Photoinduced Electron Transfer in the Strong Coupling Regime: Waveguide-Plasmon Polaritons

Molecular Polaritons for Chemistry, Photonics and Quantum Technologies

Molecular polaritons are quasiparticles resulting from the hybridization between molecular and photonic modes. These composite entities, bearing characteristics inherited from both constituents, exhibit modified energy levels and wave functions, thereby capturing the attention of chemists in the past decade. The potential to modify chemical reactions has spurred many investigations, alongside efforts to enhance and manipulate optical responses for photonic and quantum applications. This Review centers on the experimental advances in this burgeoning field. Commencing with an introduction of the fundamentals, including theoretical foundations and various cavity architectures, we discuss outcomes of polariton-modified chemical reactions. Furthermore, we navigate through the ongoing debates and uncertainties surrounding the underpinning mechanism of this innovative method of controlling chemistry. Emphasis is placed on gaining a comprehensive understanding of the energy dynamics of molecular polaritons, in particular, vibrational molecular polaritons─a pivotal facet in steering chemical reactions. Additionally, we discuss the unique capability of coherent two-dimensional spectroscopy to dissect polariton and dark mode dynamics, offering insights into the critical components within the cavity that alter chemical reactions. We further expand to the potential utility of molecular polaritons in quantum applications as well as precise manipulation of molecular and photonic polarizations, notably in the context of chiral phenomena. This discussion aspires to ignite deeper curiosity and engagement in revealing the physics underpinning polariton-modified molecular properties, and a broad fascination with harnessing photonic environments to control chemistry.

Machine Learning Framework for Modeling Exciton Polaritons in Molecular Materials

A light-matter hybrid quasiparticle, called a polariton, is formed when molecules are strongly coupled to an optical cavity. Recent experiments have shown that polariton chemistry can manipulate chemical reactions. Polariton chemistry is a collective phenomenon, and its effects increase with the number of molecules in a cavity. However, simulating an ensemble of molecules in the excited state coupled to a cavity mode is theoretically and computationally challenging. Recent advances in machine learning (ML) techniques have shown promising capabilities in modeling ground-state chemical systems. This work presents a general protocol to predict excited-state properties, such as energies, transition dipoles, and nonadiabatic coupling vectors with the hierarchically interacting particle neural network. ML predictions are then applied to compute the potential energy surfaces and electronic spectra of a prototype azomethane molecule in the collective coupling scenario. These computational tools provide a much-needed framework to model and understand many molecules' emerging excited-state polariton chemistry.

Modeling the near-field effect on molecular excited states using the discrete interaction model/quantum mechanical method

Strong light-matter interactions significantly modify the optical properties of molecules in the vicinity of plasmonic metal nanoparticles. Since the dimension of the plasmonic cavity approaches that of the molecules, it is critical to explicitly describe the nanoparticle junctions. In this work, we use the discrete interaction model/quantum mechanical (DIM/QM) method to model the coupling between the plasmonic near-field and molecular excited states. DIM/QM is a combined electrodynamics/quantum mechanical model that uses an atomistic description of the nanoparticle. We extend the DIM/QM method to include the local field effects in the sum-over-state formalism of time-dependent density functional theory. As a test of the method, we study the interactions between small organic chromophores and metal nanoparticles. In particular, we examine how the inclusion of multiple electronic transitions and intermolecular interactions modify the coupling between molecules and nanoparticles. Using the sum-over-state formalism of DIM/QM, we show that two-state models break down when the plasmon excitation is detuned from the molecular excitations. To gain further insight, we compare the simple coupled-dipole model (CDM) with the DIM/QM model. We find that CDM works well for simple systems but fails when going beyond the single molecule or single nanoparticle cases. We also find that the coupling depends strongly on the site of the nanoparticle in which the chromophore couples to. Our work suggests the importance of explicitly describing the cavity to capture the atomistic level local field environment in which the molecule strongly couples to.

