Quantum dynamics of the photoisomerization of a single thiacynine iodide molecule embedded in an optical microcavity was theoretically studied. The molecular model consisting of two electronic states and the reaction coordinate was coupled to a single cavity mode via the quantum Rabi Hamiltonian. We show that an electronic excitation of the molecule at cis configuration is followed by the generation of two photons in the trans configuration upon nonadiabatic isomerization. Although conditions for this phenomenon to operate in the collective strong light-matter coupling regime were found to be unfeasible for the present system, our finding suggests a new mechanism that, without ultrastrong coupling, achieves photon down-conversion by exploiting the emergent molecular dynamics arising in polaritonic architectures.
Quantum dynamics of the photoisomerization of a single thiacynine iodide molecule embedded in an optical microcavity was theoretically studied. The molecular model consisting of two electronic states and the reaction coordinate was coupled to a single cavity mode via the quantum Rabi Hamiltonian. We show that an electronic excitation of the molecule at cis configuration is followed by the generation of two photons in the trans configuration upon nonadiabatic isomerization. Although conditions for this phenomenon to operate in the collective strong light-matter coupling regime were found to be unfeasible for the present system, our finding suggests a new mechanism that, without ultrastrong coupling, achieves photon down-conversion by exploiting the emergent molecular dynamics arising in polaritonic architectures.
Molecular polaritons within the mid-infrared regime have emerged as a source for modifying and manipulating molecular and photonic properties. However, the development of new methodologies for photon generation is still a challenge in nanophotonics. We propose a molecular model based on the Holstein-quantum-Rabi Hamiltonian, which also incorporates realistic dipole moments and nonadiabatic couplings among electronic excited states, to study the ultrafast photodynamics of diatomic molecules in confined electromagnetic fields within quantized cavities. In addition to vibronic transitions due to intrinsic nonadiabatic couplings, two types of light-induced crossings emerge: one type is located at molecular nuclear geometries where the rotating wave approximation is fulfilled, and another type appears at different geometries where counter-rotating transitions may occur. We make a comprehensive study of polariton photodynamics within a time window of a few tens of femtoseconds, where dissipative mechanisms do not influence the polariton photodynamics. We stress the dramatic change of the polariton energy spectrum as a function of the Huang-Rhys factor when nonadiabatic couplings are included in the model. We conclude that both the molecular nonadiabatic couplings and, more specifically, the counter-rotating couplings in the cavity-molecule interaction play a crucial role in converting vibronic energy into photons through excited dressed states. We also show that the sign of the Huang-Rhys factor has a significant impact on this photon conversion. Our work paves the way for the development of many-photon generation powered by strong light-matter interaction, along with potential applications using alkaline earth monohydride molecules.
Abstract Strong light–matter coupling can modify the photochemistry of molecular systems. The collective dynamics of an ensemble of molecules coupled to the light field plays a crucial role in experimental observations. However, the theory of polaritonic chemistry is primarily understood in terms of single molecules, since even in small molecular ensembles the collective dynamics becomes difficult to disentangle. Understanding of the underlying ensemble mechanisms is key to a conceptual understanding and interpretation of experiments. We present a model system that simplifies the problem by mixing two-level Mg atoms with a single MgH+ molecule and investigate its collective dynamics. Our focus is on the modified chemical properties of a single diatomic molecule in the presence of an ensemble of resonant atoms as well as the structure of the major and intermediate polariton states. We present quantum dynamics simulations of the coupled vibronic–photonic system for a variable size of the atomic ensemble. Special attention is given to dissociative the dynamics of the MgH+ molecule.
We present a new theoretical framework, polarized Fock states (PFSs), to describe the coupled molecule-cavity hybrid system in quantum electrodynamics. Through the quantum light-matter interactions under the dipole Gauge, the molecular permanent dipoles polarize the photon field by displacing the photonic coordinate. Hence, it is convenient to use these shifted Fock states (termed the PFSs) to describe light-matter interactions under the strong coupling regimes. These PFSs are nonorthogonal to each other and are light-matter entangled states. They allow an intuitive understanding of several phenomena that go beyond the prediction of the quantum Rabi model, while also offering numerical convenience to converge the results with much fewer states. With this powerful new theoretical framework, we explain how molecular permanent dipoles lead to the generation of multiple photons from a single electronic excitation (down-conversion), effectively achieving the dynamical Casimir effect through the nuclear vibration instead of cavity mirror oscillations.
Strong light-matter coupling gives rise to polaritons - hybrid excitations whose mixed photonic and matter character enables control over optical, electronic and chemical properties. This Feature Article surveys the main architectures supporting polariton formation, including photonic microcavities, plasmonic nanostructures, open cavities and metasurfaces, and outlines how inorganic semiconductors, organic aggregates and hybrid systems access strong and ultrastrong coupling. Key phenomena such as coherent dynamics, vibronic interactions, dark-state reservoirs and polariton-mediated energy and electron transport are discussed, together with the experimental and theoretical tools used to study them. We highlight examples where strong coupling modifies charge transport, energy flow and chemical reactivity, and we summarize emerging regimes, including intermediate and dark-strong coupling, that broaden the landscape of hybrid light-matter physics.
The field of Polaritonic Chemistry studies molecules strongly coupled to the light field of optical cavities. The resulting modification of the potential energy landscape has been demonstrated to enable control over the chemical and physical properties of materials and molecular ensembles. This thesis extends the Tavis–Cummings model to describe molecules under strong light–matter coupling, incorporating static dipole moments and dipole self-energy contributions derived from the Pauli–Fierz Hamiltonian. These additional terms are shown to be essential for reproducing cavity-modified molecular dynamics and spectra. Simulations on the nuclear wavefunction propagation of MgH+ molecules demonstrate the impact of these terms, while an effective two-level system model accurately captures the behavior of large ensembles. Beyond single excitations, the thesis also explores the higher excitation manifolds of the Tavis-Cummings model, which reveals that allowing for more than a single excitation makes the reaction of the involved polaritons entropically more favorable. Open-system simulations highlight how the coherence of the system's initial state governs decay pathways and enables long-lived energy storage. Finally, a multilevel vibrational model coupled to a two-mode cavity is introduced, where a Raman scheme enables selective mixing of vibrational states. The simulation results show that continuous pumping can drive molecules into specific vibrational states while suppressing electronic excitation, both for single molecules and small ensembles. Together, these results provide new theoretical insights into the mechanisms by which optical cavities reshape molecular energy landscapes. The results suggest approaches for controlling photonic, electronic, and vibrational dynamics in strongly coupled systems, contributing to the theoretical foundation of polariton chemistry.
Strong coupling between molecular excitations and quantized electromagnetic fields in optical cavities provides a powerful means to control the physical and chemical properties of molecular systems. Here, we study electron transfer (ET) dynamics in cavity-coupled molecules using the numerically exact hierarchical equations of motion (HEOM) method, which captures nonperturbative and non-Markovian effects beyond standard perturbative theories. We identify distinct resonance and collective effects associated with polariton formation and show that the ET rate saturates in the strong-coupling regime, a feature not captured by perturbative approaches. We further extend the cavity-modified ET model by incorporating the nuclear-coordinate dependence of molecular electric dipole moments, which gives rise to a three-body interaction involving molecular electronic and vibrational degrees of freedom and cavity photons. This vibronic polariton formation leads to non-monotonic, oscillatory dependencies of the ET rate on the light-matter coupling strength and cavity frequency, which we attribute to quantum interference among multiple transfer pathways. These findings establish cavity-modified electron transfer as a multichannel quantum process governed by the interplay of electronic, vibrational, and photonic degrees of freedom.