Polaritonic Chemistry - Coherent Quantum Control of Molecules with Modified Vacuum Fields

4 May 2026, 08:15 → 6 May 2026, 17:30 Europe/Stockholm

Albano 3: 4205 - SU Conference Room (40 seats) (Albano Building 3)

Gerrit Groenhof (University of Jyvaskyla), Markus Kowalewski (Stockholm University), Thomas Schnappinger (Stockholm University)

Please be aware that scammers sometimes approach participants claiming to be able to provide accommodation and asking for credit card details. Do not give this information to them! If you are in any doubt about the legitimacy of an approach, please get in contact with the organizers.

Venue

Nordita, Stockholm, Sweden

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Scope

Coherent control of molecular dynamics by dressing potential energy surfaces with light has an enormous potential for photochemical applications in light harvesting, energy storage, opto-electronic and communication. In recent years this concept has been extended to modified vacuum fields as they are created by photonic structures that can confine specific light modes, such as optical cavities or nano-plasmonic structures. The increased interaction between those confined light modes and the molecules can lead to the formation of new hybrid light-matter states called polaritons. These polaritons are coherent superpositions of excitations of the molecules and of the light modes. The hybridization between light and matter not only delocalizes the excitation over many molecules but also changes their potential energy surfaces, and thus could be a new way to control chemistry, change optical material properties, and control energy transfer. Manipulating molecules with cavities requires a complete understanding of how the interaction with confined light affects the molecular dynamics. Thus, to unlock the full potential of manipulating chemistry with photonic structures, we urgently need a unified theoretical description of the strongly coupled light matter interaction that is sufficiently accurate to systematically design optical nano-structures for selectively altering reactivity. The new theoretical description needs to go significantly beyond the state of the art, as it needs to take into account the quantum nature of light. Therefore, the integration of the new theoretical framework into existing electronic structure and molecular dynamics methodologies will be a tremendous challenge and requires an interdisciplinary approach. With the workshop, we intend to overcome this challenge by bringing together researchers who are at the forefront of this field, either in theory or experiment. Ultimately, we expect to not only further our insights into strong light-matter coupling and its applications, but also to establish the new state of the art in understanding and predicting the effects of a modified vacuum on chemical reactivity.

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Themes and preliminary program schedule

We are planning a full 3 day program from May 4th to May 6th We thus recommend planning arrival on May 3rd and departure on May 7th.

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Invited speakers

Henrik Koch

Karl Borjesson

Timur Shegai

Mark Kamper Svendsen

Antonio Fernández Domínguez

Tönu Pullerits

Anton Zasedatelev

Tao Li

Johannes Feist

Norah Hoffmann

Anael Ben-Asher

Oriol Vendrell

Agnes Vibok

Marit Fiechter

Tal Schwartz

Remi Avriller

Dominik Sidler

Blake Simpkins

Stefano Corni

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Accommodation

Nordita has reserved a block rooms at the following hotel:

Elite Hotel Arcadia | 20 Superior double room for single use - For details and reserving a room, click here. Last day to book your room is 4th of April. All reservations, date adjustments and/or cancellations are to be made by the interested participant directly with the hotels.

Please be aware that scammers sometimes approach participants claiming to be able to provide accommodation and asking for credit card details. Do not give this information to them! If you are in any doubt about the legitimacy of an approach, please get in contact with the organizers.

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Application/Registration

Registration to be considered for on-site participation will close April 19th. Registrants will receive an on-site/remote participation confirmation from the organizers after this date.

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Sponsored by:

Monday 4 May

08:15 Registration and Coffee

Monday Morning

1 Welcome and Introduction

2 The Many Faces of Strong Light–Matter Interactions: From Non-Thermal Aging to Quantum Twin Polaritons

Strong light–matter interactions span a wide range of questions, from the nature of the cavity itself (e.g., microcavities versus plasmonic cavities), to local versus collective coupling effects, and the distinction between classical and quantum behavior, to name just a few. In this talk, I will focus on two somewhat complementary perspectives that highlight these different facets.

First, I will present a classical viewpoint and show how cavities can be used to control aging in disordered materials. Aging is a defining feature of disordered materials such as glasses, plastics, and pharmaceuticals, often limiting long-term stability and performance. Whereas aging is typically controlled through global parameters such as temperature or pressure, here we show it can instead be tuned selectively using light. Specifically, when a supercooled liquid is coupled to an optical cavity, the system undergoes what we call non-thermal aging: aging induced by light without any change in temperature. Importantly, the cavity-coupled liquid behaves as if it were structurally colder than its surroundings, enabling a cavity cooling protocol.

Second, I will turn to a more quantum perspective and discuss the quantum–classical divide in polaritonic systems. Here we introduce a new feature, the twin polariton: an additional splitting beyond the primary polariton resonance that originates from vacuum field fluctuations. This feature can persist in the many-molecule limit under symmetric initial conditions, but most importantly follows the same linear dependence on coupling strength as the primary splitting. This establishes a novel mechanism by which a quantum feature (the twin polariton) can be tuned through a classical one (the primary polariton), offering new opportunities to probe and control the fundamental nature of polaritonic systems.

Speaker: Norah Hoffmann (New York University)

3 Quantum dynamics and electronic structure of polaritonic Hamiltonians with MCTDH: things we learned so far

Recent results on ensembles of the rotational-vibrational-electronic Shin-Metiu
model using variational tree-tensor network quantum dynamics will be discussed
first. This model captures rovibronic couplings exactly and beyond the
Born-Oppenheimer approximation, within the Pauli-Fierz Hamiltonian. Inspecting
the fully converged ground state for a range of coupling strengths we find out
that the cavity induces local modifications in individual molecules. However,
the extent of these modifications depends only on the per-molecule coupling
strength up until each molecule reaches the ultrastrong coupling regime.
Specific ansatze for the wavefunction let us investigate the
cavity-Born-Oppenheimer approximation as a limiting case.
Next, the application of MCTDH to excitonic-polaritonic and photo-chemical
problems will be shortly illustrated as well. MCTDH emerges as a robust and
flexible tool to study certain types of polaritonic Hamiltonians, delivering
benchmarks for approximate numerical methods and analytical results alike.

