Spanning Strong to Weak Normal Mode Coupling between Vibrational and Fabry–Pérot Cavity Modes through Tuning of Vibrational Absorption Strength

Abstract

Designing coupled vibrational-cavity polariton systems to modify chemical reaction rates and paths requires an understanding of how this coupling depends on system parameters (i.e., absorber strength, modal distribution, and vibrational absorber and cavity line widths). Here, we evaluate the impact of absorption coefficient and cavity design on normal mode coupling between a Fabry–Pérot cavity and a molecular vibration. For a vibrational band of urethane in a polymer matrix, the coupling strength increases with its concentration so that the system spans the weak and strong coupling regimes. The experimentally determined Rabi splitting values are in excellent agreement with an analytical expression derived for classical coupled oscillators that includes no fitting parameters. Also, the cavity mode profile is altered through choice of mirror type, with metal mirrors resulting in stronger confinement and thus coupling, while dielectric stack mirrors provide higher transmission for a given cavity quality factor and decreased coupling due to greater mode penetration into the dielectric mirror. In addition to polymers, the cavities can couple to molecular vibrational bands of dissolved species in solution, which greatly expands the range of systems that can be explored. Finally, longer path length cavities are used to demonstrate the path length independence of the coupling strength. The ability to adjust the cavity line width, through the use of higher order modes, represents a route to match the cavity dephasing time to that of the molecular vibration and may be applied to a range of molecular systems. Understanding the roles of cavity design and validating empirical and analytical descriptions of absorber properties on coupling strength will facilitate application of these strong coupling effects to enable currently unreachable chemistries.