In this dissertation, the application of advanced spectroscopy, such as two-dimensional infrared (2D IR) spectroscopy to vibrational-polaritons challenges and advances our understanding in both fields, as mentioned detailly in the introduction. In the fourth chapter, 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.
Besides revealing the polariton-dark modes interactions, the spectroscopy tools has also been employed to study the optical nonlinearities of the molecular vibrational polaritons. In the fifth chapter, the control of vibrational polariton coherent nonlinearities has been fulfilled by manipulation of macroscopic parameters such as cavity longitudinal length or molecular concentration. In chapter six, we studied the long-lived dynamics of vibrational polaritons in various solvent environments. 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 in highly nonpolar solvents, despite the fact that the LP lifetime remains within 5 ps, suggesting the hidden intermediate states in the LP to dark mode pathway.
Based on the knowledge of the molecular vibrational polaritons, two novel applications have been shown in chapter seven and eight. In chapter seven, a dual-cavity system has been employed to explore the intercavity polaritonic interactions enabled by molecular vibrations. The combination of 2D IR and imaging system has offered an extra dimension (spatial) to view the possible propagation of polaritonic nonlinear responses in space. In chapter eight, the polariton-enabled intermolecular vibrational energy transfer has been studied in chapter five, providing a new transfer pathway to the molecular systems with naturally weak dipole-dipole coupling strength.
The research mentioned in this dissertation has reviewed the current challenge and invoked new questions raised by exciting results in the hope of stimulating more fundamental studies. The novel chemical physics properties of MVPs grant a bright future of applications in new chemistry/biochemistry, energy devices, novel optics, and quantum computation.