Lower Polariton Research Articles

Detuning Effects on the Reverse Intersystem Crossing from Triplet Exciton to Lower Polariton.

The lower polariton (LP) can reduce the energy barrier of the reverse intersystem crossing (rISC) process from T1 to harvest triplet energy for fluorescence. Based on a Tavis-Cummings model including both singlet and triplet excitons, both coupled with quantized photons, we derive here a comprehensive rISC rate formalism. We found that the latter consists of three contributions: the one originated from spin-orbit coupling as first obtained by Martinez-Martinez et al. ( J. Chem. Phys. 2019, 151, 054106), the one from light-matter coupling of Ou et al. ( J. Am. Chem. Soc. 2021, 143, 17786), and the cross-term first reported here. We apply the formalism to investigate the experimentally observed barrier-free rISC (BFrISC) process in cavity devices with DABNA-2 molecular thin film. We found it can be attributed to the detuning effect. The rISC rates can be increased by orders of magnitude through changing the detuning energy to realize the BFrISC process. In addition, the BFrISC rates exhibit a maximum as a function of the incident angle and the doping concentration. The formalism provides a solid ground for molecular design toward highly efficient cavity-promoted light-emitting materials.

Spin-orbit coupling in organic microcavities: Lower polariton splitting, triplet polaritons, and disorder-induced dark-state relaxation

Using an extended Tavis-Cummings model, we study the effect of the spin-orbit coupling between the singlet and the triplet molecular excitons in organic microcavities in the strong coupling regime. The model is solved in the single excitation space for polaritons, which contains the bright (permutation symmetric) singlet and triplet excitons, as well as the dark bands consisting of the nonsymmetric excitons of either type. We find that the spin-orbit coupling splits the lower polariton into two branches, and also creates a triplet polariton when the cavity mode is in resonance with the triplet excitons. The optical absorption spectrum of the system that can reveal this splitting in experiments is presented and the effect of disorder in exciton energies and couplings is explored. An important consequence of the disorder in the spin-orbit coupling -- a weak coupling between the otherwise decoupled bright and dark sectors -- is explored and detailed calculations of the squared transition matrix elements between the dark bands and polaritons are presented along with derivation of some approximate yet quite accurate analytical expressions. This relaxation channel for the dark states contains an interference between two transition paths that, for a given polariton state, suppresses the relaxation of one dark band and enhances it for the other.

Efficient Anisotropic Polariton Lasing Using Molecular Conformation and Orientation in Organic Microcavities

AbstractOrganic exciton‐photon polariton lasers are promising candidates for the efficient generation of coherent light at room temperature. While their thresholds are now comparable with those of conventional organic photon lasers, tuning of molecular conformation and orientation as a means to control fundamental properties of their emission and thus further enhance performance remains largely unexplored. Here, a two‐fold reduction in the threshold of a microcavity polariton laser based on an active layer of poly(9,9‐dioctylfluorene) (PFO) is achieved when 15% β‐phase conformation is introduced. In addtion, taking advantage of the liquid crystalline properties of PFO, a thin photoalignment layer is used to induce nematic alignment of the polymer chains. The resulting transition dipole moment orientation increases the Rabi energy, bringing the system into the ultra‐strong coupling regime and facilitating anisotropic polariton lasing with an eight‐fold reduction in absorbed threshold, down to 1.14 pJ (0.36 µJ cm−2) for the direction parallel to the orientation, with no emission along the orthogonal direction. This represents the first demonstration of anisotropic polariton lasing in conjugated polymer microcavities and a lower threshold than current organic vertical cavity surface‐emitting photon and polariton lasers. Dipole orientation offers new opportunities for switchable, more efficient polaritonic devices, and observation of fundamental effects at low polariton numbers.