Understanding Polaritonic Chemistry from Ab Initio Quantum Electrodynamics

In this review, we present the theoretical foundations and first-principles frameworks to describe quantum matter within quantum electrodynamics (QED) in the low-energy regime, with a focus on polaritonic chemistry. By starting from fundamental physical and mathematical principles, we first review in great detail ab initio nonrelativistic QED. The resulting Pauli-Fierz quantum field theory serves as a cornerstone for the development of (in principle exact but in practice) approximate computational methods such as quantum-electrodynamical density functional theory, QED coupled cluster, or cavity Born-Oppenheimer molecular dynamics. These methods treat light and matter on equal footing and, at the same time, have the same level of accuracy and reliability as established methods of computational chemistry and electronic structure theory. After an overview of the key ideas behind those ab initio QED methods, we highlight their benefits for understanding photon-induced changes of chemical properties and reactions. Based on results obtained by ab initio QED methods, we identify open theoretical questions and how a so far missing detailed understanding of polaritonic chemistry can be established. We finally give an outlook on future directions within polaritonic chemistry and first-principles QED.

Sub-picosecond collapse of molecular polaritons to pure molecular transition in plasmonic photoswitch-nanoantennas

Molecular polaritons are hybrid light-matter states that emerge when a molecular transition strongly interacts with photons in a resonator. At optical frequencies, this interaction unlocks a way to explore and control new chemical phenomena at the nanoscale. Achieving such control at ultrafast timescales, however, is an outstanding challenge, as it requires a deep understanding of the dynamics of the collectively coupled molecular excitation and the light modes. Here, we investigate the dynamics of collective polariton states, realized by coupling molecular photoswitches to optically anisotropic plasmonic nanoantennas. Pump-probe experiments reveal an ultrafast collapse of polaritons to pure molecular transition triggered by femtosecond-pulse excitation at room temperature. Through a synergistic combination of experiments and quantum mechanical modelling, we show that the response of the system is governed by intramolecular dynamics, occurring one order of magnitude faster with respect to the uncoupled excited molecule relaxation to the ground state.

Femtosecond Autocorrelation of Localized Surface Plasmons

Plasmon electronic dephasing lifetime is one of the most important characteristics of localized surface plasmons, which is crucial both for understanding the related photophysics and for their applications in photonic and optoelectronic devices. This lifetime is generally shorter than 100 fs and measured using the femtosecond pump-probe technique, which requires femtosecond laser amplifiers delivering pulses with a duration even as short as 10 fs. This implies a large-scale laser system with complicated pulse compression schemes, introducing high-cost and technological challenges. Meanwhile, the strong optical pulse from an amplifier induces more thermal-related effects, disturbing the precise resolution of the pure electronic dephasing lifetime. In this work, we use a simple autocorrelator design and integrate it with the sample of plasmonic nanostructures, where a femtosecond laser oscillator supplies the incident pulses for autocorrelation measurements. Thus, the measured autocorrelation trace carries the optical modulation on the incident pulses. The dephasing lifetime can be thus determined by a comparison between the theoretical fittings to the autocorrelation traces with and without the plasmonic modulation. The measured timescale for the autocorrelation modulation is an indirect determination of the plasmonic dephasing lifetime. This supplies a simple, rapid, and low-cost method for quantitative characterization of the ultrafast optical response of localized surface plasmons.

Vibrational coupling with O-H stretching increases catalytic efficiency of sucrase in Fabry-Pérot microcavity

Vibrational strong coupling (VSC) has been reported as a polariton-based method for modulating the rate of biochemical reactions. Herein, we studied how VSC modulates the sucrose hydrolysis. By monitoring the refractive index-induced shift of Fabry-Pérot microcavity, in which the catalytic efficiency of sucrose hydrolysis can be increased at least two times, as VSC was tuned to resonate with the stretching vibration of O-H bonds. This research provides new evidence for applying VSC in life sciences, which holds great promise to improving enzymatic industries.

