report experimental 2D infrared (2D IR) spectra of coherent light-matter excitations--molecular vibrational polaritons. The application of advanced 2D IR spectroscopy to vibrational polaritons challenges and advances our understanding in both fields. First, the 2D IR spectra of polaritons differ drastically from free uncoupled excitations and a new interpretation is needed. Second, 2D IR uniquely resolves excitation of hybrid light-matter polaritons and unexpected dark states in a state-selective manner, revealing otherwise hidden interactions between them. Moreover, 2D IR signals highlight the impact of molecular anharmonicities which are applicable to virtually all molecular systems. A quantum-mechanical model is developed which incorporates both nuclear and electrical anharmonicities and provides the basis for interpreting this class of 2D IR spectra. This work lays the foundation for investigating phenomena of nonlinear photonics and chemistry of molecular vibrational polaritons which cannot be probed with traditional linear spectroscopy.

Selective vibrational energy transfer between molecules in the liquid phase, a difficult process hampered by weak intermolecular forces, is achieved through polaritons formed by strong coupling between cavity photon modes and donor and acceptor molecules. Using pump-probe and two-dimensional infrared spectroscopy, we found that the excitation of the upper polariton, which is composed mostly of donors, can efficiently relax to the acceptors within ~5 picoseconds. The energy-transfer efficiency can be further enhanced by increasing the cavity lifetime, suggesting that the energy transfer is a polaritonic process. This vibrational energy-transfer pathway opens doors for applications in remote chemistry, sensing mechanisms, and vibrational polariton condensation.

The modification of vibrational dynamics is essential for controlling chemical reactions and IR photonic applications. The hybridization between cavity modes and molecular vibrational modes provides a new way to control molecular dynamics. In this work, we study the dynamics of molecular vibrational polaritons in various solvent environments. We find the dynamics of the polariton system is strongly influenced by the nature of the solvents. While the relaxation from upper polariton (UP) to dark modes is always fast (<5 ps) regardless of the medium, lower polariton (LP) in low polarity solvents shows much slower transfer (10–30 ps) into dark modes, despite the fact that the LP lifetime remains within 5 ps. This result suggests that in the latter media, the energy pumped into the LP is first transferred into intermediate states which only subsequently decay into dark modes. In contrast, in solvent environments that strongly interact with the solute, the LP population relaxes into the dense dark state manifold within a much faster time scale. We propose the intermediate state to be the high-lying excited states of dark modes, which are effectively populated by LP via, e.g., ladder-climbing. Such population in the high-lying states can be retained for tens of picoseconds, which could be pertinent to recently observed cavity-modified chemistry.

Optical nonlinearities are key resources in the contemporary photonics toolbox, relevant to quantum gate operations and all-optical switches. Chemical modification is often employed to control the nonlinear response of materials at the microscopic level, but on-the-fly manipulation of such response is challenging. Tunability of optical nonlinearities in the mid-IR is even less developed, hindering its applications in chemical sensing or IR photonic circuitry. Here, we report control of vibrational polariton coherent nonlinearities by manipulation of macroscopic parameters such as cavity longitudinal length or molecular concentration. Further 2D IR investigations reveal that nonlinear dephasing provides the dominant source of the observed ultrafast polariton nonlinearities. The reported phenomena originate from the nonlinear macroscopic polarization stemming from strong coupling between microscopic molecular excitations and a macroscopic photonic cavity mode. RESULTS. Polariton bleach dependence on cavity longitudinal length.To investigate the optical nonlinearities of vibrational polaritons, we measure their third-order nonlinear susceptibilities by femtosecond IR pumpprobe spectroscopy. The hybrid light-matter system consists of a FP microcavity filled with an ensemble of asymmetric carbonyl stretch modes originating from W(CO)6 molecules in a hexane solution (14). At zero waiting time, when IR pump and probe pulses overlap, we see a notable reduction in the intensity of the polariton transmission (Figs. 1b). This reduction gives rise to absorptive features in the differential transmission spectra (Figs. 1c). Qualitatively, the observed behavior resembles the well-known "photon blockade effect" in the single-emitter quantum regime of the Jaynes-Cummings model (18)(19)(20): when a photon excites the emitter-cavity system, the latter is blocked from further interactions with incoming photons of the same frequency. Here, we observe a similar effect, although it is in the ensemble regime and the mechanism for reduced transmission is quite different from the single oscillator case. We hereafter term this phenomenon "polariton bleach". Note this effect is not a trivial coherent artifact from the overlap between the pump and probe pulses. The IR pulse duration is ~100 fs, while the bleach lasts for less than 5 ps (which is also the approximate lifetime of the cavity photon), indicating the effect is intrinsic to the lifetime of the cavity polaritons. The polariton bleach concept provides a foundation for developing mid-IR photonic devices in the ensemble regime of light-matter strong coupling. Chen for his help on designing Figure 1 and Jiaxi Wang for her help on the experiments.

Conformation-specific direct interfacial electron transfer is observed by the first ultrafast electric field–induced VSFG.