Vibrational Strong Coupling Controlled by Spatial Distribution of Molecules within the Optical Cavity

Similar to excitonic materials interacting with optical cavity fields, vibrational absorbers coupled to resonantly matched optical modes can exhibit new hybridized energy states called cavity polaritons. The delocalized nature of these hybrid polaritonic states can potentially modify a material’s physical and chemical characteristics, with the promise of a significant impact on reaction chemistry. In this study, we investigate the relationship between the spatial distribution of vibrational absorbers and the cavity mode profile in vibrational strong coupling by systematically varying the location of a 245-nm-thick poly­(methyl methacrylate) (PMMA) film within a few-micrometer-thick Fabry–Perot cavity. Angle-tuning the cavity reveals that the first- and second-order cavity resonances couple to molecular absorption lines of PMMA (the CO and C–H stretching bands at 1731 and 2952 cm^–1, respectively), resulting in quantifiable vacuum Rabi splittings in the dispersion response. These splittings, as extracted from experiment, transfer-matrix calculations, and an analytical treatment, display a consistent and strong dependence on the molecular spatial distribution within a cavity. Furthermore, we demonstrate the response of two physically separated molecular layers by measuring and calculating the vacuum Rabi splitting for cavities loaded with single and widely spaced pairs of PMMA layers. The results provide evidence that extended cavity polariton modes sample these separate layers simultaneously and, more broadly, provide guidance for controlling the coupling strength, and potentially chemical reactivity, of a given region through modification of the cavity mode profile or through introducing a remotely located molecular layer

Quantification of Efficient Plasmonic Hot-Electron Injection in Gold Nanoparticle–TiO₂ Films

Excitation of localized surface plasmons in metal nanostructures generates hot electrons that can be transferred to an adjacent semiconductor, greatly enhancing the potential light-harvesting capabilities of photovoltaic and photocatalytic devices. Typically, the external quantum efficiency of these hot-electron devices is too low for practical applications (<1%), and the physics underlying this low yield remains unclear. Here, we use transient absorption spectroscopy to quantify the efficiency of the initial electron transfer in model systems composed of gold nanoparticles (NPs) fully embedded in TiO₂ or Al₂O₃ films. In independent experiments, we measure free carrier absorption and electron–phonon decay in the model systems and determine that the electron-injection efficiency from the Au NPs to the TiO₂ ranges from about 25% to 45%. While much higher than some previous estimates, the measured injection efficiency is within an upper-bound estimate based on a simple approximation for the Au hot-electron energy distribution. These results have important implications for understanding the achievable injection efficiencies of hot-electron plasmonic devices and show that the injection efficiency can be high for Au NPs fully embedded within a semiconductor with dimensions less than the Au electron mean free path

Optical Dark-Field and Electron Energy Loss Imaging and Spectroscopy of Symmetry-Forbidden Modes in Loaded Nanogap Antennas

We have produced large numbers of hybrid metal–semiconductor nanogap antennas using a scalable electrochemical approach and systematically characterized the spectral and spatial character of their plasmonic modes with optical dark-field scattering, electron energy loss spectroscopy with principal component analysis, and full wave simulations. The coordination of these techniques reveal that these nanostructures support degenerate transverse modes which split due to substrate interactions, a longitudinal mode which scales with antenna length, and a symmetry-forbidden <i>gap-localized transverse</i> mode. This gap-localized transverse mode arises from mode splitting of transverse resonances supported on both antenna arms and is confined to the gap load enabling (i) delivery of substantial energy to the gap material and (ii) the possibility of tuning the antenna resonance <i>via</i> active modulation of the gap material’s optical properties. The resonant position of this symmetry-forbidden mode is sensitive to gap size, dielectric strength of the gap material, and is highly suppressed in air-gapped structures which may explain its absence from the literature to date. Understanding the complex modal structure supported on hybrid nanosystems is necessary to enable the multifunctional components many seek

Low-Loss, Extreme Subdiffraction Photon Confinement via Silicon Carbide Localized Surface Phonon Polariton Resonators

Plasmonics provides great promise for nanophotonic applications. However, the high optical losses inherent in metal-based plasmonic systems have limited progress. Thus, it is critical to identify alternative low-loss materials. One alternative is polar dielectrics that support surface phonon polariton (SPhP) modes, where the confinement of infrared light is aided by optical phonons. Using fabricated 6H-silicon carbide nanopillar antenna arrays, we report on the observation of subdiffraction, localized SPhP resonances. They exhibit a dipolar resonance transverse to the nanopillar axis and a monopolar resonance associated with the longitudinal axis dependent upon the SiC substrate. Both exhibit exceptionally narrow linewidths (7–24 cm^–1), with quality factors of 40–135, which exceed the theoretical limit of plasmonic systems, with extreme subwavelength confinement of (λ_res³/<i>V</i>_eff)^1/3 = 50–200. Under certain conditions, the modes are Raman-active, enabling their study in the visible spectral range. These observations promise to reinvigorate research in SPhP phenomena and their use for nanophotonic applications