Phase change materials (PCMs) are seeing tremendous interest for their use in reconfigurable photonic devices. Understanding the volume of the phase change is critical to the construction of devices. In this work, we demonstrate the ability to crystallize and reamorphize Sb2S3 thin films using a 405 nm laser and nondestructively quantify partial volumetric crystallization with nanoscale resolution via near-field scanning optical microscopy (NSOM). We provide an analytical description of the NSOM measurements and correlate the results with a model of laser/Sb2S3 interactions that includes laser irradiance power, thermal transport, and phase change kinetics. The results demonstrate a relationship between computational modeling and physical device behavior, which is critical for the creation of phase change-based devices.

The goal of this chapter is to accelerate the ability of research groups to contribute quickly, accurately, and substantively, to the field of cavity-modified materials behaviors. The opportunity to tune fundamental chemical and physical properties with an engineered optical environment has inspired images of designer cavity-catalysis, enhanced energy technologies, and directed bio-assembly. Although numerous demonstrations of cavity-modified chemistry have been made, hesitancy remains due to a lack of mechanistic understanding, experimentally precarious results, and the general enigmatic character of this phenomenon. In fact, corroborating results have been met with doubt by even close colleagues. However, just as error bars are slowly shaved away, measurement by measurement, to eventually reveal the sought-after truth, so too must we continue to evaluate cavity-induced effects, troubleshooting and improving experimental techniques, refining analytical tools, and listening to and addressing legitimate criticism.

In this article, we developed a theory describing surface exciton polaritons (SEPs) that accounts for the spatial dispersion of the dielectric constant connected with exciton momentum. Due to strong coupling between light and bulk excitons in the frequency separation, & hbar;omega LT, between the longitudinal and transverse excitons, the SEP is formed and behaves as partially light and partially matter. The dispersion of the SEP was found through a combined solution of Maxwell's and Thomas-Hopfield's equations. The analytical theory describes SEPs at any bulk exciton/vacuum interface and provides its complete dispersion if one knows & hbar;omega LT, the exciton effective mass, M, and the high-frequency dielectric constant, kappa infinity. The presented theory is in excellent agreement with the only numerical modeling of this problem, which was conducted for SEPs at a ZnO/vacuum interface. Calculations show the spatial dispersion of the dielectric constant leads to rather small broadening of the photon-like quasi-particle and suggest using SEPs for long-range coherence transfer. The theory was used to describe SEP dispersion in CsPbCl3 and CsPbBr3 perovskites.

In this paper, we have developed a theory describing surface exciton polariton (SEPs) that accounts for the spatial dispersion of the dielectric constant connected with exciton momentum. Due to strong coupling between light and bulk excitons in the frequency separation, ħω_LT, between the longitudinal and transverse exciton, the SEP is formed and behaves at partially light and partially matter. The dispersion of the SEP was found through a combined solution of Maxwell's and Thomas-Hopfield's equations. The analytical theory describes SEPs at any bulk exciton/vacuum interface and provides its complete dispersion if one knows ħω_LT, the exciton effective mass, M, and the high frequency dielectric constant, κ_∞. The presented theory is in excellent agreement with the only numerical modeling of this problem, which was conducted for SEPs at a ZnO/vacuum interface. Calculations show the spatial dispersion of the dielectric constant leads to rather small broadening of the photon-like quasi-particle and suggests using SEPs for long-range coherence transfer.

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. We find excellent agreement between this model and experimental results for prototypical systems exhibiting both homogeneous and inhomogeneous broadening. This work implies that reservoir excitation is solely responsible for all optical response measured after the polariton modes dephase and represents an important new method for predicting and interpreting the nonlinear optical response of molecular vibrational polaritons.

Reports of modified materials processes under vibrational strong coupling (VSC), which include cavity-mediated vibrational energy transfer, modified chemical product selectivity, and altered solvent-solute interaction forces, have been met with a great deal of skepticism due to several irreproducible results and the lack of an accepted theoretical framework. In this work, we add some insight by identifying an optical measurement artifact arising when a distribution of cavity lengths is probed. This artifact can alter extracted chemical reaction rates and, therefore, has important implications for numerous previously reported, and contested, examples of cavity-modified chemistry. We then use these revelations to inform best practices for carrying out reliable measurements in cavities.

