Hybrid Light-matter States Research Articles

Cooperative Conical Intersection Dynamics of Two Pyrazine Molecules in an Optical Cavity.

Hybrid light-matter states in optical cavities, known as polaritons, offer a novel means of manipulating and controlling photochemical processes. We investigate the cooperative cavity photochemistry of two pyrazine molecules undergoing conical intersection dynamics and interacting with a single cavity photon mode by exact quantum dynamics. When the cavity mode is coupled to the electronic transition between the ground and excited states, we find an enhanced polaritonic splitting and collective dark states. These features dominate the cooperative polariton dynamics and can be observed in the transient absorption spectrum.

Near-Field Spectroscopy of Cylindrical Phonon-Polariton Antennas.

Surface phonon polaritons (SPhPs) are hybrid light-matter states in which light strongly couples to lattice vibrations inside the Reststrahlen band of polar dielectrics at mid-infrared frequencies. Antennas supporting localized surface phonon polaritons (LSPhPs) easily outperform their plasmonic counterparts operating in the visible or near-infrared in terms of field enhancement and confinement thanks to the inherently slower phonon-phonon scattering processes governing SPhP decay. In particular, LSPhP antennas have attracted considerable interest for thermal management at the nanoscale, where the emission strongly diverts from the usual far-field blackbody radiation due to the presence of evanescent waves at the surface. However, far-field measurements cannot shed light on the behavior of antennas in the near-field region. To overcome this limitation, we employ scattering-scanning near-field optical microscopy (sSNOM) to unveil the spectral near-field response of 3C-SiC antenna arrays. We present a detailed description of the behavior of the antenna resonances by comparing far-field and near-field spectra and demonstrate the existence of a mode with no net dipole moment, absent in the far-field spectra, but of importance for applications that exploit the heightened electromagnetic near fields. Furthermore, we investigate the perturbation in the antenna response induced by the presence of the AFM tip, which can be further extended toward situations where for example strong IR emitters couple to LSPhP modes.

Electrical Control of Hybrid Monolayer Tungsten Disulfide-Plasmonic Nanoantenna Light-Matter States at Cryogenic and Room Temperatures.

Hybrid light-matter states-polaritons-have attracted considerable scientific interest recently, motivated by their potential for development of nonlinear and quantum optical schemes. To realize such states, monolayer transition metal dichalcogenides (TMDCs) have been widely employed as excitonic materials. In addition to neutral excitons, TMDCs host charged excitons, which enables active tuning of hybrid light-matter states by electrical means. Although several reports demonstrated charged exciton-polaritons in various systems, the full-range interaction control attainable at room temperature has not been realized. Here, we demonstrate electrically tunable charged exciton-plasmon polaritons in a hybrid tungsten disulfide (WS2) monolayer-plasmonic nanoantenna system. We show that electrical gating of monolayer WS2 allows tuning the oscillator strengths of neutral and charged excitons not only at cryogenic but also at room temperature, both at vacuum and atmospheric pressure. Such electrical control enables a full-range tunable switching from strong neutral exciton-plasmon coupling to strong charged exciton-plasmon coupling. Our experimental findings allow discussing beneficial and limiting factors of charged exciton-plasmon polaritons, as well as offer routes toward realization of charged polaritonic devices at ambient conditions.

On the SN2 reactions modified in vibrational strong coupling experiments: reaction mechanisms and vibrational mode assignments.