Substrate-mediated plasmon hybridization toward high-performance light trapping

High-performance light trapping in metamaterials and metasurfaces offers prospects for the integration of multifunctional photonic components at subwavelength scales. However, constructing these nanodevices with reduced optical losses remains an open challenge in nanophotonics. Herein, we design and fabricate aluminum-shell-dielectric gratings by integrating low-loss aluminum materials with metal-dielectric-metal designs for high-performance light trapping featuring nearly perfect light absorption with broadband and large angular tuning ranges. The mechanism governing these phenomena is identified as the occurrence of substrate-mediated plasmon hybridization that allows energy trapping and redistribution in engineered substrates. Furthermore, we strive to develop an ultrasensitive nonlinear optical method, namely, plasmon-enhanced second-harmonic generation (PESHG), to quantify the energy transfer from metal to dielectric components. Our studies may provide a mechanism for expanding the potential of aluminum-based systems in practical applications.

Optimal Geometry for Plasmonic Hot-Carrier Extraction in Metal-Semiconductor Nanocrystals

Plasmon-induced energy and charge transfer from metal nanostructures hold great potential for harvesting solar energy. Presently, the efficiencies of charge-carrier extraction are still low due to the competitive ultrafast mechanisms of plasmon relaxation. Using single-particle electron energy loss spectroscopy, we correlate the geometrical and compositional details of individual nanostructures to their carrier extraction efficiencies. By removing ensemble effects, we are able to show a direct structure-function relationship that permits the rational design of the most efficient metal-semiconductor nanostructures for energy harvesting applications. In particular, by developing a hybrid system comprising Au nanorods with epitaxially grown CdSe tips, we are able to control and enhance charge extraction. We show that optimal structures can have efficiencies as high as 45%. The quality of the Au-CdSe interface and the dimensions of the Au rod and CdSe tip are shown to be critical for achieving these high efficiencies of chemical interface damping.

Tailored Dispersion of Spectro-Temporal Dynamics in Hot-Carrier Plasmonics

Ultrafast optical switching in plasmonic platforms relies on the third-order Kerr nonlinearity, which is tightly linked to the dynamics of hot carriers in nanostructured metals. Although extensively utilized, a fundamental understanding on the dependence of the switching dynamics upon optical resonances has often been overlooked. Here, all-optical control of resonance bands in a hybrid photonic-plasmonic crystal is employed as an empowering technique for probing the resonance-dependent switching dynamics upon hot carrier formation. Differential optical transmission measurements reveal an enhanced switching performance near the anti-crossing point arising from strong coupling between local and nonlocal resonance modes. Furthermore, entangled with hot-carrier dynamics, the nonlinear correspondence between optical resonances and refractive index change results in tailorable dispersion of recovery speeds which can notably deviate from the characteristic lifetime of hot carriers. The comprehensive understanding provides new protocols for optically characterizing hot-carrier dynamics and optimizing resonance-based all-optical switches for operations across the visible spectrum.

Controllable Patterning of Metallic Photonic Crystals for Waveguide-Plasmon Interaction

Waveguide-plasmon polaritons sustained in metallic photonic crystal slabs show fascinating properties, such as narrow bandwidth and ultrafast dynamics crucial for biosensing, light emitting, and ultrafast switching. However, the patterning of metallic photonic crystals using electron beam lithography is challenging in terms of high efficiency, large area coverage, and cost control. This paper describes a controllable patterning technique for the fabrication of an Ag grating structure on an indium-tin oxide (ITO) slab that enables strong photon-plasmon interaction to obtain waveguide-plasmon polaritons. The Ag grating consisting of self-assembled silver nanoparticles (NPs) exhibits polarization-independent properties for the excitation of the hybrid waveguide-plasmon mode. The Ag NP grating can also be annealed at high temperature to form a continuous nanoline grating that supports the hybrid waveguide-plasmon mode only under transverse magnetic (TM) polarization. We tuned the morphology and the periodicity of the Ag grating through the concentration of silver salt and the photoresist template, respectively, to manipulate the strong coupling between the plasmon and the waveguide modes of different orders.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Probing Charge Transport Kinetics in a Plasmonic Environment with Cyclic Voltammetry

Possible modifications in electrochemical reaction kinetics are explored in a nanostructured plasmonic environment with and without additional light illumination using a cyclic voltammetry (CV) method. In nanostructured gold, the effect of light on anodic and cathodic currents is much pronounced than that in a flat system. The electron-transfer rate shows a 3-fold increase under photoexcitation. The findings indicate a possibility of using plasmonic excitations for controlling electrochemical reactions.

