During the past decades, strong coupling of matter excitations with confined electromagnetic modes have been shown to greatly modify material properties. Notably, polaritonic excitations are known to exhibit long-range energy transport characteristics. Moreover, while Anderson localization suppresses transport in one-dimensional systems with short-range interaction [1], such hybrid light-matter excitations have been proven robust against disorder [2]. Surprisingly, in recent studies of disordered polaritonic systems [3,4], an improvement of the transport characteristics has been observed when increasing the disorder strength, instead of the expected suppression. The interplay between disorder and strong light-matter coupling is then highly nontrivial, and its understanding is of primary importance, since disorder is always present in experimental setups.

In this work, we study such interplay by considering a disordered one-dimensional chain of lossy dipoles coupled to a multimode optical cavity through a microscopically-derived Hamiltonian. Such a disordered system hosting polaritonic excitations may be realized experimentally in a wide range of systems with strong light-matter coupling, from plasmonic and dielectric nanoparticles to ultracold atoms or molecules embedded in a photonic cavity.

By analyzing both the eigenspectrum and the driven-dissipative transport properties of our system, we find that in the strong-coupling regime, increasing disorder leads uncoupled dark states to acquire a photonic part, allowing them to inherit polaritonic long-range transport characteristics. Crucially, we show that this disorder-enhanced transport mechanism is increasingly noticeable when the considered dipoles are lossier.

Single-photon sources play a vital role in light-based quantum-information systems. One well-known design for such sources is a coupled emitter-cavity system. Single-photon emission is then achieved through two mechanisms: the destructive interference between the emitter and the cavity decays (unconventional antibunching [1,2]) and the photon blockade phenomenon (conventional antibunching [2.3]). While the destructive interference mechanism commonly emerges in the weak-coupling regime, the photon blockade mechanism requires a strong emitter-cavity coupling. Such strong coupling induces a breakdown of the harmonicity of the system and as a result prevents the absorption of subsequent photons. We here propose a novel non-Hermitian photon blockade mechanism, which stems from the anharmonicity of the system but works in the weak-coupling regime. We demonstrate an implementation of this idea using hybrid metallodielectric cavities that incorporate photon modes with different loss rates, and show that high-purity single-photon emission at high repetition rates could be achievable in such systems.

Contemporary nanophotonics research has focused recently on the ability of non-linear optical processes to mediate and transform optical signals in a myriad of novel devices, including optical modulators, transducers, color filters, photodetectors, photon sources, and ultrafast optical switches. The inherent weakness of optical nonlinearity at smaller-scales (e.g., nano- to micro-meter) has, however, hindered the realization of efficient miniaturized devices, and strategies for enhancing their throughput via nanoengineering remain limited. Here, we demonstrate a novel mechanism by which second harmonic generation, a prototypical non-linear optical phenomenon, from individual lithium niobate particles can be significantly enhanced through nonradiative coupling to the plasmon resonances of embedded gold nanoparticles. A single-particle experimental investigation of mesoporous lithium niobate particles coated with a dispersed layer of $\sim$10 nm diameter gold nanoparticles, together with a theoretical exploration of the energy exchange between the constituents of the hybrid structures, shows that a $\sim$32-fold enhancement of SHG response can be achieved without introducing large, lossy plasmonic nanoantennas to mediate photon transfer to or from the non-linear material. This work highlights the limitations of current Purcell-type strategies for enhancing non-linear optical phenomena and proposes a route through which a new class of smaller-size non-linear optical platforms can be designed to maximize non-linear efficiencies through near-field energy exchange.

Figure: A diagram of the energy transfer pathways of light within the system. Clockwise from left: incident light (dark red), SHG (blue arrows) through energy transfer from nonlinear polarization (blue vector field) to Mie resonances (halo), Prucell-enhanced SHG, and near-field enhanced SHG.

