Nonlinear Signal Enhancement Arising from Weakly Coupled Systems

Vibrational strong coupling (VSC) occurs when the coupling of a vibrational mode and a cavity mode result in new transitions denoted vibration-cavity polaritons. Vibration-cavity polaritons show promise as a means of modifying reaction kinetics and altering product ratios, yet the mechanisms by which these modifications occur are poorly understood. Techniques like 2DIR measure vibrational energy redistribution among reservoir modes and polaritonic modes, and find use in deciphering the dynamics of systems under VSC. In this work, we use 2DIR to instead measure the dynamics of systems under weak coupling. Inserting a molecule with a vibrational probe (e.g. caronbyl, nitrile) into a cavity can enhance the nonlinear infrared signal by over an order of magnitude. The signal enhancement manifests as increased amplitude in two-dimensional infrared (2D-IR) and pump-probe spectra, and it occurs despite the lack of energetically-split polaritonic modes commonly associated with VSC. We use our measurements to estimate a limit of detection for dilute weakly-coupled solutions containing species such as W(CO)₆ and NaSCN, and then compare our experimental results with calculated 2DIR spectra. The data shown herein also describe the impact of cavity linewidth and molecular linewidth on signal enhancement factor. A more thorough understanding of the weak coupling regime could give rise to new methods of chemical sensing, especially dilute systems, and extend 2DIR to previously inaccessible species with particularly weak transitions.