2, National Research Council, Washington, District of Columbia, United States
We will focus on using light-matter interactions in an effort to alter the chemical behavior of a target molecular species. This is done through cavity coupling to a molecular vibration. Coupling vibrational transitions to resonant optical modes creates vibrational polaritons shifted from the uncoupled molecular resonances and provides a convenient way to modify the energetics of molecular vibrations and to explore controlling chemical reactivity[i] and energy relaxation.[ii] Here, we experimentally and numerically describe strong coupling between a Fabry-Pérot cavity and several molecular species (e.g., poly-methylmethacrylate, thiocyanate, hexamethyl diisocyanate)[iii] and investigate the transition from the strong to weak coupling regimes. Furthermore, we map the influence of molecule/cavity mode overlap by systematically altering the position of a molecular slab throughout the first and second order cavity resonances with results agreeing well with analytical and transfer matrix predictions.
In the time domain, we use pump-probe infrared spectroscopy to characterize the dynamics of vibration-cavity polaritons for the CO vibrational band of W(CO)6.2 At very early times, we observe quantum beating between the two polariton states, which may account for a lower degree of vibrational excitation observed. After the quantum beating, we interpret our observations as excited-state absorption from polariton modes and uncoupled reservoir modes. The polariton mode relaxes ten times more quickly than the uncoupled vibrational mode and it exhibits a cavity tuning-dependent lifetime which we believe is a result of modifying the relative fractions of cavity and molecular character comprising the polariton. We show that energy relaxation times depend on cavity-vibration coupling and thereby may be a viable way to control the frequency and lifetime of vibration-cavity polaritons and, therefore, may provide opportunities to influence chemical reactivity. This work points out the possibility of systematic and predictive modification of the excited-state kinetics of vibration-cavity polariton systems. Opening the field of polaritonic coupling to vibrational species promises to be a rich arena amenable to a wide variety of infrared-active bonds that can be studied in steady state and dynamically.
[i] A. Thomas, J. George, A. Shalabney, M. Dryzhakov, S. J. Varma, J. Moran, T. Chervy, X. Zhong, E. Devaux, C. Genet, J. A. Hutchison, T. W. Ebbesen, Angew. Chem. Int. Ed. Engl. (2016)
[ii] A.D. Dunkelberger, B.T. Spann, K.P. Fears, B.S. Simpkins, and J.C. Owrutsky, “Modified Relaxation Dynamics in Coupled Vibration-cavity Polaritons”, Nature Communications 7, 13504 (2016)
[iii] B. S. Simpkins, K. P. Fears, W. J. Dressick, B. T. Spann, A.D. Dunkelberger, and J. C. Owrutsky, , ACS Photonics, 2, 1460 (2015)