Disorder is an intrinsic attribute of any realistic molecular system; recently it was suggested that in molecular ensembles strongly coupled to photonic cavities, disorder may lead to delocalization [1], and modification of transport [2] and chemical reaction rates [3]. When the coupling strength largely exceeds the molecular inhomogeneity, polaritons are unaffected by the disorder [4]. However, in many realizations, such homogeneous limit does not apply. We investigated vibrational polaritons in an open cavity involving localized molecular ensembles with a systematically modified disorder. Experimental spectroscopic results are in a good agreement with the numerical model involving a generalized Tavis-Cummings-like Hamiltonian accounting for molecular disorder. We observed an increase in Rabi-splitting and modification of the polariton bandwidths, suggesting that enhanced delocalization of the reservoir states occurs via an admixture of the cavity mode. Another manifestation of the delocalization within the reservoir is observed in third-order nonlinear two-dimensional spectroscopic experiments, where ultrafast quantum dynamics of vibrational polaritons is measured [5]. In contrast to localized states, where polariton-to-reservoir excitation transfer is expected to be the dominant relaxation pathway [6], we observed strong reservoir-to-polariton transfer, suggesting a collective process involving correlated many-body states [7,8]. Our results provide new exciting insights into dynamics of polaritons involving disordered molecules.

Light-induced nonadiabatic phenomena arise when molecules or molecular ensembles are exposed to resonant external electromagnetic fields. The latter can either be classical laser or quantized cavity radiation fields, which can couple to either the electronic, nuclear or rotational degrees of freedom of the molecule. In the case of quantized radiation fields, the light-matter coupling results in the formation of two new hybrid light-matter states, namely the upper and lower “polaritons”. Light-induced avoided crossings and light-induced conical intersections between polaritons exist as a function of the vibrational and rotational coordinates of single molecules [1]. For ensembles of N molecules, the N − 1 dark states between the two optically active polaritons feature, additionally, so-called collective conical intersections, involving the coordinates of more than one molecule to form [2]. Here, we study the competition between intramolecular and collective light-induced nonadiabatic phenomena by comparing the escape rate from the Franck-Condon region of a single molecule and of a molecular ensemble coupled to a cavity mode [3]. In situations where the polaritonic gap would be large and the dark-state decay channels could not be reached effectively, the presence of a seam of light-induced conical intersection between the polaritons facilitates again the participation of the dark manifold, resulting in a cooperative effect that determines the overall non-radiative decay rate from the upper into the lower polaritonic states.

Common quantum-mechanical models for polaritonic chemistry lack mechanisms that populate dark states. In this work we explicitly model a process that does; disorder from stochastic energy variations the in matter systems inside the optical cavity. Such fluctuations typically arise due to local environment interactions, and the effect enters into the equations of motion as dephasing operators. Viewed through the lens of polaritonic states, a reservoir of previously inaccessible states has thus been opened up, and basic entropic arguments would imply that the remaining polaritonic states have less population to share. In other words, excess energy will be trapped in dark states, which would (e.g.) inhibit cavity output. On the other hand, in a product-basis of the sub-systems, before polaritonic diagonalisation, it is hard to see how small phase shifts would trap energy. In this work we investigate this tension, and we find that the analysis from the polaritonic perspective provides the correct intuition.

The interaction of an atom with the electromagnetic environment supported by a macroscopic body affects its structure and induces both spontaneous emission and Casimir-Polder energy shifts of the atomic levels. The possibility to control the properties of the atom by tuning the parameters of the macroscopic body has drawn attention recently [1]. Usually, the effects of the environment are treated separately for each atomic level. However, for groups of near-degenerate states, the induced shifts can become comparable to the energy differences between levels. In that case, the standard approach fails and it becomes necessary to treat the environment-induced interaction between the levels, leading to off-diagonal terms in the field-induced decay and energy shifts. We here present a way to treat such systems and show that the dynamics of an atom close to a macroscopic structure can noticeably deviate from those predicted by the standard diagonal formulation of the theory. Our approach is based on the framework of macroscopic quantum electrodynamics [2], together with a recent Lindblad master equation formalism that can treat near-degenerate levels while avoiding some of the problems of the standard Bloch-Redfield equation, leading to a trace-preserving master equation even without performing a secular approximation [3]. While this method includes the well-known Casimir-Polder potentials in the diagonal energy shifts, it importantly also describes off-diagonal contributions, especially relevant in near-degenerate atomic subspaces. Due to these off-diagonal contributions, the effective atomic eigenstates are linear combinations of the free-space eigenstates. The mixing of the atomic eigenstates has interesting physical consequences on, for instance, the atomic decay rates, and yet, as far as we know, these off-diagonal terms have not been discussed in the literature. We have simulated the fine structure of a hydrogen atom coupled to a silver nanoparticle that supports plasmon-polaritonic resonances. We quantify the amount of Casimir-Polder-induced mixing of the eigenstates through the so-called participation ratio. We can also gauge the impact of the off-diagonal Casimir-Polder terms on the atomic dynamics by obtaining the eigenenergies and decay rates from the master equation and studying them as a function of the distance between the hydrogen atom and the nanoparticle. Noticeably, avoided crossings become a new marked feature of the energy spectrum of the atom, a clear signature of mixed eigenstates. As for the decay rates, the nanoparticle offers new decay channels that become enhanced as the atom approaches it, leading to the naive expectation that the atomic decay rates will increase in a monotonic fashion, closely related to the well-known emission quenching due to nanoparticles. Our results show that the real evolution of the decay rates becomes more complicated as a consequence of the off-diagonal contributions. In particular, for an adequate set of parameters and distance range, some decay rates can be shown to actually decrease as the atom approaches the nanoparticle, again, thanks to the state mixing induced by the off-diagonal Casimir-Polder potential. Such effects are not present at all in traditional Casimir-Polder treatments lacking the off-diagonal terms and are necessary to include in order to properly understand the physics behind quantum state manipulation applications.

