Polariton States Research Articles

Perturbation theoretical approaches to strong light-matter coupling in ground and excited electronic states for the description of molecular polaritons.

Quantum chemical methods for the description of molecular polaritonic states in the strong coupling regime based on the Pauli-Fierz Hamiltonian are introduced. Based on a quantum electrodynamic Hartree-Fock (QED-HF) reference, a QED Møller-Plesset perturbation theory of second order for the electronic ground state and a second order quantum electrodynamic algebraic diagrammatic construction scheme for the polarization propagator [QED-ADC(2)] for excited electronic states have been derived, implemented, and tested for polaritons in hydrogen fluoride. Analogous approaches based on a standard non-polaritonic HF reference are also presented and thoroughly compared, both algebraically and numerically, to those based on the QED-HF reference. Furthermore, a promising route to approximate QED-ADC methods based on a unitary transformation of the algebraic expression into a restricted state space is outlined showing excellent agreement in second order with QED-ADC(2). All presented novel methods are compared to and tested against other existing abinitio approaches, mostly QED coupled cluster theory, including single and double excitations, and show qualitative agreement at a reduced computational effort.

All-Optical Control of Rotational Exciton Polaritons Condensate in Perovskite Microcavities

Exciton polariton condensation is a macroscopic quantum state that can be efficiently controlled by an external field through a nonlinear interaction. In an annular potential landscape, a rotational polariton flow with different orbit indexes can be created spontaneously. Control of their superposition states with symmetry broken is of fundamental interest for exotic quantum states. By adopting the nonresonant ring-shaped pumping in a planar perovskite microcavity, we demonstrate the linear coupled counter-rotating polariton states with ordered phase changes and a vortex pairs-petal state at the condensed threshold. The azimuthal indices and inherent flowing velocity can be well controlled by the diameter of the pumping ring and the detuning energy. At high pump density, the petal state becomes unstable and a single vortex generates, indicating the role of the polariton–polariton interaction and exciton depletion in controlling the superposition of the rotational polariton flow. Broken cylindrical symmetry is realized by a pumping ring with radial slots and the consequent dark soliton. An additional phase jump and fractional orbit index of polariton rotation can thus be obtained. Our results suggest the efficient all-optical control of rotational polariton flow and their superposition states with a flexible diameter and a broken cylindrical symmetry at room temperature, which is promising for practical polaritonic circular current-based applications with topological and chiral properties.

Rovibrational Polaritons in Gas-Phase Methane.

Polaritonic states arise when a bright optical transition of a molecular ensemble is resonantly matched to an optical cavity mode frequency. Here, we lay the groundwork to study the behavior of polaritons in clean, isolated systems by establishing a new platform for vibrational strong coupling in gas-phase molecules. We access the strong coupling regime in an intracavity cryogenic buffer gas cell optimized for the preparation of simultaneously cold and dense ensembles and report a proof-of-principle demonstration in gas-phase methane. We strongly cavity-couple individual rovibrational transitions and probe a range of coupling strengths and detunings. We reproduce our findings with classical cavity transmission simulations in the presence of strong intracavity absorbers. This infrastructure will provide a new testbed for benchmark studies of cavity-altered chemistry.

Quantitative Investigation of the Rate of Intersystem Crossing in the Strong Exciton-Photon Coupling Regime.

Strong interactions between excitons and photons lead to the formation of exciton-polaritons, which possess completely different properties compared to their constituents. The polaritons are created by incorporating a material in an optical cavity where the electromagnetic field is tightly confined. Over the last few years, the relaxation of polaritonic states has been shown to enable a new kind of energy transfer event, which is efficient at length scales substantially larger than the typical Förster radius. However, the importance of such energy transfer depends on the ability of the short-lived polaritonic states to efficiently decay to molecular localized states that can perform a photochemical process, such as charge transfer or triplet states. Here, we investigate quantitatively the interaction between polaritons and triplet states of erythrosine B in the strong coupling regime. We analyze the experimental data, collected mainly employing angle-resolved reflectivity and excitation measurements, using a rate equation model. We show that the rate of intersystem crossing from the polariton to the triplet states depends on the energy alignment of the excited polaritonic states. Furthermore, it is demonstrated that the rate of intersystem crossing can be substantially enhanced in the strong coupling regime to the point where it approaches the rate of the radiative decay of the polariton. In light of the opportunities that transitions from polaritonic to molecular localized states offer within molecular photophysics/chemistry and organic electronics, we hope that the quantitative understanding of such interactions gained from this study will aid in the development of polariton-empowered devices.

