Local vs Collective Interactions in Polaritonic Chemistry
Polaritonic Chemistry - a novel Scientific Challenge
The field of « polaritonic chemistry » is an emerging field of research that opens new ways to control the properties and dynamics of molecules and materials by engineering their sourrounding electromagnetic vacuum by micro- and nano-cavities . A striking experimental fact is that, even in absence of any external driving source, the electromagnetic cavity modes can couple to a molecular transition (electronic or vibrational) and strongly influence the matter subsystem. These changes are commonly accompanied by the formation of hybrid light-matter excitations called polaritons . In this regime of strong light-matter coupling, materials get dressed by the cavity modes and thus can partially inherit properties of the confined light field . This phenomenon has an impact on both optical and chemical properties [2,3] of materials and molecules.
Pionneering experiments reported a wealth of different properties that are altered significantly in optical cavities, for instance, the slowing-down of a photochromic chemical reaction by electronic strong coupling , a change in the kinetics and thermodynamics of a ground-state deprotection reaction due to vibrational strong coupling , the modification of electronic conductivity of an organic material coupled to surface plasmons , the increase by one order of magnitude of the rate of energy transfer in a donor-acceptor bridge , and more recently, a modification of the superconducting gap of a material upon coupling to surface plasmon polaritons . Importantly, many of these effects have been observed at standard ambient conditions and are hence technologically very promising.
This paves the way to design a new generation of materials in confined electromagnetic environments, and taylor their physico-chemical properties at the nanoscale through strong light-matter coupling.
In parallel to this flourishing experimental activities, a wealth of theoretical works has appeared that try to describe these cavity-induced modifications. A common theoretical approach is to use phenomenological models such as the Holstein-Tavis-Cummings model . These models aim at explaining the observed changes of, e.g., the chemical reactivity, by describing how collective strong-coupling of molecules to the cavity-mode modifies the potential-energy surface for the chemical reactions , and thus the corresponding reaction activation energy. The concept of collective polariton potential energy surfaces has emerged as a cornerstone of various theories explaning the strong change in the kinetics of photoreactions , and electron-transfer reactions . Recently, alternative theoretical methods have emerged that aim at explaining the observed experimental results by an ab-initio solution of the fundamental Hamiltonian of Quantum Electrodynamics (QED) and which have created the new field of ab-initio QED . These approaches enable to describe accurately and quantitatively both the electronic and vibrational structure of the molecules at the microscopic level, the coupling to the electromagnetic cavity mode and damping by the electromagnetic fluctuations but are restricted in ensemble size [14,15].
Some recent disagreements and controversial issues were reported [3,16,17] between theories and experiments addressing, for instance, the role of vibrational strong coupling in the modification of ground-state chemical reactivity observed in various experiments [5,18]. While the experiments performed in polaritonic chemistry opened a completely new field of research and drove the interest of the scientific community, it appears that there is, on the experimental side, a strong need to perform further experiments (e.g. issue of reproducibility ), and on the theoretical side, to develop a general picture and qualitative understanding of the physical mechanism responsible for cavity-modified chemistry. Both of these aspects are needed to advance the field of polaritonic chemistry, and to explore new ways of engineering material properties in confined electromagnetic environments.
Issues adressed in this Workshop
This worshop aims at clarifying and scrutinizing the different theoretical and experimental scenarios and methods for investigating the light-matter strong-coupling mechanism in polaritonic chemistry. We structure the worskhop following three main topics of the field, for which open questions are formulated :
i) Day 1. “Collective vs Local” Light-Matter Strong-Coupling Mechanisms
i a.) The actual mechanism responsible for changes in chemical reactivity due to strong-coupling in electromagnetic microcavities is still unclear. On one hand, a collective interaction mechanism is postulated, by which molecular populations would interact in phase with the same electromagnetic cavity mode (Dicke-like mechanism), while on the other hand, recent ab-initio QED calculations support the picture of a local strong-coupling scenario, by which single molecules couple strongly to the local field created by the interaction between other molecules and the cavity mode . In other words, the following fundamental question is formulated and will be actively debated :
“How to reconcile the description of polaritons as collective and spatially extended light-matter excitation, with the locality implied by chemical reactions (bond-breaking in a specific molecule) ?”