In this talk I present a method (1) capable of simulating many-body systems with strong coupling to multiple environments. This uses mean-field theory in conjunction with matrix product operator methods. I apply the method to the problem of organic polariton lasing, investigating the phase diagram and spectrum of a model of a cavity coupled to many molecules, with each molecule having a continuous spectrum of vibrational degrees of freedom. I also discuss the nature of the mean-field theory and the role of bright and dark exciton states in this approach.

Figure: Summary slide for talk including results from (1).

The near-field enhancement in plasmonic platforms can induce strong changes in the molecular electronic density at both the ground and excited state level. These effects are highly non-homogeneous in space, as they depend on both the nanoparticles geometries and the molecular structure, position and orientations with respect to the plasmonic platform. As a consequence, the model descriptions of the electromagnetic field and the molecular dipole approximation are extremely limited tools to grasp the complexity of these systems[1]. In this poster, we firstly present a quantisation strategy for the plasmonic modes building on a polarizable continuum description of the nanoparticle[2]. By these means, we obtain a representation of the plasmonic modes as electrostatic response charges. We then compute the quantum coupling between the quantised plasmons and a molecule described at quantum chemical level[3], highlighting the changes in the molecular electronic structure induced by the presence of the nanoparticles. Finally, we extend this coupling framework to the case of multiple molecules, investigating the optical properties of molecules collectively coupled to a realistic nanoparticle[4].

Figure: Nanotip on a support interacting with a single free-base porphyrin molecule (left) and nano disk interacting with a manifold of merocyanine molecules (right).

Two-dimensional spectroscopy (2DS) is a tool used for the study of radiation-assisted molecular processes with femtosecond resolution [1]. It is based on the interaction of three delayed pulses (two pump and one probe pulse) with a target system, and can access the third-order polarization response. We theoretically apply 2DS to a polaritonic system of N molecules interacting with a quantized mode of the electromagnetic field within a nanocavity [2]. The hybrid light-matter states (polaritons) in the coupled system are described by the Tavis-Cummings model in the strong coupling regime. Photonic losses are included as a Lindblad term in the master equation, while molecular vibrations are represented using the Bloch-Redfield formalism to appropriately describe the vibration-induced decay and pumping between the system states.

To obtain the 2D spectra we derive a simple and efficient computational procedure based on a set of pseudo-analytical equations obtained by expanding the states of the system in the Liouvillian eigenbasis. These equations are quite general and can be used for any dissipative system with diagonalizable Liouvillian. Our analytical formulas allow us to understand some characteristics of the 2D spectra in terms of both the Liouvillian and the Hamiltonian eigenstates.

Using our approach, we find that 2D spectra recorded at different waiting times T (between the pump and probe pulses) show that the diagonal signal associated with the lower polariton (LP) decreases more rapidly than that of the upper polariton (UP), a counterintuitive effect since the LP has a longer lifetime than the UP and is additionally refilled by vibration-induced decay from molecular dark states (non-radiative eigenstates of the Tavis-Cummings model). Interestingly, this effect also appears in recent experiments on polaritons involving molecular J-aggregates [3]. We explain the physical reason behind these observations through a careful analysis of the different contributions to the 2D spectra, and show that dark states indeed play a crucial (albeit indirect) role. We also discuss the importance of different excited-state manifolds to the observed spectra.