Recent experiments have reported modified chemical reactivity under vibrational strong coupling (VSC) in microfluidic Fabry-Pérot cavities. In particular, the reaction rate of nucleophilic substitution reactions at silicon centers (SN2@Si) has been altered when a vibrational mode of the reactant was coupled to a confined light mode in the strong coupling regime. In this situation, hybrid light-matter states known as polaritons are formed and seem to be responsible for the modified chemical kinetics. These results are very encouraging for future applications of polaritonic chemistry to catalyze chemical reactions, with the ability to manipulate chemical phenomena without any external excitation of the system. Still, there is no theory capable of explaining the mechanism behind these results. In this work we address two points that are crucial for the interpretation of these experiments. Firstly, by means of electronic structure calculations we report the reaction mechanism in normal conditions of the two recently modified SN2@Si reactions, obtaining in both cases a triple-well PES where the rate-determining step is due to the Si-C and Si-O bond cleavage. Secondly, we characterize in detail the normal modes of vibration of the reactants. In the VSC experiments, reaction rates were modified only when specific vibrations of the reactants were coupled to a cavity mode. We find that these vibrations are highly mixed among the different fragments of the reactants leading to a completely new assignment of the IR peaks coupled to cavity modes in the original experimental works. Our results are fundamental for the interpretation of the VSC experiments given that in the absence of a theory explaining these results, the current phenomenological understanding relies on the assignment of the character of the vibrational IR peaks.

Inverting singlet and triplet excited states using strong light-matter coupling.

In organic microcavities, hybrid light-matter states can form with energies that differ from the bare molecular excitation energies by nearly 1 eV. A timely question, given the recent advances in the development of thermally activated delayed fluorescence materials, is whether strong light-matter coupling can be used to invert the ordering of singlet and triplet states and, in addition, enhance reverse intersystem crossing (RISC) rates. Here, we demonstrate a complete inversion of the singlet lower polariton and triplet excited states. We also unambiguously measure the RISC rate in strongly coupled organic microcavities and find that, regardless of the large energy level shifts, it is unchanged compared to films of the bare molecules. This observation is a consequence of slow RISC to the lower polariton due to the delocalized nature of the state across many molecules and an inability to compete with RISC to the dark exciton reservoir.

Non-adiabatic molecular dynamics of molecules in the presence of strong light-matter interactions.

When the interaction between a molecular system and confined light modes in an optical or plasmonic cavity is strong enough to overcome the dissipative process, hybrid light-matter states (polaritons) become the fundamental excitations in the system. The mixing between the light and matter characters modifies the photophysical and photochemical properties. Notably, it was reported that these polaritons can be employed to control photochemical reactions, charge and energy transfer, and other processes. In addition, according to recent studies, vibrational strong coupling can be employed to resonantly enhance the thermally-activated chemical reactions. In this work, a theoretical model and an efficient numerical method for studying the dynamics of molecules strongly interacting with quantum light are developed based on nonadiabatic excited-state molecular dynamics. The methodology was employed to study the cis-trans photoisomerization of a realistic molecule in a cavity. Numerical simulations demonstrate that the photochemical reactions can be controlled by tuning the properties of the cavity. In the calculated example, the isomerization is suppressed when polaritonic states develop a local minimum on the lower polaritonic state. Moreover, the observed reduction of isomerization is tunable via the photon energy and light-molecule coupling strength. However, the fluctuation in the transition dipole screens the effect of light-matter, which makes it harder to tune the photochemical properties via the coupling strength. These insights suggest quantum control of photochemical reactions is possible by specially designed photonic or plasmonic cavities.

Tracking Polariton Relaxation with Multiscale Molecular Dynamics Simulations.

When photoactive molecules interact strongly with confined light modes in optical cavities, new hybrid light–matter states form. They are known as polaritons and correspond to coherent superpositions of excitations of the molecules and of the cavity photon. The polariton energies and thus potential energy surfaces are changed with respect to the bare molecules, such that polariton formation is considered a promising paradigm for controlling photochemical reactions. To effectively manipulate photochemistry with confined light, the molecules need to remain in the polaritonic state long enough for the reaction on the modified potential energy surface to take place. To understand what determines this lifetime, we have performed atomistic molecular dynamics simulations of room-temperature ensembles of rhodamine chromophores strongly coupled to a single confined light mode with a 15 fs lifetime. We investigated three popular experimental scenarios and followed the relaxation after optically pumping (i) the lower polariton, (ii) the upper polariton, or (iii) uncoupled molecular states. The results of the simulations suggest that the lifetimes of the optically accessible lower and upper polaritons are limited by (i) ultrafast photoemission due to the low cavity lifetime and (ii) reversible population transfer into the “dark” state manifold. Dark states are superpositions of molecular excitations but with much smaller contributions from the cavity photon, decreasing their emission rates and hence increasing their lifetimes. We find that population transfer between polaritonic modes and dark states is determined by the overlap between the polaritonic and molecular absorption spectra. Importantly, excitation can also be transferred ”upward” from the lower polariton into the dark-state reservoir due to the broad absorption spectra of the chromophores, contrary to the common conception of these processes as a ”one-way” relaxation from the dark states down to the lower polariton. Our results thus suggest that polaritonic chemistry relying on modified dynamics taking place within the lower polariton manifold requires cavities with sufficiently long lifetimes and, at the same time, strong light–matter coupling strengths to prevent the back-transfer of excitation into the dark states.

