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Strong light–matter coupling provides a promising path for the control of quantum matter where the latter is routinely described from first principles. However, combining the quantized nature of light with this ab initio tool set is challenging and merely developing as the coupled light–matter Hilbert space is conceptually different and computational cost quickly becomes overwhelming. In this work, we provide a nonperturbative photon-free formulation of quantum electrodynamics (QED) in the long-wavelength limit, which is formulated solely on the matter Hilbert space and can serve as an accurate starting point for such ab initio methods. The present formulation is an extension of quantum mechanics that recovers the exact results of QED for the zero- and infinite-coupling limit and the infinite-frequency as well as the homogeneous limit, and we can constructively increase its accuracy. We show how this formulation can be used to devise approximations for quantum-electrodynamical density-functional theory (QEDFT), which in turn also allows us to extend the ansatz to the full minimal-coupling problem and to nonadiabatic situations. Finally, we provide a simple local density–type functional that takes the strong coupling to the transverse photon degrees of freedom into account and includes the correct frequency and polarization dependence. This QEDFT functional accounts for the quantized nature of light while remaining computationally simple enough to allow its application to a large range of systems. All approximations allow the seamless application to periodic systems.

In this article, we review strong light-matter coupling at the interface of materials science, quantum chemistry, and quantum photonics. The control of light and heat at thermodynamic limits enables exciting new opportunities for the rapidly converging fields of polaritonic chemistry and quantum optics at the atomic scale from a theoretical and computational perspective. Our review follows remarkable experimental demonstrations that now routinely achieve the strong coupling limit of light and matter. In polaritonic chemistry, many molecules couple collectively to a single-photon mode, whereas, in the field of nanoplasmonics, strong coupling can be achieved at the single-molecule limit. Theoretical approaches to address these experiments, however, are more recent and come from a spectrum of fields merging new developments in quantum chemistry and quantum electrodynamics alike. We review these latest developments and highlight the common features between these two different limits, maintaining a focus on the theoretical tools used to analyze these two classes of systems. Finally, we present a new perspective on the need for and steps toward merging, formally and computationally, two of the most prominent and Nobel Prize-winning theories in physics and chemistry: quantum electrodynamics and electronic structure (density functional) theory. We present a case for how a fully quantum description of light and matter that treats electrons, photons, and phonons on the same quantized footing will unravel new quantum effects in cavity-controlled chemical dynamics, optomechanics, nanophotonics, and the many other fields that use electrons, photons, and phonons.

The vacuum Rabi splitting of polaritonic eigenmodes in semiconductor microcavities scales with the square root of the oscillator strength, as predicted by the coupled oscillator model and confirmed in many experiments. We show here that the square root law is no more applicable if the Rabi splitting becomes comparable or larger than the stop-band width of the Bragg mirrors forming the cavity. Once the oscillator strength becomes large enough, the material hosting excitons hybridises with the quasi-continuum microcavity Bragg modes lying outside of the stop-band, thus forming a novel kind of polaritonic resonance. We study this physics considering both two- and three-dimensional excitonic materials embedded in the microcavity. We highlight the varied phenomenology of those polaritons and develop a theoretical understanding of their most peculiar features.

The vacuum Rabi splitting (VRS) in molecular polaritons stands as a fundamental measure of collective light–matter coupling. Despite its significance, the impact of molecular disorder on VRS is not fully understood yet. This study delves into the complexities of VRS amidst various distributions and degrees of disorder. Our analysis provides precise analytical expressions for linear absorption, transmission, and reflection spectra, along with a “sum” rule, offering a straightforward protocol for extracting accurate collective light–matter coupling values from experimental data. Importantly, our study cautions against directly translating large VRS to the onset of ultrastrong coupling regime. Furthermore, for rectangular disorder, we witness the emergence of narrow side bands alongside a broad central peak, indicating an extended coherence lifetime even in the presence of substantial disorder. These findings not only enhance our understanding of VRS in disordered molecular systems but also open avenues for achieving prolonged coherence lifetimes between the cavity and molecules via the interplay of collective coupling and disorder.

Exciton–polariton systems, formed through the strong coupling of excitons and photons, provide a unique platform for investigating quantum coherence and collective dynamics in solid-state systems. These hybrid quasiparticles combine photonic and excitonic characteristics, enabling phenomena such as Rabi oscillations, long-distance coherent energy transfer, ballistic energy transport, and Bose–Einstein condensation. Their ability to sustain macroscopic quantum coherence, alongside their sensitivity to environmental and system-engineering factors, highlights their potential for advancing both fundamental quantum science and practical applications, including nanophotonics, energy harvesting, and quantum technologies. This review aims to offer a comprehensive exploration of coherent phenomena in exciton–polariton systems, spanning theoretical foundations, experimental realizations, and applications. Key topics include the dynamics of strong light–matter coupling, the role of vibrational modes and energetic disorder, and the interplay between coherence and dissipation. Advances in ultrafast spectroscopy and quantum electrodynamics models have been pivotal in uncovering polaritonic behavior and optimizing system performance. Despite significant progress, challenges remain in maintaining coherence and addressing the effects of dissipation and disorder. By overcoming these hurdles, exciton–polariton systems promise transformative technological applications and deeper insights into quantum phenomena, positioning them as a cornerstone in the future of quantum science and technology.