[1] Krupp, N.; Huber, M.; Luo, C.; Vendrell, O.
First Principles Simulation of the Collective Rovibronic Ground State in a Cavity.
Phys. Rev. Res. 2026, 8, 013118. https://doi.org/10.1103/tcpr-1wrh.

[2] Krupp, N.; Groenhof, G.; Vendrell, O.
Quantum Dynamics Simulation of Exciton-Polariton Transport.
Nat Commun 2025, 16, 5431. https://doi.org/10.1038/s41467-025-61298-9.

[3] Mellini, F.; Vendrell, O.
Competition between Coherent Ultrafast Energy Redistribution and Photochemistry in the Collective Strong Coupling Regime: The Role of Static Disorder.
J. Phys. Chem. Lett. 2025, 16, 6155–6162. https://doi.org/10.1021/acs.jpclett.5c01117.

Speaker: Oriol Vendrell (University of Heidelberg)

10:05 Coffee

4 Raman scattering of phonon polaritons under nanoscale confinement: the role of structure and environment

Strong light-matter coupling in polar materials gives rise to phonon polaritons – hybrid quasiparticles with mixed photonic and vibrational character. Here, we show that polar nanocrystals exhibit Raman scattering that depends sensitively on their structure and electromagnetic environment. This spectral dispersion enables refractive index sensing in the mid-infrared via visible-wavelength Raman spectroscopy and draws parallels with molecular systems under vibrational strong coupling. Importantly, Raman scattering emerges only under nanoscale confinement of phonon polaritons, within a particle size range of ∼ 40 – 400 nm, bounded by volume limitations at smaller sizes and polariton coherence constraints at larger scales. In optimal structures, self-hybridization arises between localized phonon modes and surface phonon polaritons hosted by the same nanoparticle. Finally, we show that depositing only a few nanometers of alumina (∼ 10−4 of the free-space wavelength) enables tuning and complete suppression of surface phonon polaritons in this system, as probed by Raman scattering. Our findings establish polar nanocrystal Raman spectroscopy as a versatile probe of mid-infrared polaritonics and open routes to optical sensing inaccessible to surface- and tip-enhanced Raman scattering as well as noble-metal and graphene plasmonic platforms [1].

ACKNOWLEDGEMENT
This work was supported by Swedish Research Council (VR project, grant No. 2022-03347), Chalmers Area of Advance Nano, 2D-TECH VINNOVA competence center (Ref. 2024-03852), Olle Engkvist foundation (grant No. 211-0063), and the Knut and Alice Wallenberg Foundation (KAW, grant No. 2019.0140).

REFERENCES
[1] Zograf et al. arXiv, arXiv:2603.25895

Speaker: Timur Shegai (Chalmers University of Technology)

5 Stimulated cooling in non-equilibrium Bose–Einstein condensate

We report on the experimental observation of stimulated cooling in the non-equilibrium Bose–Einstein condensate (BEC) of weakly interacting exciton-polaritons from approximately room temperature down to 20 K. Our results indicate that the temperature of the weakly interacting Bose gas is universally set by the density-dependent chemical potential, revealing a defining property of non-equilibrium BECs. We demonstrate that the stimulated nature of the cooling process directly governs the emergence of quantum coherence of the condensate and shapes the dissipative properties of the excited states.

Speaker: Anton Zasedatelev (Aalto University)

12:00 Lunch Break

Monday Afternoon

6 Cavity-Mediated Long-Range Cooperative Coupling between Localized Plasmons and Molecular Excitons

Over the years, strong light-matter coupling has been observed and studied in a plethora of configurations, with microcavities and plasmonic nanostructures being among the most common geometries. Recently, however, hybrid electromagnetic modes that integrate plasmonic and photonic contributions have been suggested as a versatile platforms with distinct advantages for tailoring light–matter interactions.
Here, we present such hybrid fields, formed through the coupling between localized surface plasmon resonances of Au nanoparticles and Fabry–Pérot microcavity modes. Furthermore, by using this hybrid platform, we demonstrate that the cavity field can give rise to cooperative coherent coupling between the spatially separated plasmonic nanoparticles and the molecules, by mediating long-range dipole-dipole interactions. As we show, this cavity-mediated coupling also enhances the plasmon–exciton mixing, as compared to their direct interaction.
This configuration opens new avenues for tailoring light–matter interactions at the nanoscale, by supporting novel hybrid plexcitonic states that incorporate contributions from plasmons, excitons, and extended cavity fields.

Speaker: Prof. Tal Schwartz (Tel Aviv University)

7 Hybrid Light–Matter States and Energy Flow in Molecular Systems

Optical microcavities provide a versatile platform for tailoring light–matter interactions and thereby modifying excitation dynamics in molecular systems. In this presentation, we discuss how embedding excitonic materials in Fabry–Pérot microcavities reshapes their relaxation pathways through the formation of hybrid light–matter states, polaritons. Using photosynthetic light-harvesting complexes as a model system, we show how the cavity modifies excitation relaxation and inter-complex energy transfer. In the strong-coupling regime, population dynamics are governed by rapid redistribution between polaritonic branches and dark states, leading to altered lifetimes and transfer efficiencies. Remarkably, even in the weak-coupling regime, cavity-mediated connectivity between spatially separated complexes can enhance energy transfer. The relaxation dynamics can be quantitatively described using rate-equation approaches and Redfield theory, revealing how transfer rates scale with the square of the light–matter coupling strength in accordance with Fermi’s golden rule, while the role of dark states and wavefunction overlaps becomes crucial for understanding the dynamics.