THz quantum optics is seen as a potential powerful tool [1,2] because the THz frequencies are in the same order of magnitude as the transition frequencies in many molecules. The initial hurdle lies in creating a potent source of THz radiation, a THz laser. In this project, we follow the proposal in [3], where this is achieved with an ensemble of weak-coupling quantum dots. We, however, study a strongly-coupled single colloidal quantum dot confined in a SiC dimer nanocavity. We show that this configuration can function as a laser, but can also exhibit antibunching or superbunching. We therefore prove that our platform acts as a versatile, tunable source of nonclassical THz radition.

Dipole-coupled subwavelength quantum emitter arrays respond cooperatively to external light fields as they may host collective delocalized excitations (a form of excitons) with super- or subradiant character. Deeply subwavelength separations typically occur in molecular ensembles, where in addition to photon-electron interactions, electron-vibron couplings and vibrational relaxation processes play an important role. We provide analytical and numerical results on the modification of super- and subradiance in molecular rings of dipoles including excitations of the vibrational degrees of freedom. While vibrations are typically considered detrimental to coherent dynamics, we show that molecular dimers or rings can be operated as platforms for the preparation of long-lived dark superposition states aided by vibrational relaxation. In closed ring configurations, we extend previous predictions for the generation of coherent light from ideal quantum emitters to molecular emitters, quantifying the role of vibronic coupling onto the output intensity and coherence.

Figure: (a) The equilibrium mismatch R between the groundand excited state electronic potential landscapes along agiven nuclear coordinate leads to the standard Franck-Condonphysics with a branching of transitions into different vibrational levels. The electron-vibron coupling is schematicallyrepresented by the link, at coupling strength λ, between anelectronic transition operator σ and a bosonic vibrationalmode operator b. (b) Schematics of a molecular ring wheremutual interactions are mediated by the electromagnetic vacuum at coherent/incoherent rates Ωij and Γij. The inset showsbranching of electronic transitions between the manifolds ofvibrational levels. (c) Preparation of an entangled moleculardimer with subwavelength separation d « λ0 via an impingingshort laser pulse. (d) Schematics of a molecular nanoscalelight source where the central gain molecule is incoherentlypumped and coherently coupled to the symmetric eigenmodeof the ring molecules. The ring provides an effective resonatorenhancement leading to the emission of coherent laser light.

At high excitation densities, exciton-polaritons in organic microcavities can show Bose-Einstein condensation (BEC) at room temperature [1], but the thresholds range from P_th ~ 2.2 to 500 μJ cm^-2, which are too high for electrical pumping. Recently, it has been demonstrated that maximizing the polariton relaxation rate (W_ep) is one way to accumulate efficiently lower polaritons (LPs) and decrease the polariton condensation threshold [2]. However, the currently reported W_ep via a radiative pumping of spin-singlets is about W_ep = 5 × 10⁷ s^-1 [2], and the use of spin-triplets is essential to further decrease the threshold. If triplet excitons can be efficiently converted to the LP state, the LP density will further increase due to the relaxation from the triplet in addition to the spin-singlets to the LP state (Fig. 1 a). So far, although the dynamics of the reverse intersystem crossing process from the spin-triplet to the LP state have been studied [3, 4], the influence of phosphorescence emission on the polariton relaxation process has not been fully explored. In this study, we focused on the microcavities containing ν-DABNA (Fig. 1 b, c) and discuss the role of phosphorescence emission in the polariton relaxation process through time-resolved emission measurement. As a result, we demonstrated that LPs were directly relaxed from triplet states via a radiative pumping of phosphorescence.

Figure: (a) Schematic illustrations of exciton-polariton and polariton relaxation (DS: dark state, LP: lower polariton, UP: upper polariton, and T₁ : spin-triplet states).(b) Chemical structure of N7 ,N7 ,N13,N13,5,9,11,15-octaphenyl-5,9,11,15-tetrahydro-5,9,11,15-tetraaza-19b, 20b-diboradinaphtho[3,2,1-de:1’,2’,3’-jk]pentacene-7,13-diamine (ν-DABNA).(c) Schematic image of the planar microcavity (DBR: distributed Brag reflectors).

We have developed a stochastic formalism to describe collective reactivity in polaritonic chemistry. In this approach, the stochastic behavior of many bistable systems (toy model for vibrational ground state of a molecule) is modified by the coupling to a single cavity vacuum field. The coupling modifies the reaction rates and, after a certain threshold, causes a spontaneous symmetry breaking in the stationary state. However, this effect is crucially dependent on the distribution of the light-matter couplings. When the values of the couplings greatly vary, the bifurcation shows up as a phase separation, and there is no apparent modification to the reaction rates. We discuss how this rich phenomenology connects to the existing and future experiments in the field.