Exciton-plasmon coupling between two-dimensional transition metal dichalcogenides and metallic nanostructures has attracted much attention as a means of creating room temperature polaritons and controlling the optoelectronic properties of these hybrid quasiparticles. Prior investigations of strained monolayer transition metal dichalcogenides, that host single photon emission, coupled to plasmonic nanostructures have remained in the weak coupling regime where the interaction is characterized by Purcell enhancement of emission rates. Here we achieve site-specific coupling between spatially localized excitons in nanoindented WSe2 and localized surface plasmon resonances hosted by Au nanodiscs, thereby producing mode splitting within the scattered light spectrum. This establishes nanoindentation as a means of tailoring the topography of two-dimensional materials around plasmonic resonators. By tuning the localized surface plasmon resonance via disc diameter, we observe an avoided crossing between the exciton and plasmon resonances with room temperature mode splitting of 78 +/- 4 meV. We describe the exciton-plasmon interaction with a self-consistent theoretical framework based on cavity electrodynamics and arrive at an exciton-plasmon coupling strength of 23 +/- 5 meV. The apparent discrepancy between the mode splitting and coupling strength arises from enhanced excitonic absorption, a phenomenon that dominates when the coupling strength is less than the line width and is responsible for larger than expected mode splitting in scattering spectra. Our observations are consistent with full-wave electromagnetic models and place the exciton-plasmon system within the intermediate coupling regime. Enhanced absorption plays a key role in shaping the scattering spectra from plasmonic platforms where the resonator is often much more lossy than the exciton and we therefore recommend that it should be accounted for when estimating the coupling strength based on scattering data from such systems. We also suggest pathways to increase the coupling strength for achieving strong coupling between plasmons and the strain localized excitons that contribute to single photon emission.

Here, we review the design of optical cavities, transient and modulated responses, and theoretical models relevant to vibrational strong coupling (VSC). While planar Fabry-Perot cavities remain the most common choice for experiments involving vibrational polaritons, other choices including plasmonic and phononic nanostructures, extended lattice resonances, and wavelength-scale three-dimensionally confined dielectric cavities have unique advantages, which are discussed. Next, we review the nonlinear response to laser excitation of VSC systems revealed by transient pump-probe and 2DIR techniques. The assignment of various features observed in these experiments has been an important topic with significant recent progress and controversy. The modulation of VSC systems by various means such as ultrafast pulses and electrochemical methods is also described. Finally, theoretical approaches to understanding the physics and chemistry of VSC systems are reviewed with an eye toward their applicability and usefulness. These fall into two main categories: (1) solving for the eigenmodes of the system and (2) evolutionary techniques including the transfer-matrix method and its generalizations. The need for quantum optical methods of describing VSC systems is critically evaluated in light of current experimental work, and we discuss circumstances which necessitate consideration of the full in-plane dispersion of the Fabry-Perot cavities.

Data to reproduce Figures in paper "Modification of ground state chemical reactivity via light-matter coherence in infrared cavities" published in DOI:0000/0000000

Quasi-2D perovskites are recognized as a promising material for lightweight photovoltaic applications. For these applications, it is critical to understand how mechanical stresses affect the perovskite's optoelectronic properties. Here, we discuss how applied stress impacts the photo-luminescence spectrum and lifetime of quasi-2D perovskite thin films and how these effects depend on interlayer cation choice (butylammonium or phenethylammonium) and film processing (hot-casting versus postdeposition annealing). We find that compressive stress induces an enhancement and spectral redshift (>10 meV per percent strain) in the emission from bulk-like grains in the film. Moreover, the charge carrier recombination lifetime is greatly enhanced (up to a factor of 14 with only 1% strain). We show that the phenethylammonium-based perovskites are more robust against strain-induced modifications compared to those containing the butylammonium cation. These results highlight how strongly strain can modify the optoelectronic properties of quasi -2D perovskite thin films and how cation choice and film processing can have important practical implications for developing lightweight photovoltaics.

