Revisiting cavity-coupled 2DIR: A classical approach implicates reservoir modes.

Significant debate surrounds the origin of nonlinear optical responses from cavity-coupled molecular vibrations. Several groups, including our own, have previously assigned portions of the nonlinear response to polariton excited-state transitions. Here, we report a new method to approximate two-dimensional infrared spectra under vibrational strong coupling, which properly accounts for inhomogeneous broadening.

Comment on "Isolating Polaritonic 2D-IR Transmission Spectra".

This Viewpoint responds to the analysis of 2D IR spectra of vibration cavity polaritons in the study reported in (Duan et al. , 12, 11406). That report analyzed 2D IR spectra of strongly coupled molecules, such as W(CO) and nitroprusside anion, based on subtracting a background signal generated by polariton filtered free space signals.

Extremophilic behavior of catalytic amyloids sustained by backbone structuring.

Enzyme function relies on the placement of chemistry defined by solvent and self-associative hydrogen bonding displayed by the protein backbone. Amyloids, long-range multi-peptide and -protein materials, can mimic enzyme functions while having a high proportion of stable self-associative backbone hydrogen bonds. Though catalytic amyloid structures have exhibited a degree of temperature and solvent stability, defining their full extremophilic properties and the molecular basis for such extreme activity has yet to be realized.

Vibration-Cavity Polariton Chemistry and Dynamics.

Molecular polaritons result from light-matter coupling between optical resonances and molecular electronic or vibrational transitions. When the coupling is strong enough, new hybridized states with mixed photon-material character are observed spectroscopically, with resonances shifted above and below the uncoupled frequency. These new modes have unique optical properties and can be exploited to promote or inhibit physical and chemical processes.

Comparative analysis of polaritons in bulk, dielectric slabs, and planar cavities with implications for cavity-modified reactivity.

We examine closely the differences between the densities of vibrational states of bulk, slab, and cavity polariton modes under weak and moderate inhomogeneous broadening. While existing theoretical treatments are often based on a comparative analysis of "bare" vibrations and cavity polaritons, in the strong-coupling regime, only differences between slab/bulk polaritons on the one hand and cavity polaritons on the other hand are meaningful since "bare" vibrations are not observed experimentally. We find that polaritons in cavities significantly detuned from resonance with molecular transitions at zero in-plane wavevector do not differ appreciably from bulk polaritons in their density of vibrational states.

Hyperbolic phonon polariton resonances in calcite nanopillars.

We report the first experimental observation of hyperbolic phonon polariton (HP) resonances in calcite nanopillars, demonstrate that the HP modes redshift with increasing aspect ratio (AR = 0.5 to 1.1), observe a new, possibly higher order mode as the pitch is reduced, and compare the results to both numerical simulations and an analytical model.

Excited-state vibration-polariton transitions and dynamics in nitroprusside.

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.

Power of Aerogel Platforms to Explore Mesoscale Transport in Catalysis.

We describe the opportunity to deploy aerogels-an ultraporous nanoarchitecture with co-continuous networks of meso/macropores and covalently bonded nanoparticulates-as a platform to address the nature of the electronic, ionic, and mass transport that underlies catalytic activity. As a test case, we fabricated Au||TiO junctions in composite guest-host aerogels in which ∼5 nm Au nanoparticles are incorporated either directly into the anatase TiO network (Au "in" TiO, Au-TiO aerogel) or deposited onto preformed TiO aerogel (Au "on" TiO, Au/TiO aerogel). The metal-meets-oxide nanoscale interphase as visualized by electron tomography feature extended three-dimensional (3D) interfaces, but Au-TiO aerogels impose a greater degree of Au contact with TiO particles than does the Au/TiO form.

Negligible Effect of Vibrational Polaritons on Chemical Reaction Rates via the Density of States Pathway.

We show that the polariton density of states in planar optical cavities strongly coupled to vibrational excitations remains much lower than the density of vibrational states even at the frequency of the lower and upper polaritons under most practical circumstances. The polariton density of states is higher within a narrow window only when the inhomogeneous line width is at least an order of magnitude smaller than the Rabi splitting. Therefore, modification of reaction rates via the density-of-states pathway appears small or negligible for the scenarios reported in the literature.

Nanosecond transient absorption studies of the pH-dependent hydrated electron quenching by HSO.

The large standard reduction potential of an aqueous solvated electron (eaq-, E° = -2.9 V) makes it an attractive candidate for reductive treatment of wastewater contaminants. Using transient absorption spectroscopy, the nanosecond to microsecond dynamics of eaq- generated from 10 mM solutions of Na2SO3 at pH 4 to 11 in H2O and D2O are characterized, resulting in the determination that between pH 4 and 9 it is the HSO3-, and not H+ as previously postulated by others, that effectively quenches eaq-.

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. 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.

Two-dimensional infrared spectroscopy of vibrational polaritons.

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.

Ultrafast Transmission Modulation and Recovery via Vibrational Strong Coupling.

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.