We then turn to plexcitons, hybrid states formed by coupling molecular excitons to plasmonic resonances. We discuss how interference between electric and magnetic dipolar modes leads to Fano-like spectral features that can mimic or compete with genuine Rabi splitting. A rigorous treatment based on non-Hermitian Hamiltonians within the response-function formalism enables clear distinction between interference-dominated and strong-coupling regimes, both in linear and nonlinear
spectroscopy.

Finally, we present recent results on chiral plexcitonic systems based on helicoidal plasmonic nanostructures coupled to molecular J-aggregates. Here, helicity-dependent interference between electric and magnetic dipole channels governs both steady-state spectra and ultrafast relaxation dynamics.

Together, these examples illustrate how optical and plasmonic cavities provide powerful means to engineer excitation relaxation, energy transport, and nonlinear optical response.

Wu, F.; Nguyen- Phan, T. C.; Cogdell, R.; Pullerits, T. Efficient Cavity-Mediated Energy Transfer between Photosynthetic Light Harvesting Complexes from Strong to Weak Coupling Regime. Nat. Commun. 2025, 16, 1–9.

Wu, F.; Finkelstein-Shapiro, D.; Wang, M.; Rosenkampff, I.; Yartsev, A.; Pascher, T.; Nguyen- Phan, T. C.; Cogdell, R.; Börjesson, K.; Pullerits, T. Optical Cavity-Mediated Exciton Dynamics in Photosynthetic Light Harvesting 2 Complexes. Nat. Commun. 2022, 13, 6864

Rosenkampff, I.; Pullerits, T. Microcavity-Enhanced Exciton Dynamics in Light-Harvesting Complexes: Insights from Redfield Theory. J. Chem. Phys. 2025, 163, 044305

Speaker: Tönu Pullerits (Chemical Physics, Lund University)

8 Cavity-modified exciton-exciton annihilation in disordered molecular systems

Recent experiments have shown contradictory effects of strong light-matter coupling on exciton-exciton annihilation (EEA) in organic molecular systems. Here, I will present the results of numerical simulations of polariton dynamics, which reveal the role of strong coupling in changing the EEA rate. These results suggest that in systems with poor exciton mobility, strong coupling allows to partially overcome disorder via delocalisation of excitons owing to the interaction with the common cavity mode. This leads to an enhanced connectivity between excitons and, consequently, to an increase in the EEA rate. Conversely, in systems with high exciton mobility, in which disorder has a much smaller effect on excitation energy transfer, excitons can interact strongly even without coupling to the cavity photons at the exciton densities at which EEA typically occurs. In this case, the EEA rate can be even lower than in bare molecules due to the existence of a competing decay channel associated with photon leakage through the cavity mirrors. We also find that in the weak coupling regime, the EEA rate is always suppressed due to this decay channel regardless of the exciton transport properties. Our simulations resolve the experimental controversy on the effect of strong coupling on EEA and provide guidance for minimising the EEA rate towards a more feasible realisation of Bose-Einstein condensation of polaritons.

Speaker: Ilia Sokolovskii (University College London)

14:50 Coffee Break

9 Resonant modification of Van der Waals interactions under vibrational strong coupling

Experiments have demonstrated that chemical properties, such as reaction rates and equilibrium constants, can be modified under vibrational strong coupling (VSC). However, the mechanism that facilitates this remains ill-understood.
In my talk, I will show how under VSC, Van der Waals interactions can be resonantly modified, and how collective effects and disorder come into play here. I will demonstrate that these VSC-modified Van der Waals interactions can lead to rate enhancement in a simple model system, and discuss the implications of these findings in relation to experiment.

Speaker: Marit Fiechter (ETH Zürich)

10 Dynamic Fabry–Pérot Cavities for Rigorous Study of Chemical Reactivity Under Vibrational Strong Coupling

We present an experimentally and analytically rigorous reexamination of the urethane addition reaction under vibrational strong coupling (VSC) initially reported by Ahn & Simpkins in 2023. Our current results indicate no modification of chemical reaction rates between cavity-coupled and uncoupled systems. The specific causes for the previously measured differences are under investigation. However, we have uncovered two important measurement features that must be accounted for to make reliable measurements of this sort. First, we find, counterintuitively, that the reaction rates extracted from cavity-coupled transmittance spectra can be more reliable than those obtained from extracavity (i.e., control) reaction spectra, because pathlength information is more easily and readily accessible in the cavity spectra. Changes in pathlength caused by solution injection and the subsequent relaxation can result in systematic errors of the extracted concentration depending on the sign and magnitude of the change. We demonstrate that using an expanded solution dielectric function, which provides pathlength information encoded in solvent vibrational bands, reduces the disparity between extracavity and cavity rates. Secondly, we investigate the effect of pathlength nonuniformity on reaction rates extracted from cavity-coupled spectra and find that the analytical approach used by Ahn & Simpkins which mimics spatial/temporal broadening of cavity modes by artificially reducing the simulated quality factor can systematically overestimate molecular concentration. We show that when the simulated cavity transmittance is modeled to reflect the true underlying pathlength distribution, the correct molecular concentration is obtained. When pathlength instability and nonuniformity are accounted for using these methods, the difference between cavity and extracavity reaction rates (previously attributed to modified chemistry under VSC) vanishes.

Speaker: Blake S. Simpkins (Naval Research Laboratory, Chemistry Division)

11 Rotational polaritons in the gas phase

This presentation will provide a theoretical perspective on the topic of rotational polaritons formed in THz/microwave cavities. Some of the unique features of rotational strong coupling for the different types of molecular rotors will be presented and discussed, and a gentle introduction to the comprehensive quantum theory of rotational polaritons, based on the more general framework on rovibrational polaritons [1,2], will be given. Recent numerical results on the energetics, spectroscopy and dynamics of one- or more molecular rotors strongly coupled to multiple cavity modes will also be covered.
[1] J. Chem. Phys. 159, 014112 (2023).
[2] J. Chem. Phys. 162, 034117 (2025).