Spectral and Spatial Characteristics of the Electromagnetic Modes in a Tunable Optical Microcavity Cell for Studying Hybrid Light–Matter States

Studies of resonance interaction between matter and localized electromagnetic field in a cavity have recently attracted much interest because they offer the possibility of controllably modifying some of the fundamental material properties. However, despite the large number of such studies, these is no universal approach that would allow investigation of sets of different samples with wide variation of the main experimental parameters of the optical modes. In this work, the main optical parameters of a previously developed universal tunable microcavity cell, i.e., the Q factor and mode volume, as well as their dependence on the characteristics of cavity mirrors and spacing between them, are analyzed. The results obtained will significantly expand the scope of applications of resonance interaction between light and matter, including such effects as the enhancement of Raman scattering, long-range resonance nonradiative energy transfer, and modification of chemical reaction rates.

Turning the challenge of quantum biology on its head: biological control of quantum optical systems.

When light-harvesting complex II (LHCII), isolated from spinach, is adsorbed onto arrays of gold nanostructures formed by interferometric lithography, a pronounced splitting of the plasmon band is observed that is attributable to strong coupling of the localised surface plasmon resonance to excitons in the pigment-protein complex. The system is modelled as coupled harmonic oscillators, yielding an exciton energy of 2.24 ± 0.02 eV. Analysis of dispersion curves yields a Rabi energy of 0.25 eV. Extinction spectra of the strongly coupled system yield a resonance at 1.43 eV that varies as a function of the density of nanostructures in the array. The enhanced intensity of this feature is attributed to strong plasmon-exciton coupling. Comparison of data for a large number of light-harvesting complexes indicates that by control of the protein structure and/or pigment compliment it is possible to manipulate the strength of plasmon-exciton coupling. In strongly coupled systems, ultra-fast exchange of energy occurs between pigment molecules: coherent coupling between non-local excitons can be manipulated via selection of the protein structure enabling the observation of transitions that are not seen in the weak coupling regime. Synthetic biology thus provides a means to control quantum-optical interactions in the strong coupling regime.

Nonlinear Emission of Molecular Ensembles Strongly Coupled to Plasmonic Lattices with Structural Imperfections.

We demonstrate nonlinear emission from molecular layers strongly coupled to extended light fields in arrays of plasmonic nanoparticles in the presence of structural imperfections. Hybrid light-matter states, known as plasmon-exciton polaritons (PEPs), are formed by the strong coupling of Frenkel excitons in molecules to surface lattice resonances. These resonances result from the radiative coupling of localized surface plasmon polaritons in silver nanoparticles enhanced by diffraction on the array. By designing arrays with different lattice constants, we show that the nonlinear emission frequency is solely determined by the relaxation of exciton polaritons through vibrational quanta in the molecules. We also observe long-range spatial coherence in the samples, which supports the explanation in terms of a nonlinear collective emission of strongly coupled PEPs. In contrast to recent observations of exciton-polariton lasing and condensation in organic systems, photonic modes play a minor role at the emission frequency in our system, and this emission has an undefined momentum because of the structural imperfections. This remarkable result reveals the rich and distinct physics of strongly coupled organic molecules to photonic cavities.

Molecular Emission near Metal Interfaces: The Polaritonic Regime.