Degenerate parametric down-conversion facilitated by exciton-plasmon polariton states in a nonlinear plasmonic cavity

We study the effect of degenerate parametric down-conversion (DPDC) in an ensemble of two-level quantum emitters (QEs) coupled via near-field interactions to a single surface plasmon (SP) mode of a nonlinear plasmonic cavity. For this purpose, we develop a quantum driven-dissipative model capturing non-equilibrium dynamics of the system in which incoherently pumped QEs have transition frequency tuned near the second-harmonic response of the SPs. Considering the strong coupling regime, i.e. the SP-QE interaction rate exceeds system dissipation rates, we find a critical SP-QE coupling attributed to the phase transition between normal and lasing steady states. Examining fluctuations above the system’s steady states, we predict new elementary excitations, namely, the exciton-plasmon polaritons formed by the two-SP quanta and single-exciton states of QEs. The contribution of two-SP quanta results in the linear scaling of the SP-QE interaction rate with the number of QEs, , as opposed to the -scaling known for the Dicke and Tavis–Cummings models. We further examine how SP-QE interaction scaling affects the polariton dispersions and power spectra in the vicinity of the critical coupling. For this purpose, we compare the calculation results assuming a finite ensemble of QEs and the model thermodynamic limit. The calculated power spectra predict an interplay of coherent photon emission by QEs near the second-harmonic frequency and correlated photon-pair emission at the fundamental frequency by the SPs (i.e. the photonic DPDC effect).

Hartree method for molecular polaritons

The formation of the composite photonic-excitonic particle, known as a polariton, is a phenomenon emerging in materials possessing strong coupling to light. The organic-based materials besides the strong light-matter coupling also demonstrate strong interaction of electronic and vibrational degrees of freedom. We study the vibration-assisted polariton wave-function evolution treating both types of interactions as equally strong. Using the multiconfiguration Hartree approach we derive the equations of motion for the polariton wave function, where the vibration degrees of freedom interact with the polariton quantum field through the mean-field Hartree term. For the conventional quadratic polariton Hamiltonian and the Holstein-type vibration Hamiltonian (Tavis-Cummings-Holstein model), the obtained equations are in one-to-one correspondence with the original Schr\"odinger equation. In the second part of the paper, we show that our theory reproduces the physical properties of the polariton light emission spectrum. In particular, the theory explains experimental observations of the molecular Stokes shift in the polariton fluorescence spectra in the systems with strong light-matter coupling. We also investigate the behavior of the polariton wave function in the vicinity of the anticrossing point and demonstrate that the Hartree term can produce an infinite potential barrier of a dynamical origin, which is responsible for the formation of the mixed upper-lower polariton states. The nonlinear nature of the polariton theory reflects their collective behavior. We expect that the multiconfiguration Hartree approach being applied to polaritons and similar systems will result in a manifestation of new physical phenomena.

Reply to "Comment on: 'Isolating Vibrational Polariton 2D-IR Transmission Spectra'".

In a Comment on our recent Letter, the authors take issue with our method of refining 2D-IR transmission spectra to remove a background contribution that arises from nonpolaritonic molecules in the cavity. In our response to their Comment, we describe how our approach was motivated by the previous work of the authors, and we present a spatially dependent molecule-cavity Tavis-Cummings model that can account for the significant response from localized molecules with nonzero oscillator strengths. The telltale signature of the localized molecule response is the spectral diffusion dynamics of the bare W(CO)6 molecules in the polar butyl acetate solvent. Inhomogeneous broadening is absent from polaritonic states due to the extreme degree of exchange narrowing in coupling very large numbers of molecules to a cavity mode.

Strong coupling of multiple plasmon modes and excitons with excitation light controlled active tuning

Abstract While the strong coupling between cavity modes and quantum emitters has been investigated in various systems, multiple surface plasmon modes in single nanostructures strongly coupling with excitons are rarely explored. Here, we investigate the strong coupling between three surface plasmon modes in silver nanowires and excitons in monolayer WSe2 at room temperature. Four plasmon-exciton polariton (plexciton) states are observed in the scattering spectra. The photoluminescence (PL) spectra of the hybrid system show clear splitting due to strong coupling, and the energies of the emission corresponding to the two lower plexciton states agree with that of the scattering very well. In addition, we show that the plasmon-exciton interaction in this system can be efficiently tuned by controlling the excitation power. These results reveal the fundamental properties of strong coupling between multiple plasmon modes and excitons, deepen the understanding of the correlation between scattering and PL spectra of plasmon-exciton strong coupling systems, and open up a new way to actively control the coupling between plasmonic nanostructures and two-dimensional semiconductors.

Polaritonic Chern Insulators in Monolayer Semiconductors.