Speaker: Tamás Szidarovszky (Eötvös Loránd University)

12 Understanding Resonance‑Enabled Polaritonic Control of Hydrogen Transfer Dynamics and Relaxation

Molecular vibro-polaritons are currently discussed as a possible tool to modify ground-state reactivity. They arise when vibrational transitions couple strongly to an electromagnetic field, e.g in a cavity. We present numerical open-system quantum dynamics of thioacetylacetone (TAA) undergoing hydrogen transfer, coupled to a cavity mode and a bath. This model system, previously studied in [Fischer and Saalfrank Phys. Chem. Chem. Phys., 2023, 25, 1177], comprises an asymmetric potential energy surface (PES). By accounting for vibrational energy relaxation (VER), we provide reaction rates and corresponding photon-frequency dependent rate-profiles, exhibiting both, suppression and acceleration, depending on microscopic details.
To understand the numerical results, analytic expressions for the rate-profiles are provided by utilizing the Jaynes-Cummings model, while treating VER perturbatively. We attribute our findings to the formation of polariton states, representing the light-matter hybridization when the light mode is in resonance with a vibrational transition. Our results indicate that a full quantum treatment of both the cavity mode and the bath is required.

Speaker: Richard Gundermann (University of Potsdam, Institute of Chemistry, Karl-Liebknecht-Str. 24 - 25 D-14476 Potsdam Germany)

Poster session

13 Efficient charge transfer dynamics inside a cavity - a dynamical perspective

Charge transfer (CT) processes in donor–acceptor systems are central to photochemical and optoelectronic applications; however, the microscopic factors governing ultrafast transfer remain incompletely understood. Recent experiments on the PM605–TCNQ donor–acceptor system report ultrafast CT (~180 fs) outside an optical cavity. Upon coupling to cavity modes, the energetic driving force is reduced to approximately 0.5 eV compared to the bare system, leading to further acceleration of CT.

In this work, we have investigated different configurations using electronic structure methods and nonadiabatic dynamics. The ground-state optimized structure shows that the lowest CT lies significantly below S1 state, suggesting that the optimized structure alone does not favor ultrafast CT. However, other configurations show CT approaching S1 state and exhibit avoided crossings or conical intersections, enabling strong nonadiabatic coupling. Dynamics simulation reveal rapid population transfer from S1 to CT. Additionally, geometries slightly distorted from optimized configuration reveal proximity between S1 and higher CT states, facilitating alternative transfer pathways.

Building on these insights, we will investigate the accessibility of different configurations in solvent and establish possible pathway for ultrafast CT. Further we will investigate whether cavity-induced enhancements arise from intrinsic modifications of the system or from altered excitation pathways due to modified absorption inside cavity.

Speaker: Priyam Kumar De

14 Energy Transfer in Optical Microcavities: Theory Meets Experiment

Understanding spectroscopic signatures of polaritonic systems is essential for probing processes in polariton-controlled chemistry and photovoltaic devices. Transient absorption spectroscopy enables the study of energy transfer pathways between polaritons and dark states, as well as between dark states, which evolve on picosecond (ps) timescales, in contrast to the femtosecond (fs) decay of polaritons. We combine transient absorption measurements with theoretical modeling to investigate energy transfer from rhodamine 3B (λmax = 561 nm) to oxazine-1 (λmax = 653 nm) in a Fabry-Perot microcavity under strong coupling. By tuning cavity detuning and donor-acceptor ratios, we observe both enhancement and suppression of transfer rates. Using Redfield theory with a microscopic multi-mode Tavis-Cummings Hamiltonian, we show that transfer is enhanced when the cavity is resonant with acceptors, but suppressed when resonant with donors. This behavior arises from the degree of polariton-acceptor hybridization, which governs the efficiency of energy flow. Our results highlight cavity detuning as a key control parameter for engineering energy transfer in polaritonic systems.

Speaker: Ilmari Rosenkampff (Division of Chemical Physics, Lund University, Lund, Sweden)

15 Entangled Polariton States in the Visible and Mid-Infrared Spectral Ranges

Entanglement generation in polariton systems is fundamentally constrained by high losses and decoherence, which typically outweigh polariton nonlinearities. Here, we propose a conceptually different approach that uses optomechanical interactions, rather than polariton–polariton interactions, to generate entangled polaritons. Our double-resonant scheme relies on strong exciton-phonon coupling, found in both inorganic and molecular semiconductors, enabling room-temperature generation of spectrally disparate photon pairs. The quantum coherent and delocalized nature of polariton states inside optical cavities ensures efficient single-mode outcoupling and allows for unconditional quantum state preparation – not relying on any post-selection or projective measurements. When conditioned on exciton-polariton emission, single phonon-polariton states can be prepared that subsequently yield bright, heralded single-photon emission in the mid-IR/THz. We introduce a double-resonant optomechanical platform that enables scalable, room-temperature quantum polaritonics without relying on conventional excitonic nonlinearities.

Speaker: Vladislav Shishkov (Macroscopic Quantum Optics (MQO) Labs, Department of Applied Physics, Aalto University School of Science, FI-00076 Espoo, Finland)

16 Excited state electron transfer reaction under strong coupling conditions

Strong light–matter coupling in optical microcavities offers a promising route to manipulate molecular processes by engineering the electromagnetic vacuum field.(1,2) In this work, we propose to investigate polariton-mediated electron transfer in donor–acceptor systems, using BODIPY based donor and TCNQ acceptor embedded in silver Fabry–Pérot microcavities. This platform is designed to selectively hybridize donor excitations into polaritonic states while maintaining localized acceptor states, enabling controlled exploration of non-equilibrium energy transfer pathways.(3) The BODIPY donor will be designed to homogeneously fill the entire cavity to ensure strong coupling while the spatial distribution of the TCNQ acceptor will be varied, enabling investigation of the effects of placing the reaction at electric field nodes and antinodes.

