Two-dimensional infrared spectroscopy of vibrational polaritons

Abstract

<p id="d1836785e256">Quantum states of molecular vibrational polaritons, hybrid half-light, half-matter quasi-particles, are studied using ultrafast coherent two-dimensional infrared spectroscopy. Valuable physical insights such as existence of hidden dark states and ultrafast interactions between dark states and bright polariton states are unambiguously revealed. These results not only advance coherent two-dimensional spectroscopy, but also have significant implications in harvesting the creation of molecular vibrational polaritons for infrared photonic devices, lasing, molecular quantum simulation, as well as chemistry by tailoring potential energy landscapes. </p><p class="first" id="d1836785e259">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. </p>