We have investigated a number of molecular systems in the last few years with regard to the strong cavity coupling. While it seems to be widely accepted that collective interactions play a major in strong coupling photochemistry, other effects are less well explored. Our theoretical studies have shown that, for example, limited photon lifetimes, dipole self interactions, or vibrational strong coupling seems to play a role. We will give an overview over our studies and discuss the influence of these effects.

Self-assembled systems can be observed in molecular and biological systems with short range interactions such as van der Waals. It is possible to have stable longer range (100-200 nm separation) self-assembled microcavities with the charged metallic (in this case gold) flakes by creating an equilibrium between attractive Casimir and repulsive electrostatic forces. This system forms a tunable Fabry-Perot microcavities and allow us the modulate the system resonance both in active (laser pressure) and passive (electrostatic screening) ways [1]. Casimir torque is complicated to measure due to its small magnitude; however recently direct measurement is performed by Munday et al. [2] by using the two anisotropic material and mechanical torque observed which is induced by quantum fluctuations. In our work, we focus on the rotation effect of the lateral Casimir force and realize the self-alignment on the two triangle gold flakes. The gold flake in water solution starts rotating until it reaches the lowest energy state which is the perfect alignment with bottom triangle Au film / or another triangle flake in the solution. Self-alignment of microcavities is sensitive to electrostatic changes in the liquid surrounding media since it changes the electronic screening and breaks the equilibrium between two forces. We experimentally can obtain stable microcavities and investigate the effects of electrostatic on the lateral Casimir force by analyzing the alignment of the two triangle flakes. Self-assembled and self-aligned Casimir microcavities opens a controllable/tunable platform for various applications such as optomechanics [3], polaritonic chemistry [4] and nano- and micromechanical systems [5].

Light-matter interaction between molecules and confined electromagnetic fields is of great interest because it allows the modification of fundamental properties of the coupled system. This effect has been exploited for several different applications, such as low-threshold Bose-Einstein condensation and lasing [1-3], cavity-modified photochemical reactivity [4], and (long-range) energy transfer between donor and acceptor molecules [5-8]. By placing a donor-acceptor pair in a microcavity, delocalized polariton states are formed if the strong coupling condition is met. In this case, the efficiency of resonant energy transfer from the donor to the acceptor is changed, as a result of the appearance of new relaxation channels.

Here we investigate how a photochemical reaction of donor molecules can affect the efficiency of the energy transfer process. We first demonstrate the strong coupling regime between the chromophores and the cavity mode. Using UV illumination, we drive a photochemical reaction of the donor molecules that allow us to modify the hybrid light-matter states. By tuning the time of UV illumination, we obtain a significant enhancement of the emission intensity from the acceptor excitonic reservoir as compared to the outside-cavity case. We also present a theoretical explanation of the physical mechanism behind this experimental finding and we discuss a new way to achieve controllable modification of cavity-mediated energy transfer based on this fundamental knowledge.

Figure: Scheme of the experimental setup. The microcavity is formed by two silver mirrors. From top to bottom, the active layers are given by BRK J-aggregates in a PVA matrix (acceptor), spiropyrane/merocyanine in a PMMA matrix (donor), and a PVA spacer. Emission measurements were performed under pumping by a green 532 nm laser, while the photochemical reaction between spiropyrane and merocyanine was driven using a 365 nm UV laser.

The control of quantum systems lies at the core of many quantum technologies. In the field of coherent control, classical fields coherently drive the quantum system from a given initial state into a target state. Exploiting the quantum nature of the field to improve these control protocols has so far been mostly limited to the weak coupling regime.

In my talk, I will discuss how the quantum statistics of a bosonic field can be optimally tailored to drive an (ultra-)strongly coupled quantum system such as an atom or a molecule. This extends optimal control theory to the realm where control and target systems are both quantized and strongly coupled.

Figure: Optimal control problem under consideration: Given a quantum system (black atoms) initially prepared in state $|i\rangle$ and interacting (strongly) with a bosonic field (red shaded cavity field), which is the optimal initial state of the field $|\phi_\text{opt}\rangle$ such that, after a certain interaction time, the sample is in a predefined target state $|f\rangle$ (green atoms)?

