Weak Coupling as a Tool for Enhancing Nonlinear Infrared Signal of Chemical Species*

The existing literature contains numerous examples of vibrational strong coupling (VSC) in which the coupling of a vibrational mode and a cavity mode results in new transitions termed vibration-cavity polaritons. These vibration-cavity polaritons possess both molecular and photonic character. VSC has previously proven useful in applications such as modifying reaction kinetics and altering product ratios. Herein we explore a different regime of vibrational-cavity coupling for systems where the optical and vibrational mode are only weakly coupled. Assuming a cavity mode is resonant with the vibrational mode of interest, the act of inserting a molecule into a cavity can enhance the nonlinear signal by over an order of magnitude. This effect manifests as increased amplitude in two-dimensional infrared (2D-IR) and pump-probe spectra. The signal enhancement occurs despite the lack of energetically-split polaritonic modes commonly associated with VSC. Herein we use the carbonyl stretch of W(CO)₆ to compare the dynamics for weakly coupled molecules with bare molecules via their respective frequency-frequency correlation function (FFCF) parameters. FFCF values such as the homogeneous linewidth contribution are reported in a range of concentrations with various solvents to determine their dependence on the phenomena responsible for the signal enhancement. We also use our measurements to estimate a limit of detection for dilute weakly-coupled solutions such as W(CO)₆. A more thorough understanding of the weak coupling regime could give rise to new methods of chemical sensing for especially dilute systems and extend 2D-IR to previously inaccessible species with particularly weak transition dipole moments.