Twisted intramolecular charge transfer (TICT) and twists beyond TICT: from mechanisms to rational designs of bright and sensitive fluorophores

The twisted intramolecular charge transfer (TICT) mechanism has guided the development of numerous bright and sensitive fluorophores. This review briefly overviews the history of establishing the TICT mechanism, and systematically summarizes the molecular design strategies in modulating the TICT tendency of various organic fluorophores towards different applications, along with key milestone studies and representative examples. Additionally, we also succinctly review the twisted intramolecular charge shuttle (TICS) and twists during photoinduced electron transfer (PET), and compare their similarities and differences with TICT, with emphasis on understanding the structure-property relationships between the twisted geometries and how they can directly affect the fluorescence of the molecules. Such structure-property relationships presented herein will greatly aid the rational development of fluorophores that involve molecular twisting in the excited state.

Chemistry under Vibrational Strong Coupling

Over the past decade, the possibility of manipulating chemistry and material properties using hybrid light-matter states has stimulated considerable interest. Hybrid light-matter states can be generated by placing molecules in an optical cavity that is resonant with a molecular transition. Importantly, the hybridization occurs even in the dark because the coupling process involves the zero-point fluctuations of the optical mode (a.k.a. vacuum field) and the molecular transition. In other words, unlike photochemistry, no real photon is required to induce this strong coupling phenomenon. Strong coupling in general, but vibrational strong coupling (VSC) in particular, offers exciting possibilities for molecular and, more generally, material science. Not only is it a new tool to control chemical reactivity, but it also gives insight into which vibrations are involved in a reaction. This Perspective gives the underlying fundamentals of light-matter strong coupling, including a mini-tutorial on the practical issues to achieve VSC. Recent advancements in "vibro-polaritonic chemistry" and related topics are presented along with the challenges for this exciting new field.

Theory of molecular emission power spectra. II. Angle, frequency, and distance dependence of electromagnetic environment factor of a molecular emitter in plasmonic environments

Our previous study [S. Wang et al., J. Chem. Phys. 153, 184102 (2020)] has shown that in a complex dielectric environment, molecular emission power spectra can be expressed as the product of the lineshape function and the electromagnetic environment factor (EEF). In this work, we focus on EEFs in a vacuum-NaCl-silver system and investigate molecular emission power spectra in the strong exciton-polariton coupling regime. A numerical method based on computational electrodynamics is presented to calculate the EEFs of single-molecule emitters in a dispersive and lossy dielectric environment with arbitrary shapes. The EEFs in the far-field region depend on the detector position, emission frequency, and molecular orientation. We quantitatively analyze the asymptotic behavior of the EFFs in the far-field region and qualitatively provide a physical picture. The concept of EEF should be transferable to other types of spectra in a complex dielectric environment. Finally, our study indicates that molecular emission power spectra cannot be simply interpreted by the lineshape function (quantum dynamics of a molecular emitter), and the effect of the EEFs (photon propagation in a dielectric environment) has to be carefully considered.

Manipulating matter by strong coupling to vacuum fields

Over the past decade, there has been a surge of interest in the ability of hybrid light-matter states to control the properties of matter and chemical reactivity. Such hybrid states can be generated by simply placing a material in the spatially confined electromagnetic field of an optical resonator, such as that provided by two parallel mirrors. This occurs even in the dark because it is electromagnetic fluctuations of the cavity (the vacuum field) that strongly couple with the material. Experimental and theoretical studies have shown that the mere presence of these hybrid states can enhance properties such as transport, magnetism, and superconductivity and modify (bio)chemical reactivity. This emerging field is highly multidisciplinary, and much of its potential has yet to be explored.

