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. This approach is a viable method to explore controlling chemical reactivity. In this work, we report pump–probe infrared spectroscopy of the cavity-coupled C–O stretching band of W(CO)6 and the direct measurement of the lifetime of a vibration-cavity polariton. The upper polariton relaxes 10 times more quickly than the uncoupled vibrational mode. Tuning the polariton energy changes the polariton transient spectra and relaxation times. We also observe quantum beats, so-called vacuum Rabi oscillations, between the upper and lower vibration-cavity polaritons. In addition to establishing that coupling to an optical cavity modifies the energy-transfer dynamics of the coupled molecules, this work points out the possibility of systematic and predictive modification of the excited-state kinetics of vibration-cavity polariton systems.

We report experimental 2D infrared (2D IR) spectra of coherent light-matter excitations--molecular vibrational polaritons. The application of advanced 2D IR spectroscopy to vibrational polaritons challenges and advances our understanding in both fields. First, the 2D IR spectra of polaritons differ drastically from free uncoupled excitations and a new interpretation is needed. Second, 2D IR uniquely resolves excitation of hybrid light-matter polaritons and unexpected dark states in a state-selective manner, revealing otherwise hidden interactions between them. Moreover, 2D IR signals highlight the impact of molecular anharmonicities which are applicable to virtually all molecular systems. A quantum-mechanical model is developed which incorporates both nuclear and electrical anharmonicities and provides the basis for interpreting this class of 2D IR spectra. This work lays the foundation for investigating phenomena of nonlinear photonics and chemistry of molecular vibrational polaritons which cannot be probed with traditional linear spectroscopy.

The coherent coupling between an optical transition and a confined optical mode, when sufficiently strong, gives rise to a new pair of mixed modes separated in frequency by the vacuum Rabi splitting. Such systems have been widely investigated for electronicstate transitions such as molecular excitons coupled to surfaceplasmons and optical microcavities. However, only very recently have vibrational transitions been considered. Here we experimentally investigate the coupling between a Fabry−Perot cavity and the carbonyl stretch at an infrared frequency near 1730 cm −1 in polymethyl methacrylate. As is requisite for the "strong coupling" regime, the measured vacuum-Rabi-splitting of 132 cm −1 is much larger than the full width of either the cavity resonance (34 cm −1 ) or the inhomogeneously broadened carbonyl-stretch absorption (24 cm −1 ). With the assistance of quantitative analysis using transfermatrix methods, we provide evidence that the mixed-state resonances are relatively immune to inhomogeneous vibrational broadening and demonstrate the ability to extract splittings by convenient angle tuning of the Fabry−Perot cavity to match the vibrational frequency. 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 both statically (as here) and dynamically with ultrafast methods. Moreover, microfluidic cavities will permit the study of liquids, greatly expanding the range of assessable molecules. W hen an optical transition (e.g., a molecular excitonic absorption) is excited by a confined optical-mode oscillating at the same frequency (e.g., an optical microcavity), a strong coupling between the two resonances can occur. This coupled-oscillator system exhibits a hybridization of the two resonances to create two new eigenstates of mixed character that are each shifted from the original resonant frequency by half the vacuum Rabi splitting, Ω. Such coupled systems are variously referred to as cavity polaritons, 1 normal coupled modes, 2 more specific terms (e.g., plexitons 3 ), or simply coupled-modes. These coupled-mode systems have received widespread attention in part because the split states possess new properties such as immunity to inhomogeneous broadening 4 or altered relaxation pathways. 5,6 The coupled systems can have marked implications for emission and relaxation rates and, accordingly, have found utility in the pursuit of nanoscale organic lasing materials 7 and nearly threshold-less lasers. 8,9 Cavities strongly coupled to single optical-emitters have also been considered for use in quantum information systems. 10−13 To date, a wide variety of strategies spanning microwave to visible frequencies have been employed or proposed to produce the confined-mode, including surface-plasmon polaritons propagating on films 14,15 and on nanowires, 16 local surface plasmons, 17,18 Fabry−Perot cavities, 8 whispering gallery cavities, 11 photonic crystals, 10,19 and microwave cavities. The species coupled to...