Systems with strong light-matter interaction open up new avenues for studying topological phases of matter. Examples include exciton polaritons, mixed light-matter quasiparticles, where the topology of the polaritonic band structure arises from the collective coupling between matter wave and optical fields strongly confined in periodic dielectric structures. Distinct from light-matter interaction in a uniform environment, the spatially varying nature of the optical fields leads to a fundamental modification of the well-known optical selection rules, which were derived under the plane wave approximation. Here we identify polaritonic Chern insulators by coupling valley excitons in transition metal dichalcogenides to photonic Bloch modes in a dielectric photonic crystal slab. We show that polaritonic Dirac points, which are markers for topological phase transition points, can be constructed from the collective coupling between valley excitons and photonic Dirac cones in the presence of both time-reversal and inversion symmetries. Lifting exciton valley degeneracy by breaking time-reversal symmetry leads to gapped polaritonic bands with nonzero Chern numbers. Through numerical simulations, we predict polaritonic chiral edge states residing inside the topological gaps. Our Letter paves the way for the further study of strong exciton-photon interaction in nanophotonic structures and for exploring polaritonic topological phases and their practical applications in polaritonic devices.

Tailoring photoluminescence of WS2-microcavity coupling devices in broad visible range

Abstract Most of the previous TMDC-photon coupling devices were mainly based on A exciton due to its high oscillator strength and large exciton binding energy. Less effort has been focused on the modulation of the emission of B exciton and Rydberg states in TMDCs, especially in monolayer WS2. Here, we demonstrate that the photoluminescence (PL) emission of WS2-microcavity coupling devices can be tailored in a broad visible wavelength range (490 nm–720 nm). In contrast to the intrinsic PL emission of monolayer WS2, 25-fold enhanced B exciton emission and significant PL emission from the 2s Rydberg state can be observed. From the transient absorption (TA) measurements, the strongly coupled hybrid states based on B exciton can be remarkably fingerprinted. Furthermore, the strongly enhanced PL emission from the coupled B exciton has been demonstrated due to the strongly increased lower polariton (LP) state population and the internal conversion pathway being blocked in the strong coupling regime. Besides, the remarkable PL emission from the 2s Rydberg state is also revealed and confirmed by the additional ground state bleaching signal in TA spectra. These physical mechanisms about tailoring the PL emission in low dimensional TMDCs can provide significant references for constructing highly efficient optoelectronic devices.

Quasistationary Polariton States in Mesocavities

When the strength of light–matter interaction (the Rabi splitting) in mesocavities is comparable to the energy spacing between the cavity modes, an exciton mode is coupled simultaneously to a number of optical modes. It has recently been demonstrated that a nonmonotonic dependence of the population of polariton states in mesocavities on the pump intensity is possible. Here, it is shown that an additional quasistationary state may appear in the hysteresis region and the time spent by the system in this state depends on the pump intensity.

Ultrafast dynamics in plasmon-exciton core-shell systems: the role of heat.

Strong coupling between plasmons and excitons gives rise to new hybrid polariton states with potential applications in various fields. Despite a plethora of research on plasmon-exciton systems, their transient behaviour is not yet fully understood. Besides Rabi oscillations in the first few femtoseconds after optical excitation, coupled systems show interesting non-linear features on the picosecond time scale. Here, we conclusively show that the source of these features is heat that is generated inside the particles. Until now, this hypothesis was only based on phenomenological arguments. We investigate the role of heat by recording the transient spectra of plasmon-exciton core-shell nanoparticles with excitation off the polariton resonance. We present analytical simulations that precisely recreate the measurements solely by assuming an initial temperature rise of the electron gas inside the particles. The simulations combine established strategies for describing uncoupled plasmonic particles with a recently published model for static spectra. The simulations are consistent for various excitation powers, confirming that heating of the particles is indeed the root of the changes in the transient signals.

Multi-resonance TADF in optical cavities: suppressing excimer emission through efficient energy transfer to the lower polariton states

Strong light-matter coupling is exploited as a way to reduce aggregation-induced excimer losses in multi-resonance thermally activated delayed fluorescence emitters.

Enabling multiple intercavity polariton coherences by adding quantum confinement to cavity molecular polaritons

In this study, the "particle in a box" idea, which was broadly developed in semiconductor quantum dot research, was extended into mid-infrared (IR) cavity modes by applying lateral confinement in an optical cavity. The discrete cavity modes hybridized with molecular vibrational modes, resulting in a quartet of polariton states that can support multiple coherence states in the IR regime. We applied tailored pump pulse sequences to selectively prepare these coherences and verified the multi-coherence existence. The simulation based on Lindblad equation showed that because the quartet of polariton states resided in the same cavity, they were specifically robust toward decoherence caused by fluctuations in space. The multiple robust coherences paved the way for entangled states and coherent interactions between cavity polaritons, which would be critical for advancing polariton-based quantum information technology.

