Development of a Spacerless Flow-Cell Cavity for Vibrational Polaritons

Ultrafast Spectroscopy under Vibrational Strong Coupling in Diphenylphosphoryl Azide

Strong coupling of cavity photons and molecular vibrations creates vibrational polaritons that have been shown to modify chemical reactivity and alter material properties. While ultrafast spectroscopy of vibrational polaritons has been performed intensively in metal complexes, ultrafast dynamics in vibrationally strongly coupled organic molecules remain unexplored. Here, we report ultrafast pump-probe measurement and two-dimensional infrared spectroscopy in diphenylphosphoryl azide under vibrational strong coupling. Early time oscillatory structures indicate coherent energy exchange between the two polariton modes, which decays in ∼2 ps. We observe a large transient absorptive feature around the lower polariton, which can be explained by the overlapped excited-state absorption and derivative-shaped structures around the lower and upper polaritons. The latter feature is explained by the Rabi splitting contraction, which is ascribed to a reduced population in the ground state. These results reassure the previously reported spectroscopic theory to describe nonlinear spectroscopy of vibrational polaritons. We have also noticed the influence of the complicated layer structure of the cavity mirrors. The penetration of the electric field distribution into the layered structure of the dielectric-mirror cavities can significantly affect the Rabi splitting and the decay time constant of polaritonic systems.

Optical Cavity Design and Functionality for Molecular Strong Coupling

Optical cavity/molecule strong coupling offers attractive opportunities to modulate photochemical or photophysical processes. When atoms or molecules are placed in an optical cavity, they can coherently exchange photonic energy with optical cavity vacuum fields, entering the strong coupling interaction regime. Recent work suggests that the thermodynamic and kinetic properties of molecules can be significantly changed by strong coupling, resulting in the emergence of intriguing photochemical and photophysical phenomena. As more and more physico-chemical systems are studied under strong coupling conditions, optical cavities have also advanced in their sophistication, responsiveness, and (multi)functionality. In this review, we highlight some of these recent developments, particularly focusing on Fabry-Perot microcavities.