Reaction-rate modifications for chemical processes due to strong coupling between reactant molecular vibrations and the cavity vacuum have been reported; however, no currently accepted mechanisms explain these observations. In this work, reaction-rate constants were extracted from evolving cavity transmission spectra, revealing resonant suppression of the intracavity reaction rate for alcoholysis of phenyl isocyanate with cyclohexanol. We observed up to an 80% suppression of the rate by tuning cavity modes to be resonant with the reactant isocyanate (NCO) stretch, the product carbonyl (CO) stretch, and cooperative reactant-solvent modes (CH). These results were interpreted using an open quantum system model that predicted resonant modifications of the vibrational distribution of reactants from canonical statistics as a result of light-matter quantum coherences, suggesting links to explore between chemistry and quantum science.

Modifications of chemical reaction rates via strong light-matter coupling at mid-infrared regime has attracted significant interest in physics and chemistry in recent years. However, despite numerous efforts, there is currently no general theory that can fully describe the available experimental evidence. We implement an open quantum system model to describe the suppression of the intracavity reaction rate for alcoholysis of phenyl isocyanate with cyclohexanol in infrared Fabry-Perot cavities. The model considers the three molecular modes observed in the infrared absorption spectrum of phenyl isocyanate in the region of interest. The results point out that suppression of intracavity reaction rates is modified when a cavity is resonant with specific molecular modes of the reactant [1]. We derive analytical expressions using a reduced model that explains the role of light-matter coherences and molecular disorder in the modifications of intracavity chemical processes. Our findings significantly improve our understanding of cavity-modified chemistry by tuning cavity modes with different molecular modes. [1] W.Ahn, J.F. Triana, F. Recabal, F. Herrera, B.S. Simpkins . Chemrxiv, wb6vs (2022)

This Viewpoint responds to the analysis of 2D IR spectra of vibration cavity polaritons in the study reported in The Journal of Physical Chemistry Letters (Duan et al. 2021, 12, 11406). That report analyzed 2D IR spectra of strongly coupled molecules, such as W(CO)6 and nitroprusside anion, based on subtracting a background signal generated by polariton filtered free space signals. They assigned the resulting response as being due to excited polaritons. We point out in this Viewpoint that virtually all of the response can be properly reproduced using the physics of transmission through an etalon containing a material modeled with a complex dielectric function describing the ground- and excited-state absorber populations. Furthermore, such a coupled system cannot be described as a scaled sum of the bare molecular and cavity responses.

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. One remarkable result is that vibrational strong coupling to cavities can alter reaction rates and product branching ratios with no optical excitation whatsoever. In this work we review the ability of vibration-cavity polaritons to modify chemical and physical processes including chemical reactivity, as well as steady-state and transient spectroscopy. We discuss the larger context of these works and highlight their most important contributions and implications. Our goal is to provide insight for systematically manipulating molecular polaritons in photonic and chemical applications.

Herein, we find that TiN sputter-deposited on GaN displayed the desired optical properties for plasmonic applications. While this is a positive result indicating the possible use of p- or n-type GaN as a collector of plasmonically generated hot carriers, the interfacial properties differed considerably depending on doping conditions. On p-type GaN, a distinct Schottky barrier was formed with a barrier height of ~0.56 eV, which will enable effective separation of photogenerated electrons and holes, a typical approach used to extend their lifetimes. On the other hand, no transport barrier was found for TiN on n-type GaN. While the lack of spontaneous carrier separation in this system will likely reduce unprompted hot carrier collection efficiencies, it enables a bias-dependent response whereby charges of the desired type (e.g., electrons) could be directed into the semiconductor or sequestered in the plasmonic material. The specific application of interest would determine which of these conditions is most desirable.

2D perovskites have broad technological appeal because of their tunable mechanical, optical, and electrical properties. For flexible optoelectronic applications, it is necessary to determine how mechanical stresses affect their optoelectronic properties. We compare the impact of strain on the photoluminescence (PL) spectra and charge carrier recombination rates of two different 2D perovskite materials, synthesized using either phenethylammonium or butylammonium cations. Both perovskite materials exhibit strong PL enhancement, redshifts of the PL emission wavelength, and longer recombination lifetimes for compressive strains of ≲1%. These results are discussed in relation to the materials’ band structures and trap states.