Despite the several demonstrations of molecular chemistry being altered by optical microcavities, a detailed understanding of how this works remains unclear. In particular, the common Travis-Cummings model supposes that for a large number N of molecular modes, there should only be 2 polariton modes and the remaining N-1 dark states, those that are essentially unaffected by the cavity, should dominate in a reaction (1). However, this analysis no longer holds when molecules are coupled to more than a single cavity mode (2). We demonstrate that the molecular coupling to the continuum of cavity modes in the commonly used 2D cavities reduces the dark state to polariton ratio by orders of magnitude.

We derive a qualtiative model of the expected ratio of polariton states to dark states. The figure is an example of our calculated results, showing the molecular positions of one instance of the simulation, and the resulting polariton dispersion. Despite being modeled with fully localized excitons with random positions, the molecular polariton dispersion follows nearly exactly the traditional polariton dispersion of a quantum well in a microcavity.

Furthermore, we compare trends in dark state fraction for different cavity dimensionality (0D, 1D, and 2D). We also investigate the effects of positional disorder and energetic disorder on the polariton to dark state ratio. These results facilitate modeling and understanding of polariton chemistry, clarifying plausibility of controlling chemical reactions with optical cavities.

Figure: Simulated dispersion of 501 molecules in a microcavity with uniform random positions, averaged over 10 instances. The blue dashed line shows the cavity dispersion with minimum $E_{C0}$. The cavity minimum is on resonance with the molecular resonance $E_X$, shown by the orange dashed line. The red dashed lines show a polariton dispersion that agrees well with the simulation results, despite being modeled separately using a uniform coupling between the $E_X$ and cavity resonance at each value of $k$.

Surface-enhanced Raman scattering (SERS) allows the fingerprinting of single molecules via their vibrational degrees of freedom. Inspired by its analogy with the field of cavity optomechanics [1], a first model was proposed considering the optomechanical dynamics between plasmonic electric fields and molecular vibrations [2]. This molecular optomechanical approach allowed both, to describe new effects in the field of SERS arising from the dynamical backaction and to offer new possibilities in cavity optomechanics due to the resulting large coupling strengths involved – orders of magnitude larger than in previous configurations. Despite recent experimental works evidencing such optomechanical nature of SERS, large spectral discrepancies have arisen with current theoretical predictions [3] that call for new mechanisms for its understanding. Inspired by the microscopic molecular Hamiltonian [4], in this work we propose an optomechanical SERS model that considers the internal mechanisms of the molecule. In this model, the electronic transitions involved in the Raman processes are treated as a set of two-level systems that mediate the interaction between plasmons and molecular vibrations via electron-vibron couplings [4]. Since such electronic levels typically lie in the ultra-violet range, we can adiabatically eliminate them and recover the original optomechanical Hamiltonian [2]. Beyond such adiabatic approximation, we further consider a near-resonant transition coexisting with the off-resonant ones, showing interesting cooperative behaviours such as enhancements of anti-Stokes lines and modifications of spectral widths. For ultrastrong interaction scenarios with electron-vibron couplings close to the mechanical frequency, we consider the resulting phonon-dressed states in the master equation that show incoherent contributions to the anti-Stokes peaks, decisive for understanding the spectrum. Our model shows the importance of treating the molecular degrees of freedom with equal footing in SERS and offers new perspectives of the mechanisms involved in molecular optomechanics.

We theoretically investigate the circular dichroism of a helicity-preserving Fabry-Pérot cavity made of two dielectric metamirrors. The latter are designed to act, in a narrow frequency range, as efficient polarization cross-converters in transmission for one polarization, and almost perfect reflectors for the other polarization. The resulting cavity mode is circularly polarized and decoupled (at resonance) from the outside of the cavity. Despite this decoupling, a Pasteur medium hosted inside the cavity can still couple efficiently to both the outside of the cavity and the helicity-preserving mode, inheriting a partial chiral character. The consequence of this mechanism is twofold: it increases the intrinsic chiroptical response of the molecules by two orders of magnitude and it allows for the formation of chiral polaritons upon entering the regime of strong light-matter coupling.

^* These authors contributed equally.

Planar microcavities, formed by dielectric and plasmonic materials, are able to modify material properties under the strong coupling regime which can be described by cavity quantum electrodynamics. Among others, changes in chemical rates, superfluidity, and energy transport have been reported [1]. Recently, magnetic materials came into play by experimental findings showing alterations of magnetic properties [2]. Motivated by them, we report on the theoretical foundation of cavity-mediated spin-spin interactions between magnetic nanoparticles, hosted in the structure shown in Fig. 1(a). By developing novel theoretical and rigorous methodological tools capable to account for the collective strong coupling between many optical modes and matter states, we describe how cavity polaritons can strongly couple with both local phonons and spins, a schematic illustration of which is depicted in Fig. 1(b). Our results indicate, that indeed, cavity photons can generate spin-spin interactions exceeding the conventional nearest- and next-nearest-neighbors ones not only in strength but also in range. At the same time, under appropriate conditions, symmetric Heisenberg-like and antisymmetric Dzyaloshinskii—Moriya-like interaction can be reached.