Placing a material inside an optical cavity can enhance transport of excitation energy by hybridizing excitons with confined light modes into polaritons, which have a dispersion that provides these light–matter quasi-particles with low effective masses and very high group velocities. While in experiments, polariton propagation is typically initiated with laser pulses, tuned to be resonant either with the polaritonic branches that are delocalized over many molecules, or with an uncoupled higher-energy electronic excited state that is localized on a single molecule, practical implementations of polariton-mediated exciton transport into devices would require operation under low-intensity incoherent light conditions. Here, we propose to initiate polaritonic exciton transport with a photo-acid, which upon absorption of a photon in a spectral range not strongly reflected by the cavity mirrors, undergoes ultra-fast excited-state proton transfer into a red-shifted excited-state photo-product that can couple collectively with a large number of suitable dye molecules to the modes of the cavity. By means of atomistic molecular dynamics simulations we demonstrate that cascading energy from a photo-excited donor into the strongly coupled acceptor-cavity states via a photo-chemical reaction can indeed induce long-range polariton-mediated exciton transport.

The emerging field of spinoptronics has a potential to supersede the functionality of modern electronics, while a proper description of strong light-matter coupling pose the most intriguing questions from both fundamental scientific and technological perspectives. In this paper we address a highly relevant issue for such a development. We theoretically explore spin dynamics on the surface of a 3D topological insulator (TI) irradiated with an off-resonant high-frequency electromagnetic wave. The strong coupling between electrons and the electromagnetic wave drastically modifies the spin properties of TI. The effects of irradiation are shown to result in anisotropy of electron energy spectrum near the Dirac point and suppression of spin current and are investigated in detail in this work.

It remains unclear how the collective strong coupling of cavity-confined photons to the electronic transitions of molecular chromophore leverages the distinct properties of the polaritonic constituents for future technologies. In this study, we design, fabricate, and characterize multiple types of Fabry-Pérot (FP) mirco-resonators containing copper(II) tetraphenyl porphyrin (CuTPP) to show how cavity polariton formation affects radiative relaxation processes in the presence of substantial non-Condon vibronic coupling between two of this molecule’s excited electronic states. Unlike the prototypical enhancement of Q state radiative relaxation of CuTPP in a FP resonator incapable of forming polaritons, we find the light emission processes in multimode cavity polariton samples become enhanced for cavity-exciton energy differences near those of vibrations known to mediate non-Condon vibronic coupling. We propose the value of this detuning is consistent with radiative relaxation of Herzberg-Teller polaritons into collective molecular states coupled to the cavity photon coherently. We contrast the feature stemming from light emission from the HT polariton state with those that occur due to polariton-enhanced light absorption. Our results demonstrate the landscape of molecular and photonic interactions enabled by cavity polariton formation using complex chromophores and how researchers can design resonators to leverage these interactions to characterize and control polaritonic properties.

Optical control is achieved on the excited state energy transfer between spatially separated donor and acceptor molecules, both coupled to the same optical mode of a cavity. The energy transfer occurs through the formed hybrid polaritons and can be switched on and off by means of ultraviolet and visible light. The control mechanism relies on a photochromic component used as donor, whose absorption and emission properties can be varied reversibly through light irradiation, whereas in-cavity hybridization with acceptors through polariton states enables a 6-fold enhancement of acceptor/donor contribution to the emission intensity with respect to a reference multilayer. These results pave the way for synthesizing effective gating systems for the transport of energy by light, relevant for light-harvesting and light-emitting devices, and for photovoltaic cells.

Two-dimensional semiconductors are considered intriguing materials for photonic applications, thanks to their stunning optical properties and the possibility to manipulate them at the nanoscale. In this review, we focus on transition metal dichalcogenides and low-dimensional hybrid organic-inorganic perovskites, which possess the same characteristics related to planar confinement of their excitons: large binding energies, wide exciton extension, and high oscillator strength. We describe their optoelectronic properties and their capability to achieve strong coupling with light, with particular attention to polariton-polariton interactions. These aspects make them very attractive for polaritonic devices working at room temperature, in view of the realization of all-optical logic circuits in low-cost and easy-to-synthesize innovative materials.