Molecular vibrations can couple to optical cavities to create new hybrid states called polaritons. The magnitude of this coupling, measured as the vacuum Rabi splitting (Ω), correlates with modified materials processes such as photon emission, molecular energy transfer, and chemical reaction rates. In this talk, I will first discuss active control of cavity coupling strength. We demonstrate active tuning of excitonic strong coupling in a system where organic dyes strongly couple to propagating surface plasmon polaritons (SPPs) and modulation of vibrational strong coupling in a Fabry-Perot cavity coupled to an organic charge shuttling molecule. Next, I will discuss results indicating modified chemical reaction rates for an alcoholysis addition reaction forming urethane monomers. Cavity tuning was used to selectively couple to reactant, solvent, and product vibrational modes resulting in a chemical response that is cavity tuning dependent. Lastly, and in light of the search for an understanding of the mechanisms leading to modified chemical and physical properties, I will present a theoretical description of the density of polariton states relative to molecular dark states. This work will also discuss the differences between polariton states generated in microcavities, slabs, and in the bulk.

Quantum emitters in the so-called strong coupling regime, create new hybrid light-matter states called polaritons. Recently, strong coupling between molecular vibrations and optical cavity modes has been shown to alter chemical reaction rates, product ratios, and charge exchange equilibria. In this work, we examine a cavity-modified addition reaction by monitoring alcoholysis of phenyl isocyanate with cyclohexanol, which yields urethane monomers. Since the reactants, products, and solvent all have strong vibrational modes, we examine the impact of cavity coupling to each and identify bands most active in modifying this reaction. A strong cavity-tuning dependence was found with reaction rate constants decreasing by a factor of 5 for cavities tuned to the reactant isocyanate stretch mode (NCO) and reduced by half for cavities tuned to product carbonyl stretch modes as well as for cavities tuned to CH modes shared by both the solvent and reactants. The reaction progress was tracked by extracting the time-dependent reactant concentration from fits to cavity-coupled transmission spectra. Quantitative measure of reactant concentration was necessity for analyzing this second order reaction. Our results extend the understanding of cavity-modified chemistry by examining the impact of coupling to reactant, solvent, and product modes within the same system, extracting reactant concentration directly from fits to strongly coupled spectra, and by providing rigorous verification of the effect itself.

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. Only cavity polaritons with sufficiently weak inhomogeneous broadening and tuned to resonance near normal incidence display a pronounced density-of-state enhancement. These results shed light on the heretofore puzzling observations of modified chemical reactivity only at zero detuning and supply a new baseline for assessing the explanatory power of proposed theories of cavity-modified chemistry.

TiN and ZrN refractory transition metal nitride nanoparticles (NPs) have recently emerged as an alternative to noble metals in plasmonic applications. However, plasmon-driven photocatalysis by ZrN NPs is largely unexplored. In this study, optical properties, morphology, crystal structure and surface composition of in-house synthesized and commercial ZrN nanoparticles (NPs) are vigorously characterized in order to select the best candidate material for evaluation of activity towards CH3OH photoelectrochemical oxidation. The photocatalytic activity of TiO2-supported ZrN NPs is compared to that of TiN/TiO2 as a function of NP loading and illumination wavelength. Our results indicate that optical properties and photocatalytic activity of ZrN/TiO2 are strongly affected by ZrN surface oxidation and agglomeration. We found that under visible illumination, both in-house synthesized 17 nm ZrN and commercial 30 nm TiN NPs promote TiO2 activity for CH3OH oxidation, while under visible + UV excitation, an inhibition effect is observed. The differences between the TiN/TiO2 and ZrN/TiO2 interfaces are discussed and the mechanisms of promotion/inhibition of TiO2 photocatalytic activity by ZrN and TiN NPs are proposed. Electromagnetic simulations are used to facilitate interpretation of experimental extinctions and photocatalytic activities.