We aim to employ angle-resolved reflectivity measurements to establish the formation of upper and lower polariton branches and to quantify the strength of light–matter coupling. Ultrafast transient absorption spectroscopy will be used to track population dynamics and probe potential modifications to donor-to-acceptor transfer under strong coupling conditions. By tuning cavity detuning and coupling strength, we seek to control spectral overlap and the energetic driving force for charge separation.

This approach will allow us to investigate whether polaritonic states enable new pathways for electron transfer that are inaccessible in conventional donor–acceptor systems governed by short-range interactions. More broadly, this work aims to establish modified vacuum fields as a tool for influencing molecular dynamics, opening new directions for coherent quantum control in chemical systems.

1) T. W. Ebbesen, Acc. Chem. Res., 2016, 49, 2403–2412.
2) Bhuyan, R.; Mony, J.; Kotov, O.; Castellanos, G. W.; Gómez Rivas, J.; Shegai, T. O.; Börjesson, K., Chem. Rev., 2023, 123, 10877–10919.
3) Rashidi, K.; Michail, E.; Salcido-Santacruz, B.; Paudel, Y.; Menon, V. M.; Sfeir, M. Y., Nat. Nanotechnol., 2025, 20, 1618–1624.

Speaker: Thrisha Swaminathan (University of Gothenburg)

17 Indirect Probing of Light-Induced Nonadiabatic Dynamics in Lossy Nanocavities

Light-induced nonadiabatic effects can arise from the interaction of a molecule with the quantized electromagnetic field of a Fabry−Pérot or plasmonic nanocavity. In this context, the quantized radiation field mixes the vibrational, rotational, and electronic degrees of freedom. In this work, we investigate the photodissociation dynamics of a rotating hydrogen molecule within a lossy plasmonic nanocavity. We highlight that, due to significant cavity loss, the dynamics are governed by an infinite number of light-induced conical intersections. We also examine the dissociation dynamics of fixed-in-space molecules by neglecting rotation, employing both the Lindblad master and non-Hermitian lossy Schrödinger equations. Additionally, we incorporate the effects of rotation within the parameter range of perfect agreement using the non-Hermitian lossy Schrödinger method. Furthermore, we show that in the absence of photon losses, there is a close correspondence between the classical Floquet description and the radiation field model.

Speaker: Gábor Halász (University of Debrecen)

18 Introducing vibrational quantization in classical molecular dynamics under strong light-matter coupling

Recent experiments have shown that vibrational strong coupling (VSC), a cavity quantum electrodynamics phenomenon, can alter chemical reactivity by changing kinetics, mechanisms, and product distributions.[1-4] VSC arises from the formation of hybrid light–matter states when molecular vibrations couple to photonic cavity modes, enabling modification of molecular wavefunctions without external illumination. While this possibility of controlling reactivity has generated major excitement, no predictive theoretical framework exists. Despite growing experimental evidence, it remains unclear when and how VSC affects reactions, posing a key fundamental challenge and limiting potential applications. Our work aims to model VSC with atomistic details using a classical molecular dynamics (MD) approach. Since vibrational degrees of freedom are not quantized in classical mechanics, we propose to overcome that limitation by combining classical molecular mechanics (MM) or semi-classical quantum mechanics/molecular mechanics (QM/MM) potentials with a biasing potential, effectively imposing quantization on the relevant vibrational transitions in MD simulations. I will present benchmark results demonstrating the introduction of vibrational quantization in classical MD simulations in the absence of a cavity. These results show that the
biasing potential can successfully reproduce quantized vibrational behavior within a classical framework. This methodology provides a foundation for extending the approach to systems inside optical cavities, enabling the study of chemical reactivity under VSC conditions.

References

1. Garcia-Vidal, F. J., Ciuti, C. & Ebbesen, T. W. Science 373, eabd0336 (2021).
2. Thomas, A. et al. Science 363, 615–619 (2019).
3. Xiang, B. et al. Science 368, 665–667 (2020).
4. Pang, Y. et al. Angew. Chem. Int. Ed. 59, 10436–10440 (2020)

Speaker: Shivani Verma (University of Jyväskylä)

19 Polariton Composition in a Dispersive Tavis-Cummings Mode

A detailed understanding of polariton composition is essential for unraveling the mechanisms underlying polaritonic chemistry. This work addresses vibropolaritons formed by infrared-active molecules in Fabry-Pérot cavities. In most theoretical descriptions, the polariton composition is modeled using the Jaynes–Cummings framework, or its extension to many emitters, the Tavis–Cummings model. However, the dispersive nature of the cavity modes is often neglected. We present a model that explicitly incorporates multiple cavity normal modes together with their dispersion relations within a single Hamiltonian framework. This treatment reveals a significantly richer polariton composition than predicted by conventional models. In particular, individual lower and upper polariton states within the polariton bands are shown to arise from coherent superpositions of several cavity modes. Remarkably, the contributing modes may originate from different dispersion branches. Our findings suggest that individual polariton states can be selectively accessed under different external illumination angles. Moreover, our results indicate that photon absorption and re-emission between cavity modes plays an important role in the formation of vibropolaritons in Fabry-Pérot cavities. This mechanism leads to a more intricate polariton composition.