Static and Time-Resolved Terahertz Measurements of Photoconductivity in Solution-Deposited Ruthenium Dioxide Nanofilms.

Thin-film ruthenium dioxide (RuO) is a promising alternative material as a conductive electrode in electronic applications because its rutile crystalline form is metallic and highly conductive. Herein, a solution-deposition multi-layer technique is employed to fabricate ca. 70 ± 20 nm thick films (nanoskins) and terahertz spectroscopy is used to determine their photoconductive properties.

Quantification of Efficient Plasmonic Hot-Electron Injection in Gold Nanoparticle-TiO 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 TiO or AlO films.

Plasmonic Aerogels as a Three-Dimensional Nanoscale Platform for Solar Fuel Photocatalysis.

We use plasmonic Au-TiO aerogels as a platform in which to marry synthetically thickened particle-particle junctions in TiO aerogel networks to Au∥TiO interfaces and then investigate their cooperative influence on photocatalytic hydrogen (H) generation under both broadband (i.e., UV + visible light) and visible-only excitation.

Transient Optical and Terahertz Spectroscopy of Nanoscale Films of RuO.

Solution-deposited nanoscale films of RuO ("nanoskins") are effective transparent conductors once calcined to 200 °C. Upon heating the nanoskins to higher temperature the nanoskins show increased transmission at 550 nm. Electronic microscopy and X-ray diffraction show that the changes in the optical spectrum are accompanied by the formation of rutile RuO nanoparticles.

Aspect-ratio driven evolution of high-order resonant modes and near-field distributions in localized surface phonon polariton nanostructures.

Polar dielectrics have garnered much attention as an alternative to plasmonic metals in the mid- to long-wave infrared spectral regime due to their low optical losses. As such, nanoscale resonators composed of these materials demonstrate figures of merit beyond those achievable in plasmonic equivalents. However, until now, only low-order, phonon-mediated, localized polariton resonances, known as surface phonon polaritons (SPhPs), have been observed in polar dielectric optical resonators.

Atomic-scale photonic hybrids for mid-infrared and terahertz nanophotonics.

The field of nanophotonics focuses on the ability to confine light to nanoscale dimensions, typically much smaller than the wavelength of light. The goal is to develop light-based technologies that are impossible with traditional optics. Subdiffractional confinement can be achieved using either surface plasmon polaritons (SPPs) or surface phonon polaritons (SPhPs).

Mie resonance-enhanced light absorption in periodic silicon nanopillar arrays.

Mie-resonances in vertical, small aspect-ratio and subwavelength silicon nanopillars are investigated using visible bright-field µ-reflection measurements and Raman scattering. Pillar-to-pillar interactions were examined by comparing randomly to periodically arranged arrays with systematic variations in nanopillar diameter and array pitch. First- and second-order Mie resonances are observed in reflectance spectra as pronounced dips with minimum reflectances of several percent, suggesting an alternative approach to fabricating a perfect absorber.

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.

Identification of a Methane Oxidation Intermediate on Solid Oxide Fuel Cell Anode Surfaces with Fourier Transform Infrared Emission.

Fuel interactions on solid oxide fuel cell (SOFC) anodes are studied with in situ Fourier transform infrared emission spectroscopy (FTIRES). SOFCs are operated at 800 °C with CH4 as a representative hydrocarbon fuel. IR signatures of gas-phase oxidation products, CO2(g) and CO(g), are observed while cells are under load.

High-Temperature Chemistry in Solid Oxide Fuel Cells: In Situ Optical Studies.

Solid oxide fuels cells (SOFCs) are promising devices for versatile and efficient power generation with fuel flexibility, but their viability is contingent upon understanding chemical and material processes to improve their performance and durability. Newly developed in situ optical methods provide new insight into how carbon deposition varies with different hydrocarbon and alcohol fuels and depends on operating conditions. Some findings, such as heavier hydrocarbon fuels forming more carbon than lighter fuels, are expected, but other discoveries are surprising.

Vibrational circular-dichroism spectroscopy of homologous cyclic peptides designed to fold into β helices of opposite chirality.

Cyclic β-helical peptides have been developed as model structured biomolecules for examining peptide adsorption and conformation on surfaces. As a key prerequisite to circular-dichroism (CD) analysis of these model peptides on surfaces, their conformations and the corresponding vibrational spectra in the 1400-1800 cm⁻¹ range were analyzed by vibrational circular-dichroism (VCD) spectroscopy in solution. The two model peptides ("β Leu and β Val") were examined in chloroform, where they each fold into a homogeneous well-defined antiparallel double-stranded β-helical species, as determined previously by NMR and electronic CD spectroscopy.

In situ optical studies of solid-oxide fuel cells.

Thermal imaging and vibrational spectroscopy have become important tools for examining the physical and chemical changes that occur in real time in solid-oxide fuel cells (SOFCs). Imaging techniques can resolve temperature differences as fine as 0.1 degrees C across a SOFC electrode at temperatures higher than 600 degrees C.