Theory for Nonlinear Spectroscopy of Vibrational Polaritons

Molecular polaritons have gained considerable attention due to their potential to control nanoscale molecular processes by harnessing electromagnetic coherence. Although recent experiments with liquid-phase vibrational polaritons have shown great promise for exploiting these effects, significant challenges remain in interpreting their spectroscopic signatures. In this letter, we develop a quantum-mechanical theory of pump-probe spectroscopy for this class of polaritons based on the quantum Langevin equations and the input-output theory. Comparison with recent experimental data shows good agreement upon consideration of the various vibrational anharmonicities that modulate the signals. Finally, a simple and intuitive interpretation of the data based on an effective mode-coupling theory is provided. Our work provides a solid theoretical framework to elucidate nonlinear optical properties of molecular polaritons as well as to analyze further multidimensional spectroscopy experiments on these systems

Revealing Hidden Vibration Polariton Interactions by 2D IR Spectroscopy

We report the first experimental two-dimensional infrared (2D IR) spectra of novel molecular photonic excitations - vibrational-polaritons. The application of advanced 2D IR spectroscopy onto novel vibrational-polariton challenges and advances our understanding in both fields. From spectroscopy aspect, 2D IR spectra of polaritons differ drastically from free uncoupled molecules; from vibrational-polariton aspects, 2D IR uniquely resolves hybrid light-matter polariton excitations and unexpected dark states in a state-selective manner and revealed hidden interactions between them. Moreover, 2D IR signals highlight the role of vibrational anharmonicities in generating non-linear signals. To further advance our knowledge on 2D IR of vibrational polaritons, we develop a new quantum-mechanical model incorporating the effects of both nuclear and electrical anharmonicities on vibrational-polaritons and their 2D IR signals. This work reveals polariton physics that is difficult or impossible to probe with traditional linear spectroscopy and lays the foundation for investigating new non-linear optics and chemistry of molecular vibrational-polaritons

VIBRATIONAL DYNAMICS OF TRICYANOMETHANIDE

Author Institution: Code 6111, Naval Research Laboratory, 4555 Overlook Ave SW, Washington, D.C. 20375Time-resolved and steady-state IR spectroscopy have been used to characterize vibrational spectra and energy relaxation dynamics of the CN stretching band of the tricyanomethanide (TCM, C(CN)$_{3}$$^{-}$) anion near 2170 cm$^{-1}$ in solutions of water, heavy water, methanol, formamide, dimethyl sulfoxide (DMSO) and the ionic liquid 1-butyl methyl imidazolium tetrafluoroborate ([BMIM][BF$_{4}$]). The band intensity is strong ($\sim$1500 M$^{-1}$cm$^{-1}$) and the vibrational energy relaxation times are relatively long ($\sim$5 ps in water, 12 ps in heavy water, and $\sim$30 ps in DMSO and [BMIM][BF$_{4}$]). They are longer than those previously reported for dicyanamide in the same solvents. Although the static TCM frequency generally shifts to higher frequency with more strongly interacting solvents, the shift does not follow the same trend as the vibrational dynamics. The results for the experimental frequencies and intensities agree well with results from $\it{ab}$ $\it{initio}$ calculations. Proton and electron affinities for TCM are also calculated because they are relevant to potential applications of this anion in low viscosity ionic liquids

ULTRAFAST SPECTROSCOPY OF TRANSITION METAL NANORODS

Author Institution: Chemistry Division, Naval Research Laboratory, Washington, DC 20375; Chemistry Department, United States Naval Academy, Annapolis, MD 21402Nanorods composed of transition metals were fabricated and studied by ultrafast transient absorption and static UV vis-NIR spectroscopy. Platinum, iron, cobalt, silver and rhodium high-aspect ratio nanorods were made by electrodeposition in 6 $\mu$m thick, polycarbonate templates. The nanorods were produced with aqueous plating solutions in templates with nominal pore sizes of 10 and 30 nm, resulting in rods with ~40 and ~60 nm diameters as indicated by SEM measurements. Aluminum nanorods, which cannot be electrodeposited using aqueous solutions, were fabricated in an ionic liquid-based Al plating solution. Transmission spectra show that the nanorods of each metal have a transverse surface plasmon resonance band in the 400-600 nm range and a longitudinal band in the mid-infrared. Ultrafast transient absorption measurement with 400 nm pump and 800 nm probe are used to characterize electron-phonon coupling times and coherent acoustic breathing mode oscillations. The oscillations occur on a 10-40 ps timescale and are consistent with classical expectations for acoustic breathing mode periods based on the nanorod diameters and the bulk longitudinal speed of sound for each metal. Results are consistent with those previously reported for other metals (gold, nickel, and palladium).(1) Furthermore, the dynamics for these metals are similar to those observed for smaller nanoparticles and nanorods. (1) G.M. Sando, A.D. Berry, and J.C. Owrutsky J. Chem. Phys. 127(7), 074705 August 200