Molecular complexes are held together via a variety of bonds, but they all share the common feature that their individual entities are in contact. In this work, we introduce and demonstrate the concept of a \textit{molecular optical bond}, resulting from the far-field electromagnetic coupling of several molecules via a shared mode of an optical microcavity. We discuss a collective enhancement of the vacuum Rabi splitting and study super- and sub-radiant states that arise from the cavity-mediated coupling both in the resonant and dispersive regimes. Moreover, we demonstrate a two-photon transition that emerges between the ground and excited states of the new optical compound. Our experimental data are in excellent agreement with the predictions of the Tavis-Cummings Hamiltonian and open the door to the realization of hybrid light-matter materials.
Molecular complexes are held together via a variety of bonds, but they all share the common feature that their individual entities are in contact. In this work, we introduce and demonstrate the concept of a \textit{molecular optical bond}, resulting from the far-field electromagnetic coupling of several molecules via a shared mode of an optical microcavity. We discuss a collective enhancement of the vacuum Rabi splitting and study super- and sub-radiant states that arise from the cavity-mediated coupling both in the resonant and dispersive regimes. Moreover, we demonstrate a two-photon transition that emerges between the ground and excited states of the new optical compound. Our experimental data are in excellent agreement with the predictions of the Tavis-Cummings Hamiltonian and open the door to the realization of hybrid light-matter materials.
Atoms and molecules usually hybridize and form bonds when they come in very close proximity of each other. In this work, we show that molecules can hybridize even through far-field electromagnetic interactions mediated by the shared mode of an optical microcavity. We discuss a collective enhancement of the vacuum Rabi splitting and study super- and subradiant states that arise from the cavity-mediated coupling both in the resonant and dispersive regimes. Moreover, we demonstrate a two-photon transition that emerges between the ground and excited states of the new optical compound. Our experimental data are in excellent agreement with the predictions of the Tavis-Cummings Hamiltonian and open the door to the realization of hybrid light-matter materials.
Optical microcavities and metallic nanostructures have been shown to significantly modulate the dynamics and spectroscopic response of molecular systems. We present a study of the nonlinear optics of a model consisting of N anharmonic multilevel systems (e.g., Morse oscillators) undergoing collective strong coupling with a resonant infrared microcavity. We find that, under experimentally accessible conditions, molecular systems in microcavities may have nonlinear phenomena significantly intensified due to the high quality of polariton resonances and the enhanced microcavity electromagnetic energy density relative to free space. Particularly large enhancement of multiphoton absorption happens when multipolariton states are resonant with bare molecule multiphoton transitions. In particular, our model predicts two-photon absorption cross-section enhancements by several orders of magnitude relative to free space when the Rabi splitting ${\mathrm{\ensuremath{\Omega}}}_{R}$ is approximately equal to the molecular anharmonic shift $2\mathrm{\ensuremath{\Delta}}$. Our results provide rough upper bounds to resonant nonlinear-response enhancement factors as relaxation to dark states is treated phenomenologically. Notably, ensembles of two-level systems undergoing strong coupling with a cavity (described by the Tavis-Cummings model) show no such optical nonlinearity enhancements, highlighting the rich phenomenology afforded by multilevel anharmonic systems. Similar conclusions are expected to hold for excitonic systems that share features with our model (e.g., molecular dyes with accessible ${S}_{0}\ensuremath{\rightarrow}{S}_{1}\ensuremath{\rightarrow}{S}_{2}$ transitions) and strongly interact with a UV-visible cavity.
Room-temperature cavity quantum electrodynamics with molecular materials in optical cavities offers exciting prospects for controlling electronic, nuclear and photonic degrees of freedom for applications in physics, chemistry and materials science. However, achieving strong coupling with molecular ensembles typically requires high molecular densities and substantial electromagnetic-field confinement. These conditions usually involve a significant degree of molecular disorder and a highly structured photonic density of states. It remains unclear to what extent these additional complexities modify the usual physical picture of strong coupling developed for atoms and inorganic semiconductors. Using a microscopic quantum description of molecular ensembles in realistic multimode optical resonators, we show that the emergence of vacuum Rabi splitting in linear spectroscopy is a necessary but not sufficient metric of coherent admixing between light and matter. In low-finesse multi-mode situations, we find that molecular dipoles can be partially hybridized with photonic dissipation channels associated with off-resonant cavity modes. These vacuum-induced dissipative processes ultimately limit the extent of light-matter coherence that the system can sustain.This article is part of the theme issue 'The quantum theory of light'.
Room temperature cavity quantum electrodynamics with molecular materials in optical cavities offers exciting prospects for controlling electronic, nuclear and photonic degrees of freedom for applications in physics, chemistry and materials science. However, achieving strong coupling with molecular ensembles typically requires high molecular densities and substantial electromagnetic field confinement. These conditions usually involve a significant degree of molecular disorder and a highly structured photonic density of states. It remains unclear to what extent these additional complexities modify the usual physical picture of strong coupling developed for atoms and inorganic semiconductors. Using a microscopic quantum description of molecular ensembles in realistic multimode optical resonators, we show the that the emergence of a vacuum Rabi splitting in linear spectroscopy is a necessary but not sufficient metric of coherent admixing between light and matter. In low finesse multi-mode situations we find that molecular dipoles can be partially hybridised with photonic dissipation channels associated with off-resonant cavity modes. These vacuum-induced dissipative processes ultimately limit the extent of light-matter coherence that the system can sustain.
Abstract When 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 these strongly coupled materials. 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.
Harnessing strong light-matter interactions to control chemical reactions in confined electromagnetic fields offers a promising route toward deepening our understanding of chemical dynamics at the collective quantum-mechanical level, with potential implications for future chemical synthesis paradigms. Achieving this goal, however, requires an in-depth mechanistic understanding of underlying dynamical processes. As a step in this direction, we present a systematic and numerically exact quantum dynamical study of cooperative reaction dynamics inside an optical microcavity. Using a hierarchy of model systems with increasing complexity, we elucidate how cavity-modified reactivity emerges from-and is highly sensitive to-subtle structural and environmental variations. Our models consist of optically dark reactive molecules, each represented by a symmetric double well potential, coupled to infrared-active non-reactive intramolecular or solvent vibrational modes, as well as their respective dissipative environments. Our results demonstrate that cavity-induced rate modifications arise from a delicate interplay among mode hybridization in strong-coupling regimes, the dynamical balance of all participating energy exchange processes, and quantum interference between multiple fluctuation-dissipation-mediated reaction pathways enabled by collective cavity coupling. By continuously tuning a single system parameter or introducing molecular collectivity, we observe qualitatively distinct rate modification profiles as functions of the cavity frequency, including resonant rate enhancement, resonant rate suppression, hybridization-induced peak splitting, and, notably, asymmetric Fano-type line shapes in which enhancement peaks and suppression dips coexist within a narrow resonance window, highlighting the important role of quantum interference in cavity-modified chemical reactivity.