Near-Perfect Absorption of Light by Coherent Plasmon-Exciton States

We experimentally demonstrate and theoretically study the formation of coherent plasmon-exciton states which exhibit absorption of >90% of the incident light (at resonance) and cancellation of absorption. These coherent states result from the interaction between a material supporting an electronic excitation and a plasmonic structure capable of (near) perfect absorption of light. We illustrate the potential implications of these coherent states by measuring the charge separation attainable after photoexcitation. Our study opens the prospect for realizing devices that exploit coherent effects in applications.

Prospects of Coupled Organic-Inorganic Nanostructures for Charge and Energy Transfer Applications

We review the field of organic-inorganic nanocomposites with a focus on materials that exhibit a significant degree of electronic coupling across the hybrid interface. These nanocomposites undergo a variety of charge and energy transfer processes, enabling optoelectronic applications in devices which exploit singlet fission, triplet energy harvesting, photon upconversion or hot charge carrier transfer. We discuss the physical chemistry of the most common organic and inorganic components. Based on those we derive synthesis and assembly strategies and design criteria on material and device level with a focus on photovoltaics, spin memories or optical upconverters. We conclude that future research in the field should be directed towards an improved understanding of the binding motif and molecular orientation at the hybrid interface.

Theory of molecular emission power spectra. I. Macroscopic quantum electrodynamics formalism

We study the emission power spectrum of a molecular emitter with multiple vibrational modes in the framework of macroscopic quantum electrodynamics. The theory we present is general for a molecular spontaneous emission spectrum in the presence of arbitrary inhomogeneous, dispersive, and absorbing media. Moreover, the theory shows that the molecular emission power spectra can be decomposed into the electromagnetic environment factor and lineshape function. In order to demonstrate the validity of the theory, we investigate the lineshape function in two limits. In the incoherent limit (single molecules in a vacuum), the lineshape function exactly corresponds to the Franck-Condon principle. In the coherent limit (single molecules strongly coupled with single polaritons or photons) together with the condition of high vibrational frequency, the lineshape function exhibits a Rabi splitting, the spacing of which is exactly the same as the magnitude of exciton-photon coupling estimated by our previous theory [S. Wang et al., J. Chem. Phys. 151, 014105 (2019)]. Finally, we explore the influence of exciton-photon and electron-phonon interactions on the lineshape function of a single molecule in a cavity. The theory shows that the vibronic structure of the lineshape function does not always disappear as the exciton-photon coupling increases, and it is related to the loss of a dielectric environment.

Tailorable Dynamics in Nonlinear Optical Metasurfaces

Controlling light with light is essential for all-optical switching, data processing in optical communications and computing. Until now, all-optical control of light has relied almost exclusively on nonlinear optical interactions in materials. Achieving giant nonlinearities under low light intensity is essential for weak-light nonlinear optics. In the past decades, such weak-light nonlinear phenomena have been demonstrated in photorefractive and photochromic materials. However, their bulky size and slow speed have hindered practical applications. Metasurfaces, which enhance light-matter interactions at the nanoscale, provide a new framework with tailorable nonlinearities for weak-light nonlinear dynamics. Current advances in nonlinear metasurfaces are introduced, with a special emphasis on all-optical light controls. The tuning of the nonlinearity values using metasurfaces, including enhancement and sign reversal is presented. The tailoring of the transient behaviors of nonlinearities in metasurfaces to achieve femtosecond switching speed is also discussed. Furthermore, the impact of quantum effects from the metasurface on the nonlinearities is introduced. Finally, an outlook on the future development of this energetic field is offered.