The strong coupling of a dense layer of molecular excitons with surface-plasmon modes in a metal gives rise to polaritons (hybrid light-matter states) called plexcitons. Surface plasmons cannot directly emit into (or be excited by) free-space photons due to the fact that energy and momentum conservation cannot be simultaneously satisfied in photoluminescence. Most plexcitons are also formally nonemissive, even though they can radiate via molecules upon localization due to disorder and decoherence. However, a fraction of them are bright even in the presence of such deleterious processes. In this Letter, we theoretically discuss the superradiant emission properties of these bright plexcitons, which belong to the upper energy branch and reveal huge photoluminescence enhancements compared to bare excitons, due to near-divergences in the density of photonic modes available to them. Our study generalizes the well-known problem of molecular emission next to a metal interface to the polaritonic regime.

Angle-Independent Polariton Emission Lifetime Shown by Perylene Hybridized to the Vacuum Field Inside a Fabry–Pérot Cavity

The formation of hybrid light–matter states in optical structures, manifested as a Rabi splitting of the eigenenergies of a coupled system, is one of the key effects in quantum optics. The hybrid states (exciton polaritons) have unique chemical and physical properties and can be viewed as a linear combination of light and matter. The optical properties of the exciton polaritons are dispersive by nature, a property inherited from the photonic contribution to the polariton. On the other hand, the polariton lifetime in organic molecular systems has recently been highly debated. The photonic contribution to the polariton would suggest a lifetime on the femtosecond time scale, much shorter than experimentally observed. Here, we increase the insights of light–mater states by showing that the polariton emission lifetime is nondispersive. A perylene derivative was strongly coupled to the vacuum field by incorporating the molecule into a Fabry–Pérot cavity. The polariton emission from the cavity was shown to be dispersive, but the emission lifetime was nondispersive and on the time scale of the bare exciton. The results were rationalized by the exciton reservoir model, giving experimental evidence to currently used theories, thus improving our understanding of strong coupling phenomena in molecules.

Strong Light–Matter Coupling as a New Tool for Molecular and Material Engineering: Quantum Approach

AbstractWhen atoms come together and bond, these new states are called molecules and their properties determine many aspects of our daily life. Strangely enough, it is conceivable for light and molecules to bond, creating new hybrid light–matter states with far‐reaching consequences for thesestrongly coupledmaterials. Even stranger, there is no “real” light needed to obtain the effects; it simply appears from the vacuum, creating “something from nothing.” Surprisingly, the setup required to create these materials has become moderately straightforward. In its simplest form, one only needs to put a strongly absorbing material at the appropriate place between two mirrors, and quantum magic can appear. Only recently has it been discovered that strong coupling can affect a host of significant effects at a material and molecular level, which were thought to be independent of the “light” environment: phase transitions, conductivity, chemical reactions, etc. This review addresses the fundamentals of this opportunity: the quantum mechanical foundations, the relevant plasmonic and photonic structures, and a description of the various applications, connecting material chemistry with quantum information, entanglement, nonlinear optics, and chemical reactivity. Ultimately, revealing the interplay between light and matter in this new regime opens attractive avenues for various new technologies.

Coherent Light Harvesting through Strong Coupling to Confined Light.

When photoactive molecules interact strongly with confined light modes, new hybrid light–matter states may form: the polaritons. These polaritons are coherent superpositions of excitations of the molecules and of the cavity photon. Recently, polaritons were shown to mediate energy transfer between chromophores at distances beyond the Förster limit. Here we explore the potential of strong coupling for light-harvesting applications by means of atomistic molecular dynamics simulations of mixtures of photoreactive and non-photo-reactive molecules strongly coupled to a single confined light mode. These molecules are spatially separated and present at different concentrations. Our simulations suggest that while the excitation is initially fully delocalized over all molecules and the confined light mode, it very rapidly localizes onto one of the photoreactive molecules, which then undergoes the reaction.