Designing coupled vibrational-cavity polariton systems 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 linewidths). Here, we evaluate the impact of absorption coefficient and cavity design on normal mode coupling between a FabryPérot 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 pathlength cavities are used to demonstrate the pathlength-independence of the coupling strength. The ability to adjust the cavity linewidth, 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.Coupling between an optically-driven material excitation (e.g., a semiconductor quantum dot excitonic absorption) and a confined optical-mode (e.g., an optical microcavity) can drastically alter the behavior of both modes. Strong coupling, which occurs when the two modes are in resonance and the interaction between the two exceeds the damping rate, produces two hybridized states whose fundamental character is a quantum-mechanical superposition of the two original modes. Each of these mixed-character eigenstates is shifted from the original resonant frequency by half the Rabi splitting, Ω. 1 Such coupled systems are variously referred to as cavity polaritons, 2 coupled normal modes, 1 plexcitons 3 , or simply coupled normal modes. If the system consists of a single quantum oscillator (twolevel atom, single quantum dot, etc.) interacting with a single cavity photon, nonlinear effects such as photon blocking and climbing of the Jaynes-Cummings ladder may occur. The splitting in such a system is referred to as vacuum Rabi splitting 1,4,5 as opposed to simply Rabi or polariton splitting associated with ensembles of individual oscillators, as described herein. Relaxation lifetimes, 6,7 linewidt...

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. We develop a quantum-mechanical theory of pump-probe spectroscopy for this class of polaritons based on the quantum Langevin equation 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.

Scanning Kelvin probe microscopy ͑SKPM͒ and conductive atomic force microscopy ͑C-AFM͒ have been used to image surfaces of GaN grown by molecular beam epitaxy. Detailed analysis of the same area using both techniques allowed imaging and comparison of both surface potential variations arising from the presence of negatively charged threading dislocations and localized current leakage paths associated with dislocations. Correlations between the charge state of dislocations, conductivity of current leakage paths, and dislocation type were thereby established. Analysis of correlated SKPM and C-AFM images revealed a density of negatively charged features of ϳ3ϫ10 8 cm Ϫ2 and a localized current leakage path density of ϳ3ϫ10 7 cm Ϫ2 , with approximately 25% of the leakage paths spatially correlated with negatively charged dislocation features. Based on correlated topography and previous studies quantifying the densities of edge, screw, and mixed dislocations, our results suggested that dislocations having an edge component behave as though negatively charged while pure screw dislocations are solely responsible for the observed leakage paths and are uncharged.

Strong cavity coupling to molecular vibrations creates vibration-polaritons capable of modifying chemical reaction kinetics, product branching ratios, and charge transfer equilibria. However, the mechanisms impacting these molecular processes remain elusive. Furthermore, even basic elements determining the spectral properties of polaritons, such as selection rules, transition moments, and lifetimes are poorly understood. Here, we use two-dimensional infrared and filtered pump–probe spectroscopy to report clear spectroscopic signatures and relaxation dynamics of excited vibration-polaritons formed from the cavity-coupled NO band of nitroprusside. We apply an extended multi-level quantum Rabi model that predicts transition frequencies and strengths that agree well with our experiment. Notably, the polariton features decay ~3–4 times slower than the polariton dephasing time, indicating that they support incoherent population, a consequence of their partial matter character.

Strong coupling between vibrational modes and cavity optical modes leads to the formation of vibration-cavity polaritons, separated by the vacuum Rabi splitting. The splitting depends on the square root of the concentration of absorbers confined in the cavity, which has important implications on the response of the coupled system after ultrafast infrared excitation. In this work, we report on solutions of W(CO) in hexane with a concentration chosen to access a regime that borders on weak coupling. Under these conditions, large fractions of the W(CO) oscillators can be excited, and the anharmonicity of the molecules leads to a commensurate reduction in the Rabi splitting. We report excitation fractions > 0.4, depending on excitation pulse intensity, and show drastic increases in transmission that can be modulated on the picosecond time scale. In comparison to previous experiments, the transient spectra that we observe are much simpler because excited-state transitions lie outside of the transmission spectrum of the cavity, thereby contributing only weakly to the spectra. We find that the Rabi splitting recovers with the characteristic vibrational relaxation lifetime and anisotropy decay of uncoupled W(CO), implying that polaritons are not directly involved in the relaxation we observe after the first few ps. The results help corroborate the model that we proposed to describe the results at higher concentrations and show that the ground-state bleach of cavity-coupled molecules has a broad, multisigned spectral response.