Tackling the Scalability Challenge in Plasmonics by Wrinkle-Assisted Colloidal Self-Assembly

Electromagnetic radiation of a certain frequency can excite the collective oscillation of the free electrons in metallic nanostructures using localized surface plasmon resonances (LSPRs), and this phenomenon can be used for a variety of optical and electronic functionalities. However, nanostructure design over a large area using controlled LSPR features is challenging and requires high accuracy. In this article, we offer an overview of the efforts made by our group to implement a wrinkle-assisted colloidal particle assembly method to approach this challenge from a different angle. First, we introduce the controlled wrinkling process and discuss the underlying theoretical framework. We then set out how the wrinkled surfaces are utilized to guide the self-assembly of colloidal nanoparticles of various surface chemistry, size, and shape. Subsequently, template-assisted colloidal self-assembly mechanisms and a general guide for particle assembly beyond plasmonics will be presented. In addition, we also discuss the collective plasmonic behavior in depth, including strong plasmonic coupling due to nanoscale gap size as well as magnetic mode excitation and demonstrate the potential applications of wrinkle-assisted colloidal particle assembly method in the field of mechanoresponsive metasurfaces and surface-enhanced spectroscopy. Lastly, a general perspective in the field of template-assisted colloidal assembly with regard to potential applications in plasmonic photocatalysis, solar cells, optoelectronics, and sensing devices is provided.

Hybridized Guided-Mode Resonances via Colloidal Plasmonic Self-Assembled Grating

For many photonic applications, it is important to confine light of a specific wavelength at a certain volume of interest at low losses. So far, it is only possible to use the polarized light perpendicular to the solid grid lines to excite waveguide-plasmon polaritons in a waveguide-supported hybrid structure. In our work, we use a plasmonic grating fabricated by colloidal self-assembly and an ultrathin injection layer to guide the resonant modes selectively. We use gold nanoparticles self-assembled in a linear template on a titanium dioxide (TiO₂) layer to study the dispersion relation with conventional ultraviolet-visible-near-infrared spectroscopic methods. Supported with finite-difference in time-domain simulations, we identify the optical band gaps as hybridized modes: plasmonic and photonic resonances. Compared to metallic grids, the observation range of hybridized guided modes can now be extended to modes along the nanoparticle chain lines. With future applications in energy conversion and optical filters employing these cost-efficient and upscalable directed self-assembly methods, we discuss also the application in refractive index sensing of the particle-based hybridized guided modes.

Gap-plasmon enhanced water splitting with ultrathin hematite films: the role of plasmonic-based light trapping and hot electrons

Hydrogen is a promising alternative renewable fuel for meeting the growing energy demands of the world. Over the past few decades, photoelectrochemical water splitting has been widely studied as a viable technology for the production of hydrogen utilizing solar energy. A solar-to-hydrogen (STH) efficiency of 10% is considered to be sufficient for practical applications. Amongst the wide class of semiconductors that have been studied for their application in solar water splitting, iron oxide (α-Fe₂O₃), or hematite, is one of the more promising candidate materials, with a theoretical STH efficiency of 15%. In this work, we show experimentally that by utilizing gold nanostructures that support gap-plasmon resonances together with a hematite layer, we can increase the water oxidation photocurrent by two times over that demonstrated by a bare hematite film at wavelengths above the hematite bandgap. Moreover, we achieve a six-fold increase in the oxidation photocurrent at near-infrared wavelengths, which is attributed to hot electron generation and decay in the gap-plasmon nanostructures. Theoretical simulations confirmed that the metamaterial geometry with gap plasmons that was used allows us to confine electromagnetic fields inside the hematite semiconductor and to enhance the surface photochemistry.

Direct observation of ultrafast plasmonic hot electron transfer in the strong coupling regime

Achieving strong coupling between plasmonic oscillators can significantly modulate their intrinsic optical properties. Here, we report the direct observation of ultrafast plasmonic hot electron transfer from an Au grating array to an MoS₂ monolayer in the strong coupling regime between localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs). By means of femtosecond pump-probe spectroscopy, the measured hot electron transfer time is approximately 40 fs with a maximum external quantum yield of 1.65%. Our results suggest that strong coupling between LSPs and SPPs has synergetic effects on the generation of plasmonic hot carriers, where SPPs with a unique nonradiative feature can act as an 'energy recycle bin' to reuse the radiative energy of LSPs and contribute to hot carrier generation. Coherent energy exchange between plasmonic modes in the strong coupling regime can further enhance the vertical electric field and promote the transfer of hot electrons between the Au grating and the MoS₂ monolayer. Our proposed plasmonic strong coupling configuration overcomes the challenge associated with utilizing hot carriers and is instructive in terms of improving the performance of plasmonic opto-electronic devices.