Figure: Fig. 1: (a) Planar microcavity comprised by a thin gold layer and a μm-thick polymer matrix which hosts the magnetic nanoparticles. (b) Schematic illustration of the theoretical framework, treating simultaneously the collective and local degrees of freedom, for the cavity-polariton generated spin-spin interactions which are mediated by local phonons.

Polaritons are hybrid light-matter states that emerge when the interaction between molecular excitations and the electromagnetic field is comparable to their respective frequencies. Several theoretical and experimental work suggest that polaritons provide a new platform for a variety of chemical applications such as the modification of energy transfer rates, the realization of exciton-polariton condensation, and the control of chemical reactivity. However, some of the most exciting experimental results today remain poorly understood despite all the theoretical work in the last decade. From the theoretical standpoint, the study of molecular polaritons beyond simple models is challenging due to the large dimensionality of these systems. Considering this, a very important question is to what extent single molecule models can be used for interpretation or prediction of such phenomena, given that polaritons are experimentally achieved mostly using a macroscopic number of molecules that collectively couple to light.

Here we use the Multi-Configurational Time-Dependent Hartree (MCTDH) method to study the dynamics of molecular polariton Hamiltonians in the collective strong coupling regime. By finite size-scaling of dynamics involving a few molecules, we numerically extrapolate the dynamics into the regime involving a macroscopic number of molecules

Figure: Dynamics in the collective strong coupling regime

The rate at which matter emits or absorbs light can be modified by its environment, as dramatically exemplified by the widely studied phenomenon of superradiance. The reverse process, superabsorption, is harder to demonstrate because of the challenges of probing ultrafast processes, and has only been seen for small numbers of atoms. Its central idea—superextensive scaling of absorption meaning larger systems absorb faster—is also the key idea underpinning quantum batteries. Here we implement experimentally a paradigmatic model of a quantum battery, constructed of a microcavity enclosing a molecular dye [1]. Ultrafast optical spectroscopy allows us to observe charging dynamics at femtosecond resolution to demonstrate superextensive charging rates and storage capacity, in agreement with our theoretical modelling. We find that decoherence plays an important role in stabilising energy storage.

Strong light-matter interactions are at the core of many electromagnetic phenomena. In this talk, I will give an overview of several nanophotonic systems which support polaritons - hybrids between light and matter, as well as try to demonstrate their potential usefulness in polaritonic chemistry applications. Specifically, I will show that Fabry-Perot resonators, one of the most important workhorses of nanophotonics, can spontaneously form in an aqueous solution of gold nanoflakes [1]. This effect is possible due to the balance between attractive Casimir-Lifshitz forces and repulsive electrostatic forces acting between the flakes. There is a hope that this technology is going to be useful for future developments in self-assembly, nanomachinery, and polaritonic chemistry. I will also briefly discuss the potential relevance and connection between the Casimir effect and polaritonic chemistry [2-3]. Finally, I will discuss the potential relevance of polaritons naturally existing in nanostructured resonant materials to polaritonic chemistry [4]. Several concrete examples such as a slab of the excitonic material and, most interestingly, a spherical water droplet in vacuum are shown to reach the regime of such polaritons. The abundance of cavity-free polaritons in simple and natural structures points at their relevance and potential practical importance for the emerging field of polaritonic chemistry, exciton transport, and modified material properties.

Transfer of excitation energy is a key step in light harvesting and hence of tremendous technological relevance for solar energy conversion. In bare organic materials energy transfer proceeds via incoherent hops, which restrict propagation lengths to the nanoscale. In contrast, energy transport over several micrometers has been observed in the strong coupling regime where excitations hybridise with confined light modes to form polaritons. Because polaritons inherit the group velocity of the confined light modes, their propagation is ballistic and long-ranged. However, experiments on organic micro-cavities indicate that polaritons propagate in a diffusive manner and much slower than the polariton group velocity. To resolve this controversy, we implemented atomistic multi-scale molecular dynamics simulations of Rhodamine molecules in a Fabry-Perot cavity. Our results suggest that polariton propagation is limited by the cavity lifetime and appears diffusive due to reversible population transfer between bright polaritonic states that propagate ballistically at their group velocities, and dark states that are stationary. Furthermore, because the long-lived dark states can effectively trap the excitation, propagation is observed on timescales far beyond the intrinsic polariton life-time, in particular in low-Q cavities. In addition, we discuss the influence of the cavity quality factor on the polariton propagation distance in the case of the resonant excitation of the upper polariton branch. The atomistic insights from our simulations not only help to better understand and interpret experimental observations, but also pave the way towards rational design of molecule-cavity systems for achieving coherent long-range energy transport.