Selective manipulation of electronically excited states through strong light\u2013matter interactions

Strong coupling between light and matter leads to the spontaneous formation of hybrid light–matter states, having different energies than the uncoupled states. This opens up for new ways of modifying the energy landscape of molecules without changing their atoms or structure. Heavy metal-free organic light emitting diodes (OLED) use reversed intersystem crossing (RISC) to harvest light from excited triplet states. This is a slow process, thus increasing the rate of RISC could potentially enhance OLED performance. Here we demonstrate selective coupling of the excited singlet state of Erythrosine B without perturbing the energy level of a nearby triplet state. The coupling reduces the triplet–singlet energy gap, leading to a four-time enhancement of the triplet decay rate, most likely due to an enhanced rate of RISC. Furthermore, we anticipate that strong coupling can be used to create energy-inverted molecular systems having a singlet ground and lowest excited state.

Organic Photodiodes with an Extended Responsivity Using Ultrastrong Light–Matter Coupling

In organic photodiodes (OPDs) light is absorbed by excitons, which dissociate to generate photocurrent. Here, we demonstrate a novel type of OPD in which light is absorbed by polaritons, hybrid light-matter states. We demonstrate polariton OPDs operating in the ultra-strong coupling regime at visible and infrared wavelengths. These devices can be engineered to show narrow responsivity with a very weak angle-dependence. More importantly, they can be tuned to operate in a spectral range outside that of the bare exciton absorption. Remarkably, we show that the responsivity of a polariton OPD can be pushed to near infrared wavelengths, where few organic absorbers are available, with external quantum efficiencies exceeding those of a control OPD.

A synthetic biological quantum optical system.

In strong plasmon-exciton coupling, a surface plasmon mode is coupled to an array of localized emitters to yield new hybrid light-matter states (plexcitons), whose properties may in principle be controlled via modification of the arrangement of emitters. We show that plasmon modes are strongly coupled to synthetic light-harvesting maquette proteins, and that the coupling can be controlled via alteration of the protein structure. For maquettes with a single chlorin binding site, the exciton energy (2.06 ± 0.07 eV) is close to the expected energy of the Qy transition. However, for maquettes containing two chlorin binding sites that are collinear in the field direction, an exciton energy of 2.20 ± 0.01 eV is obtained, intermediate between the energies of the Qx and Qy transitions of the chlorin. This observation is attributed to strong coupling of the LSPR to an H-dimer state not observed under weak coupling.

Multiscale Molecular Dynamics Simulations of Polaritonic Chemistry.

When photoactive molecules interact strongly with confined light modes as found in plasmonic structures or optical cavities, new hybrid light-matter states can form, the so-called polaritons. These polaritons are coherent superpositions (in the quantum mechanical sense) of excitations of the molecules and of the cavity photon or surface plasmon. Recent experimental and theoretical works suggest that access to these polaritons in cavities could provide a totally new and attractive paradigm for controlling chemical reactions that falls in between traditional chemical catalysis and coherent laser control. However, designing cavity parameters to control chemistry requires a theoretical model with which the effect of the light-matter coupling on the molecular dynamics can be predicted accurately. Here we present a multiscale quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulation model for photoactive molecules that are strongly coupled to confined light in optical cavities or surface plasmons. Using this model we have performed simulations with up to 1600 Rhodamine molecules in a cavity. The results of these simulations reveal that the contributions of the molecules to the polariton are time-dependent due to thermal fluctuations that break symmetry. Furthermore, the simulations suggest that in addition to the cavity quality factor, also the Stokes shift and number of molecules control the lifetime of the polariton. Because large numbers of molecules interacting with confined light can now be simulated in atomic detail, we anticipate that our method will lead to a better understanding of the effects of strong coupling on chemical reactivity. Ultimately the method may even be used to systematically design cavities to control photochemistry.

High-Efficiency Second-Harmonic Generation from Hybrid Light-Matter States.

We report a novel approach to modify the second order nonlinear optical (NLO) susceptibility of organic nanofiber crystals by hybridization with the optical modes of microcavities in the strong coupling regime. The wavelength dependence of the SHG efficiency displays two intense peaks corresponding to the so-formed light-matter hybrid states. Our results demonstrate an enhancement of the resonant SHG efficiency of the lower polariton by 2 orders of magnitude for the collectively coupled molecules as compared to that of the same material outside the microcavity. This study is a proof of principle that opens a new direction for NLO of organic materials in subwavelength resonators.

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.