Manipulation of the dephasing time by strong coupling between localized and propagating surface plasmon modes

Strong coupling between two resonance modes leads to the formation of new hybrid modes exhibiting disparate characteristics owing to the reversible exchange of information between different uncoupled modes. Here, we realize the strong coupling between the localized surface plasmon resonance and surface plasmon polariton Bloch wave using multilayer nanostructures. An anticrossing behavior with a splitting energy of 144 meV can be observed from the far-field spectra. More importantly, we investigate the near-field properties in both the frequency and time domains using photoemission electron microscopy. In the frequency domain, the near-field spectra visually demonstrate normal-mode splitting and display the extent of coupling. Importantly, the variation of the dephasing time of the hybrid modes against the detuning is observed directly in the time domain. These findings signify the evolution of the dissipation and the exchange of information in plasmonic strong coupling systems and pave the way to manipulate the dephasing time of plasmon modes, which can benefit many applications of plasmonics.

Enhanced water splitting under modal strong coupling conditions

Strong coupling between plasmons and optical modes, such as waveguide or resonator modes, gives rise to a splitting in the plasmon absorption band. As a result, two new hybrid modes are formed that exhibit near-field enhancement effects. These hybrid modes have been exploited to improve light absorption in a number of systems. Here we show that this modal strong coupling between a Fabry-Pérot nanocavity mode and a localized surface plasmon resonance (LSPR) facilitates water splitting reactions. We use a gold nanoparticle (Au-NP)/TiO₂/Au-film structure as a photoanode. This structure exhibits modal strong coupling between the Fabry-Pérot nanocavity modes of the TiO₂ thin film/Au film and LSPR of the Au NPs. Electronic excitation of the Au NPs is promoted by the optical hybrid modes across a broad range of wavelengths, followed by a hot electron transfer to TiO₂. A key feature of our structure is that the Au NPs are partially inlaid in the TiO₂ layer, which results in an enhancement of the coupling strength and water-oxidation efficiency. We observe an 11-fold increase in the incident photon-to-current conversion efficiency with respect to a photoanode structure with no Au film. Also, the internal quantum efficiency is enhanced 1.5 times under a strong coupling over that under uncoupled conditions.

Strong Exciton-Plasmon Coupling in Silver Nanowire Nanocavities

The interaction between plasmonic and excitonic systems and the formation of hybridized states is an area of intense interest due to the potential to create exotic light-matter states. We report herein coupling between the leaky surface plasmon polariton (SPP) modes of single Ag nanowires and excitons of a cyanine dye (TDBC) in an open nanocavity. Silver nanowires were spin-cast onto glass coverslips, and the wavevector of the leaky SPP mode was measured by back focal plane (BFP) microscopy. Performing these measurements at different wavelengths allows the generation of dispersion curves, which show avoided crossings after deposition of a concentrated TDBC-PVA film. The Rabi splitting frequencies (Ω) determined from the dispersion curves vary between nanowires, with a maximum value of Ω = 390 ± 80 meV. The experiments also show an increase in attenuation of the SPP mode in the avoided crossing region. The ability to measure attenuation for the hybrid exciton-SPP states is a powerful aspect of these single nanowire experiments because this quantity is not readily available from ensemble experiments.