In addition, the multimode character of the polaritonic states manifests itself in nontrivial spatial structures of the electromagnetic field, governed by the underlying cavity mode functions. The coherent superposition of modes with distinct wavevectors and dispersion branches gives rise to spatial interference patterns, including speckle-like intensity distributions.

Speaker: Mathis Noell (University of Potsdam)

20 Quantum dynamics simulation of exciton-polariton transport

Strong light-matter coupling offers a route to tunable and enhanced energy transport in organic materials via exciton-polaritons. These quasiparticles enable long-range ballistic flow, as observed in recent ultrafast microscopy experiments. However, experimental transport regimes vary from ballistic to diffusive, and the governing material properties remain unclear.

I will present full-quantum dynamical simulations of polariton transport in disordered organic media using the Multilayer multiconfiguration time-dependent Hartree (ML-MCTDH) method. I will discuss how vibronic interactions and static disorder influence transport. By analyzing wavepacket evolution in position and momentum space, we gain mechanistic insight into the ballistic-to-diffusive transition. Importantly, we can disentangle static and dynamic disorder, showing that static disorder primarily drives the onset of diffusion. This framework further allows us to examine the interplay between vibronic coupling and thermal disorder in shaping overall transport efficiency.

Speaker: Niclas Krupp (University of Heidelberg)

21 Rotational polaritons of the HCl dimer

In this work, the weakly bound HCl dimer is investigated in order to study rotational polaritons formed in THz/microwave cavities. The rotational levels of this near prolate symmetric top system are computed by solving the nuclear-motion Schrödinger equation using the fourth-age of quantum chemistry GENIUSH code [1-3]. In these calculations, all inter-molecular degrees of freedom are treated exactly.
[1] J. Chem. Phys. 130, 134112 (2009).
[2] J. Chem. Phys. 134, 074105 (2011).
[3] J. Chem. Phys. 147, 134101 (2017).

Speaker: János Sarka

22 Towards experimental observation of vibrational dynamics under Strong Coupling conditions by non-linear IR spectroscopy

Polaritonic chemistry bears the potential to tune molecular properties and steer chemical reactions by direct and controlled manipulation of strong light-matter interactions1, i.e. vibropolaritons formed under vibrational strong coupling (VSC) between the confined electromagnetic field in Fabry Perot cavities and molecular modes. However, experimental insights into the fundamental mechanisms and dynamics of vibropolaritons and implications for systematically tuning chemistry are still debated or lacking and difficult to obtain.
We aim to utilize coherent multidimensional vibrational spectroscopy (2D-IR) to study the ultrafast dynamics of vibropolaritions, with organic molecules. 2D-IR spectra can access vibrational lifetimes, couplings and energy transfer processes and how these are modulated upon formation of the delocalized hybrid states in contrast to pure molecular states. We have successfully established the experimental protocol to perform 2D-IR of vibropolaritons of carbonyl modes in organic molecules. These vibrational modes have a much shorter lifetime than previously studied inorganic complexes (i.e. W(CO)6)2 , hampering analysis of data as cavity contributions, rabi oscillations and spectra of uncoupled molecules all contribute to the observed data. However, first analysis shows presence of cross peaks between lower and upper polarition with dynamic features differing from the bare molecules.
References
[1] Thomas, A. et al., Tilting a Ground-State Reactivity Landscape by Vibrational Strong Coupling, Science 2019, 363 (6427), 615–619
[2] Xiang, B. et al., Intermolecular Vibrational Energy Transfer Enabled by Microcavity Strong Light–Matter Coupling, Science 2020, 368 (6491), 665–667.

Speaker: Helena Poulose (University of Potsdam)

Tuesday 5 May

Tuesday Morning

23 Vibrational dynamics of vibropolaritons formed by organic molecules

Vibrational strong coupling, i.e. the formation of polaritons between the vacuum field in Fabry Perot cavities and molecular vibrations, is discussed to allow tuning and steering chemical reactivity. However, mechanistic insights into the vibrational dynamics of the formed vibropolaritons, specifically from experiments, are still limited and not yet available for organic molecules, which are relevant in many synthesis applications.
We have started to perform 2D-IR spectroscopy on organic molecules in cavities inspired by some of the pioneering 2D-IR studies on coordination complexes under VSC, upon solving a lot of experimental constraints.
2D-IR spectra reveal vibrational dynamics of the polaritonic states and couplings between contributing states. We are able to report lifetimes of polaritons for selected organic molecules and observe strong modulation of those in comparison to the uncoupled molecules by the cavity. We find that 2D-IR spectra of vibropolaritons, comprised of signals from the cavity, the bare molecules beside the lower and upper polaritons are more and more complex to analyze the shorter the vibrational lifetime of the oscillator under investigation is.

Speaker: Henrike M. Müller-Werkmeister (Institute for Chemistry, University of Potsdam, Potsdam, Germany)

24 Multiscale simulation of molecular excitations strongly coupled to localized surface plasmons

Localized surface plasmons, collective excitations of conduction electrons in nanostructures, are associated to enhanced and spatially localized electromagnetic fields that can strongly couple the plasmonic excitations with the electronic ones of nearby molecules. The resulting hybrid excitations, called plexcitons, share with polaritons the capability of potentially modifying the molecular photophysics and photochemistry by reshaping the potential energy surfaces of the molecules, introducing new excited state decay channels and in general mediating the interaction with light.

In this talk I will present the multiscale modeling approach that we have developed to investigate optical properties and time evolution of plexcitonic systems. The model is based on an atomistic quantum chemistry description of molecules and a quantized continuum model representation of the plasmonic nanostructures, Q-PCM-NP [1]. Based on such an approach, we have investigated the microscopic picture behind collective plexciton states in dye covered nanoparticle aggregates [2], the ultrafast evolution of plexcitons [3] and could compare this quantized and the corresponding semiclassical description of molecule-plasmon interactions [4].