Demonstrations of strong and ultrastrong light-matter interaction in mid-infrared resonators have stimulated the search for control schemes that can be used to prepare target material and photonics states for a variety of applications in chemistry [1]. We propose a scheme for modifying the photon number and field quadrature statistics of a confined mid-IR electromagnetic field by modulating the vacuum frequency with femtosecond UV pulses [2]. The scheme relies on the transient modification of the carrier density of the field-confining materials and strong coupling between the confined field and an ensemble of molecular vibrations. We study variations of the Mandel Q-factor and squeezing over sub-picosecond timescales by modulating the cavity frequency ~15%, resulting in squeezing factors of order ~1 dB for a system initially prepared in the stationary ground and lower polariton states [3]. We relate the predicted variations of quantum field statistics to the type of dipolar structure and anharmonicity of the molecular vibrations. Our work can stimulate the design of novel infrared devices at room temperature for applications in quantum metrology and quantum information processing.

Over the past few years, a completely new way of modifying chemical reactivity has emerged based on creating polaritons between molecular vibrations and confined electromagnetic fields. [1] This is called vibrational strong coupling (VSC) [2] and is currently being examined by as new tool to control and catalyze chemical reactions [3-7]. VSC utilized light-matter interaction to modify material properties without the need for light as such.

Several experimental studies have demonstrated the ability of VSC to alter reaction kinetics [3-5], binding constants [6] and aggregation behavior [7]. One specific example is the 4.5x slow down of the enzymatic activity of pepsin by strongly coupling water. [5] In another study, ester hydrolysis has been observed to accelerate under VSC, highlighting the prospect of VSC as an alternative to traditional catalysis [8]. This result has however been recently contested [9]. Here, we verify the potential of VSC to catalyze ester cleavage and explore it in greater depth. We find effects of coupling different vibrations of the system and discuss the difficulties and pitfalls of VSC kinetic experiments. This work will help to understand the potential of VSC for both industrial and academic chemistry community.

In this work we show how driving the two-photon resonance of a system of interacting quantum emitters can be exploited for the generation of cavity-assisted steady-state entanglement. The fact that two interacting emitters can be coherently driven via two-photon processes has been exploited, e.g., to estimate interaction strengths and inter-molecular distances at the nanometer scale [1]. It has also been shown that, under a strong two-photon drive, the emitters can be dressed with photon pairs from the drive, developing a rich family of energy levels that translate into a complex structure in the spectrum of resonance fluorescence [2]. By coupling the dressed system to a cavity in the bad cavity limit, new processes among the two-photon dressed energy levels can be engineered. We show that, by placing particular dressed-state transitions in resonance with the cavity, these novel decay processes can stabilize the system into a highly entangled state. Since the energy of the dressed states can be tuned through the Rabi frequency of the drive, the system can be optically tuned in and out of these resonances. We also show that the stabilization of entanglement translates into particular features in the quantum optical properties of the light emitted by the system at frequencies that are well detuned from the drive, allowing us to isolate the optical signatures of entanglement by simple spectral filtering.

Figure: (a) Concurrence of the two-interacting quantum emitters. (b) Second-order correlation function of the photons emitted by the cavity. In both panels, the corresponding quantity is described in terms of the cavity detuning and the laser driving strength along with transversal cuts at the maximum value of the concurrence.

Solvation dynamics, i.e., the interaction of a solute with a surrounding solvent, is one of the most fundamental processes in nature, playing a crucial role in many chemical reactions and charge transfer processes. We derive here a simple phenomenological model for this interaction. We want to apply stochastic methods to describe the interplay between polar molecules (e.g. water) in a surrounding polarizable medium. This builds up on previous developed methods, e.g. for the description of molecules in phononic host environments. We then want to apply the developed formalism for the description of charge transfer processes, cavity-coupled molecules and the nonlinear dynamics.