Spin-orbital effect on the polariton state in traps

I discuss similitude and differences of spin-orbital effects for electrons in quantum wells with the Rashba coupling and for polaritons in semiconductor microcavities with TE-TM splitting. Contrary to the case of electron, the ground state of polariton in the trap can be non-degenerate and can possess specific polarization structure. For the case of azimuthally symmetric trap and sufficiently strong spin-orbital coupling, the ground state is either radial or azimuthal vortex, depending on the sign of the coupling constant. The effect is strongly enhanced for polaritons trapped in a ring, where even weak TE-TM splitting results in formation of vorticity and definite polarization of the ground state. The Hamiltonian for quasi-1D motion of polaritons in the ring is derived and it is shown the the dispersion of polaritons depend qualitatively on the curvature of the ring.

Tuning and Enhancing Quantum Coherence Time Scales in Molecules via Light-Matter Hybridization.

Protecting quantum coherences in matter from the detrimental effects introduced by its environment is essential to employ molecules and materials in quantum technologies and develop enhanced spectroscopies. Here, we show how dressing molecular chromophores with quantum light in the context of optical cavities can be used to generate quantum superposition states with tunable coherence time scales that are longer than those of the bare molecule, even at room temperature and for molecules immersed in solvent. For this, we develop a theory of decoherence rates for molecular polaritonic states and demonstrate that quantum superpositions that involve such hybrid light-matter states can survive for times that are orders of magnitude longer than those of the bare molecule while remaining optically controllable. Further, by studying these tunable coherence enhancements in the presence of lossy cavities, we demonstrate that they can be enacted using present-day optical cavities. The analysis offers a viable strategy to engineer and increase quantum coherence lifetimes in molecules.

Resonant tunneling diodes in semiconductor microcavities: Modeling polaritonic features in the terahertz displacement current

We develop in this work a simple qualitative quantum electron transport model, in the strong light-matter coupling regime under dipole approximation, able to capture polaritonic signatures in the time-dependent electrical current. The effect of the quantized electromagnetic field in the displacement current of a resonant tunneling diode inside an optical cavity is analyzed. The original peaks of the bare electron transmission coefficient split into two new peaks due to the resonant electron-photon interaction, leading to coherent Rabi oscillations among the polaritonic states that are developed in the system in the strong coupling regime. This mimics known effects predicted by a Jaynes-Cummings model in closed systems, and shows how a full quantum treatment of electrons and electromagnetic fields may open interesting paths for engineering new THz electron devices. The computational burden involved in the multi-time measurements of THz currents is tackled by invoking a Bohmian description of the light-matter interaction. We also show that the traditional static transmission coefficient used to characterize DC quantum electron devices has to be substituted by a new displacement current coefficient in high-frequency AC scenarios.

Optical cavity-mediated exciton dynamics in photosynthetic light harvesting 2 complexes

Strong light-matter interaction leads to the formation of hybrid polariton states and alters the photophysical dynamics of organic materials and biological systems without modifying their chemical structure. Here, we experimentally investigated a well-known photosynthetic protein, light harvesting 2 complexes (LH2) from purple bacteria under strong coupling with the light mode of a Fabry-Perot optical microcavity. Using femtosecond pump probe spectroscopy, we analyzed the polariton dynamics of the strongly coupled system and observed a significant prolongation of the excited state lifetime compared with the bare exciton, which can be explained in terms of the exciton reservoir model. Our findings indicate the potential of tuning the dynamic of the whole photosynthetic unit, which contains several light harvesting complexes and reaction centers, with the help of strong exciton-photon coupling, and opening the discussion about possible design strategies of artificial photosynthetic devices.

Non-Hermitian topological exciton-polariton corner modes

We theoretically study two-dimensional exciton-polariton lattices and predict that non-Hermitian topological corner modes can be formed under non-resonant pumping. As a generalization of the non-Hermitian skin effect, all eigenstates are localized at the two corners in our model. This is also a higher dimensional topology compared to other proposals in exciton-polariton systems and we find that it allows propagating signals in the bulk of the system to travel around defects, which is not possible in one-dimensional topological lattices or two-dimensional lattices with Hermitian edge states. Furthermore, as all polariton states are localized away from an excitation spot, the system offers an opportunity for more accurate measurement of the polariton-polariton interaction strength as the pump-induced exciton-reservoir is spatially separated from all polariton states.

Electrically tuning near-field heat flux using metal–oxide–semiconductor structure considering gradient dielectric function distribution

We build a model to determine the dependency of near-field heat flux on bias voltage using the metal–oxide–semiconductor structures considering gradient distribution of dielectric function. Quantitative dependency of near-field heat flux exchanged by two biased metal–oxide–semiconductor structures on bias voltage is established. The distribution of carrier density and the resultant dielectric function in the semiconductor layer caused by the bias are determined. The corresponding near-field heat flux is calculated using an effective multilayer model. Significant tuning performance is demonstrated, which is due to the increase or decrease in high-frequency surface polariton states induced by the injection or extraction of major carriers. This work deepens the understanding of electrical control of near-field heat transfer with metal–oxide–semiconductor structures, promising for nanoscale thermal management devices and thermal circuits.