i b.) A particular focus should be on understanding the role played by phonon symmetries when entering the vibrational strong-coupling regime . A perspective should be given between experiments and various new theoretical scenarios assigning the modification of chemical reactivity to a cavity-mediated spectral redistributing of the vibrational modes’ density of states orthogonal to the reaction coordinate . “Is the simple resonance condition between one vibrational mode and the cavity frequencies sufficient to describe the experiments, or is there a need to develop some more complete description of correlated nuclear motion of reacting molecules ?”
ii) Day 2. “Classical vs Quantum” Regimes of Light-Matter Strong Coupling
ii a.) A focus should be on the quantum nature of the collective polaritonic state in a cavity, and its corresponding role on chemistry. In particular, a question arises about the robustness of this polaritonic state to disorder, decoherence and dissipation by the (electromagnetic and solvent) environment. Many open questions naturally arise, like what is the role of non-adiabatic effects, taking into account the frequency dependence of the cavity-mode seen as a non-Markovian environment (with memory, or retardation) ? What is the role of non-equilibrium effects versus thermal ones ?
“Is there a way to build a consistent theoretical model of the thermodynamics of those states and of the chemical reaction” (the role of dark states in the entropic contribution to the reaction ) ?
ii b.) Fundamental questions should be formulated on what physical observable could be measured experimentally to unveil or not the « quantum » vs the « classical » nature of the light-matter interaction mechanism ? For that purpose, isn’t there a necessity to look for experimental quantities like the thermal noise power spectra of the cavity-mode, the autocorrelation function of the cavity-mode field or the second-order correlation function of emitted light out of the cavity  ?
“Can we look for and characterize experimentally alternative scenarios assigning the surprising robustness of the polaritonic states to the classical picture of synchronization of molecular oscillators to the main cavity-mode ? “
iii) Day 3. The Role and Breaking of Symmetries, new Directions in Light-Matter Strong Coupling
This last day is mostly an open one, and should facilitate a lively discussion towards a future evolution of the field. It deals with the idea of looking for the consequences of breaking symmetries in polaritonic chemistry. Typical examples are breaking of time-reversal symmetry (driven cavity and Floquet engineering), breaking parity (chiral molecules and chiral Fabry-Perot cavities) [23,24,25] with the idea of looking towards enhanced enantioselectivity of chemical reactions in specifically designed microcavities. In this context also the prospect of ab-initio QED simulations as a new tool to investigate strong light-matter coupling scenarios should be discussed.
How to address these issues
In order to address these various important issues of the field of polaritonic chemistry and molecular polaritons we ask the individual speakers as well as the contributors of posters to either put their results/presentations in relation to the specific topic of the day (or the other topics highlighted above) or comment on some of these topics. In this way we hope to collect various perspectives and ideas on the above fundmental topics, which subsequently can build the basis of the discussion sessions.
We hope that viewing these issues from different (experimental and theoretical) perspectives and the ensuing lively discussion can provide guidance on how we can gain a detailed understanding of the basic principles of cavity-modified chemistry and material sciences.
We are aware that the current Corona situtation evolves dynamically and we will monitor the developments closely. Nevertheless we are confident that the workshop will be an in-person meeting. If the circumenstances will not allow this, we will switch to an adapted online version of the workshop.
Further note the Corona-related restrictions that are in effect in France currently
While we expect that the rules will become less stringent in summer, a proof of full vaccination will most likely be required. We try to update you in time about the restrictions that apply at the time of the workshop.
Rémi Avriller (LOMA, CNRS and University of Bordeaux) - Organiser
Michael Ruggenthaler (Max-Planck Institute for the Structure and Dynamics of Matter) - Organiser