Plasmonic metal-semiconductor photocatalysts and photoelectrochemical cells: a review

The incorporation of plasmonic metals into semiconductors is a promising route to improve the performance of photocatalysts and photoelectrochemical cells. This article summarizes the three major mechanisms of plasmonic energy transfer from a metal to a semiconductor, including light scattering/trapping, plasmon-induced resonance energy transfer (PIRET) and hot electron injection (also called direct electron transfer (DET)). It also discusses the rational design of plasmonic metal-semiconductor heterojunctions based on the underlying plasmonic energy transfer mechanisms. Moreover, this article highlights the applications of plasmonic photocatalysts and photoelectrochemical cells in solar water splitting, carbon dioxide reduction and environmental pollutant decomposition.

Optics of exciton-plasmon nanomaterials

This review provides a brief introduction to the physics of coupled exciton-plasmon systems, the theoretical description and experimental manifestation of such phenomena, followed by an account of the state-of-the-art methodology for the numerical simulations of such phenomena and supplemented by a number of FORTRAN codes, by which the interested reader can introduce himself/herself to the practice of such simulations. Applications to CW light scattering as well as transient response and relaxation are described. Particular attention is given to so-called strong coupling limit, where the hybrid exciton-plasmon nature of the system response is strongly expressed. While traditional descriptions of such phenomena usually rely on analysis of the electromagnetic response of inhomogeneous dielectric environments that individually support plasmon and exciton excitations, here we explore also the consequences of a more detailed description of the molecular environment in terms of its quantum density matrix (applied in a mean field approximation level). Such a description makes it possible to account for characteristics that cannot be described by the dielectric response model: the effects of dephasing on the molecular response on one hand, and nonlinear response on the other. It also highlights the still missing important ingredients in the numerical approach, in particular its limitation to a classical description of the radiation field and its reliance on a mean field description of the many-body molecular system. We end our review with an outlook to the near future, where these limitations will be addressed and new novel applications of the numerical approach will be pursued.

Multimode photon-exciton coupling in an organic-dye-attached photonic quasicrystal

In this Letter, we present hybrid strong coupling between multiple photonic modes and excitons in an organic-dye-attached photonic quasicrystal. The excitons effectively interact with the photonic modes offered by the photonic quasicrystal, and multiple hybrid polariton bands are demonstrated in both experiments and calculations. Furthermore, by retrieving the measured dispersion map, we get the mixing fractions of photonic modes and excitons, and show that the polariton bands inherit not only the energy dispersion features, but also the damping behaviors from both the photonic modes and the excitons. Our investigation may inspire related studies on multimode light-matter interactions and achieve some potential applications for multimode sensors.

Hybrid Light-Matter States in a Molecular and Material Science Perspective

The notion that light and matter states can be hybridized the way s and p orbitals are mixed is a concept that is not familiar to most chemists and material scientists. Yet it has much potential for molecular and material sciences that is just beginning to be explored. For instance, it has already been demonstrated that the rate and yield of chemical reactions can be modified and that the conductivity of organic semiconductors and nonradiative energy transfer can be enhanced through the hybridization of electronic transitions. The hybridization is not limited to electronic transitions; it can be applied for instance to vibrational transitions to selectively perturb a given bond, opening new possibilities to change the chemical reactivity landscape and to use it as a tool in (bio)molecular science and spectroscopy. Such results are not only the consequence of the new eigenstates and energies generated by the hybridization. The hybrid light-matter states also have unusual properties: they can be delocalized over a very large number of molecules (up to ca. 10⁵), and they become dispersive or momentum-sensitive. Importantly, the hybridization occurs even in the absence of light because it is the zero-point energies of the molecular and optical transitions that generate the new light-matter states. The present work is not a review but rather an Account from the author's point of view that first introduces the reader to the underlying concepts and details of the features of hybrid light-matter states. It is shown that light-matter hybridization is quite easy to achieve: all that is needed is to place molecules or a material in a resonant optical cavity (e.g., between two parallel mirrors) under the right conditions. For vibrational strong coupling, microfluidic IR cells can be used to study the consequences for chemistry in the liquid phase. Examples of modified properties are given to demonstrate the full potential for the molecular and material sciences. Finally an outlook of future directions for this emerging subject is given.