[1] J. Fregoni et al., Nano Lett 21, 6664 (2021); M. Romanelli, G. Gil, S. Corni arXiv:2512.01538
[2] G. Parolin et al. Nano Lett 24, 2273 (2024)
[3] J. Kuttruff et al. Nat Commun 14, 3875 (2023)
[4] M. Romanelli and S. Corni, J Phys Chem Lett 15, 9326 (2024)

Speaker: Stefano Corni

10:00 Coffee Break

25 Polaritons in complex nanophotonic structures

I will discuss the method we have developed over the last years to correctly describe strong light-matter coupling in arbitrary nanophotonic structures. This method obtains a quantum-optics-like description using a few discrete modes while still accounting for the full complexity of light propagation and emission. As a natural consequence, this method yields quantum optical models consisting of coupled lossy modes with strong non-Hermitian character, which can enable novel applications and protocols in quantum optics. I will then discuss how these ideas can be extended to directly access photon correlations of the emitted light resolved in space, frequency, time and polarization. Finally, I will show how to integrate the method with molecular descriptions based on QM/MM models, and how using it to describe emitter arrays coupled to periodic light modes can give polaritons with unprecedented nonlinear response.

Speaker: Prof. Johannes Feist (Universidad Autónoma de Madrid)

26 Atomistic Simulations of Polaritonic Dynamics in Realistic Optical Cavities

Polaritonic chemistry involves a vast number of coupled electronic, nuclear, and photonic degrees of freedom, which limits the applicability of fully ab initio approaches. Here, we present a semiclassical simulation framework that self-consistently combines numerical solutions of Maxwell's equations for realistic optical cavities with quantum molecular dynamics at the time-dependent density-functional tight-binding (TD-DFTB) level. This approach enables atomistic simulations of large molecular ensembles interacting collectively with cavity modes, while mimicking experimental conditions. From these simulations, we can obtain cavity transmission spectra and identify polaritonic signatures, while also accessing local molecular responses that depend on molecular number, geometry, position, and orientation. Applying this framework to driven cavities under collective electronic strong coupling, we present a new mechanism of vibrational activation, whereby collective electronic Rabi oscillations coherently drive nuclear motion. The effect is maximized when the collective polaritonic splitting resonates with a molecular vibrational mode, and exhibits features consistent with a stimulated Raman–like relaxation process.

Speaker: Carlos Bustamante (Max Planck Institute for the Structure and Dynamics of Matter)

12:05 Lunch

13:05 Free time

Social activity: Social activity and Conference Dinner.

16:30 Boat trip to Fjäderholmen Island

https://maps.app.goo.gl/FesjVZZB9eDJrYfe9

17:30 Conference Dinner at Rökeriet

https://maps.app.goo.gl/bo7J5996RrDgZsXz9

20:00 Boat trip Back to City

Wednesday 6 May

Wednesday Morning

27 Frame Dissipative molecular cavity quantum dynamics

Frame Molecular cavity quantum electrodynamics examines how confined radiation modes – in a Fabry-Perot or plasmonic nano-cavity – interact with molecules. The coupling between photons and molecules gives rise to mixed light–matter hybrid states, known as polaritons, which exhibit both photonic and molecular characteristics. The use of cavities to impact molecular structure and dynamics has become popular.
Frame As cavities, in particular plasmonic nanocavities, are lossy and the lifetime of their modes can be very short, their lossy nature must be incorporated into the calculations. The non-Hermitian Schrödinger and Lindblad master equations are commonly considered as appropriate tools to describe this lossy nature [1].
Frame The present talk reviews our recent achievements in this area: i) We demonstrate how the interplay of the atomic, molecular, and photonic populations gives rise to rich dynamics in the cavity [2]; ii) We discuss the light-induced nonadiabatic dissociation dynamics of a single molecule interacting with a lossy plasmonic nanocavity under strong coupling conditions [3,4]; iii) We also investigate the role of nuclear spin statistics and the Pauli principle in polaritonic chemistry. We assume vibrational strong coupling and study effects associated with the exchange of identical particles for one and two molecules coupled to a cavity mode. Our results highlight striking quantum-dynamical consequences of the Pauli principle [5].

[1] Fábri C., Császár A., Halász G. J., Cederbaum L. S., and Vibók Á. JCP. 160, 214308, (2024).
[2] Csehi A., Szabó K., Vibók Á., Cederbaum L. S., and Halász G. J. PRL. 134, 188001, (2025).
[3] Fábri C., Csehi A., Halász G. J., Cederbaum L. S., and Vibók Á.
AVS Quantum Science. 6, 023501, (2024).
[4] Szabó K., Fábri C., Halász G. J., and Vibók Á. JPCC. 129, 5950, (2025).
[5] Fábri C., Halász G. J., Cederbaum L. S., and Vibók Á. To be published.

Speaker: Agnes Vibok (University of Debrecen)

28 Collective Electron Correlations and Novel Phases Under Vibrational Strong Coupling

Recently, we have discovered a fundamental theoretical link between the seemingly unrelated fields of polaritonic chemistry and spin glasses. Our mapping reveals a cavity‑induced spin‑glass phase of intermolecular electron correlations, which could provide the long‑sought seed for significant local chemical modifications under collective VSC. This spin‑glass mapping not only demonstrates the relevance of moving beyond the dilute‑gas approximation—where the Pauli principle becomes essential—but also suggests the existence of a phase transition in the intermolecular electron correlations. Our theoretical predictions find experimental support in recently observed Rayleigh‑scattering phase transitions under collective VSC. Finally, we explore potential implications for electron correlations in solids.

Speaker: Dr Dominik Sidler (ZHAW School of Engineering)

10:30 Coffee Break

29 Quantifying the Number of Dark, Gray, and Bright States as a Function of Spectral Overlap in Polaritonic Systems

Strong exciton-photon coupling can steer photophysical processes. However, the picture where the coupling leads to two polaritonic states and a manifold of optically dark states is now questioned. Instead, a picture where inhomogeneous broadening results in a partial photonic contribution to the dark states has gained ground. To understand the consequence of these dark and so-called gray states on polariton photophysics, they first need to be experimentally quantified. In this talk, I will start by discussing how the number of dark states is affected by the cavity mode being coupled. Then, I will use close to ideal polaritons (low energy disorder) to show that the spectral multiplication method is inadequate for simulating spectrally resolved TE and TM emissions. Instead, good agreement between simulations and experiments is received when combining the transfer matrix and source term methods. Finally, I will show that fitting coupled rate equations to experimental emission data enables the absolute value of the dark states to be attained. Furthermore, that the number of dark states increases as the exciton reservoir and lower polariton overlap decreases.

References
The effect of the relative size of the exciton reservoir on polariton photophysics
Rahul Bhuyan, Maksim Lednev, Johannes Feist, Karl Börjesson
Advanced Optical Materials, 2024, 12 (2), 2301383

Quantitative Modeling of Polaritonic Emission Using the Source Term Method
Rahul Bhuyan, Maksim Lednev, Clara Schäfer, Johannes Feist, Karl Börjesson
The Journal of Physical Chemistry Letters, 2025, 16, 6435-6441

Quantifying the Number of Dark, Gray, and Bright States as a Function of Spectral Overlap in Polaritonic Systems
Rahul Bhuyan, Ilia Sokolovskii, Clara Schäfer, Gerrit Groenhof, Karl Börjesson
Submitted

Speaker: Karl Börjesson (the University of Gothenburg)

11:45 Lunch Break

Wednesday Afternoon

30 Classical analogues to chiral light matter interactions in optical cavities: from the Foucault pendulum to the Spyrograph game

We investigate the mechanism of chiral light-matter interactions for molecular ensembles embedded inside an optical Fabry-Perot cavity. We show how to describe and compute chiroptical properties of such cavities using classical methods based on transfert matrix approaches taking into account the mirror scattering properties. Then we propose and analyse a simplified microscopic model of standard cavity and unveil the key-role of dimensionality in describing its polaritonic spectral properties. For a quasi 2D-layer configuration, we show that the interplay between molecular chirality and spatial dispersion of the cavity-modes, results in a gyrotropic coupling at the origin of a differential shift in polaritonic energy spectra. We provide finally physical analogues of such effects by analyzing the classical Newtonian motion of a fictitious Foucault pendulum. Open questions and perspectives in this field will be enlighten.

Speaker: Rémi Avriller (IPCMS, CNRS and University of Strasbourg)

31 Multiqubit state preparation with nanophotonics tools

In this talk, I will discuss different nanophotonics-based strategies to generate tailored quantum states in emitter ensembles. First, I will explore the creation of highly entangled multiqubit states by inverse engineering of their photonic environment [1]. Next, I will address the preparation of topologically protected excitonic states in emitter chains [2]. Finally, I will briefly examine the spatial dependence of photon correlations in pairs of quantum emitters placed near elementary nanophotonic structures, highlighting the conditions under which their suppression becomes independent of the internal state of the emitters themselves [3].
[1] A. Miguel-Torcal et al. Optica Quantum 5, 371-378 (2024).
[2] A. Miguel-Torcal et al. ACS Photonics 12, 5434–5442 (2025). .
[3] B. Durá-Azorín et al., Phys. Rev. Research 7, 023178 (2025).

Speaker: Antonio I. Fernández Domínguez (Universidad Autónoma de Madrid)

14:15 Coffee Break

32 Interference-induced cavity resonances and imaginary Rabi splitting

In nanophotonic environments, light–matter interactions are typically shaped by multiple overlapping electromagnetic modes that interfere with one another, giving rise to sharp spectral features such as Fano profiles. Despite this complexity, polaritons are often described using simplified single-mode cavity QED models.

In this talk, I will show that these interference features can be understood as effective electromagnetic modes: interference-induced resonances, with inherently non-Hermitian couplings to quantum emitters. I will demonstrate that such modes can hybridize with emitters to form polaritons even outside the conventional strong-coupling regime.

This leads to qualitatively new behavior: the resulting polaritons acquire different decay rates, giving rise to what we term imaginary Rabi splitting. Extending this picture to many-emitter systems, I will show that these interference-induced resonances can generate long-lived polaritons that persist beyond excitonic dark states.

Finally, I will present numerical results for realistic nanophotonic platforms, illustrating the robustness of this regime to disorder and loss. These findings point to a new route for engineering polaritonic states in complex electromagnetic environments beyond the single-mode paradigm.

Speaker: Anael Ben-Asher (Tel Aviv University)

33 Non-equilibrium cavity pumping effects for electronic molecular polaritons

A comprehensive understanding of electron–photon correlation is essential for describing the reshaping of the electronic ground state of molecular systems embedded in quantum electrodynamics (QED) environments. The strong-coupling QED Hartree–Fock (SC-QED-HF) theory addresses these aspects by providing a consistent molecular orbital framework in the strong coupling regime. However, in experimental realizations, light–matter coupling is constrained by the quantization volume of the optical cavity or photonic device. It has been suggested that pumping a cavity with quantum light, such as a laser field, may enhance the effective light–matter coupling by driving the system into a non-equilibrium state with modified properties, including altered intermolecular interactions. In this work, we develop an extension of the SC-QED-HF model that constrains the parameters describing frequency dispersion to a fixed number of photons, enabling a systematic investigation of these effects. Our results suggest that cavity pumping provides a novel route to indirectly enhance light–matter coupling through amplified interactions with quantum fields.

Speaker: Yassir El Moutaoukal (Norwegian University of Science and Technology – Trondheim, Norway)