Stimulated polariton scattering in an Yb laser pumped lithium niobate based continuous-wave singly resonant optical parametric oscillator

The effective refractive index of a dielectric medium modifies in the infrared spectral band due to a significant contribution from the vibrational modes by virtue of the presence of heavy ions. The coupling between electromagnetic (EM) modes and transverse optical (TO) vibrational modes is the underlying principle behind such a modification. This interaction manifests through the generation of polariton modes that can be observed in the Raman spectrum. However, they do not coincide with the peaks associated with the Raman scattering process. In the present work, we show that the polariton modes can be excited in a lithium niobate based continuous-wave singly resonant optical parametric oscillator (SRO) pumped by a single-frequency, near-infrared Yb-doped fiber laser. The stimulated polariton scattering process is triggered by a strong interaction between the mid-infrared resonant SRO signal beam and TO phonon modes of a lithium niobate crystal, which leads to generation of Stokes mode(s). Such mode(s) can be observed as side peak(s) in the SRO signal spectrum. The generated low energy polariton mode exhibits a dominant EM character, and this provides a plausible route to generate narrow linewidth far-infrared (FIR) or terahertz radiation by utilizing the high intra-cavity signal power in the SRO. In addition, the off-axis FIR generation process assists in reducing the thermal load on the lithium niobate crystal at high pump powers.

Control over Amplification in Exciton Polariton Condensate

Exciton polariton condensates are the most well-studied case of Bose-Einstein condensation (BEC) of quasiparticles. Together with their prominent fundamental importance, the exciton-polariton condensates have a wide spectrum of engineering applications covering interferometry and metrology, different types of SQUIDs and accelerometers, and forming a universal gate set for quantum computing via the control with external laser pulses. The efficient experimental manipulation with the polariton BEC can be realized via the bosonic final-state stimulation, matter-wave amplification, or by lasing of polaritons, but a satisfactory theoretical model for such control has not been developed yet. Here we study the polariton matter-wave amplifier based on the stimulated scattering of massive particles. The amplification of the injected quasiparticles is achieved through an elastic scattering of so-called lower polaritons (LPs). Such an amplifier has many advantages compared with a standard lasing or using a photon amplifier: it can provide a sufficient gain coefficient. To develop an efficient control algorithm for the polariton amplifier we use here the dynamical model for the LP population proposed by Ciuti, Savona, et al. in 1998. The phenomenological model for the gain coefficient is based on the experiments with cold collisions of polaritons performed by Deng, Haug, and Yamamoto in 2010 and later. We use different feedback algorithms (speed gradient vs target attractor) to track efficiently the polariton population in the amplifier. We compare the pros and cons of our alternative approaches and discuss their possible engineering applications.

Dynamics of Spin‐Dependent Polariton–Polariton Interactions in Two‐Dimensional Layered Halide Organic Perovskite Microcavities

AbstractHalf‐light and half‐matter exciton polaritons have demonstrated profound impacts on coherent quantum phenomena. Two‐dimensional (2D) organic−inorganic perovskite semiconductors, exhibiting large exciton binding energy and spin‐based behavior, are an excellent platform for the study of exciton polaritons at room temperature. However, the implementation of these fascinating coherent phenomena is still constrained by the lack of an understanding of the crucial polariton–polariton interaction effects. Here, an experimental observation of spin‐dependent polariton–polariton interactions in a prototype 2D organic−inorganic perovskite microcavity by circularly polarization‐resolved transient absorption (TA) measurements is reported. A power‐dependent blue shift of the lower polariton bands with the same spin is clearly revealed, whereas the energy band of the polariton bands with the opposite spin is almost not changed. Spectacularly, it is found that the energy band blue shift mainly takes place before the spin depolarization, which provides accurate evidence for the spin‐dependent polariton–polariton interactions. Furthermore, the strong coherent polariton–phonon coupling of perovskite microcavity has also been detected firstly in ultrafast TA dynamics. These results demonstrate that microcavity polaritons inheriting the matter properties from their excitonic constituents exhibit strong nonlinear interactions and pave the way for realizing coherent quantum phenomena at room temperature.

Buildup dynamics of room-temperature polariton condensation

By using the femtosecond angle-resolved spectroscopic imaging technique, the ultrafast buildup dynamics of room-temperature polariton condensation is explicitly visualized in a ZnO whispering gallery mode microcavity. The buildup time of polariton condensation with respect to the arrival of the femtosecond pump pulse decreases with the increasing pump power and reaches a lower limit of about 4 ps. Simulation results by numerically solving the open-dissipative Gross-Pitaevskii equation coupled to an incoherent reservoir are in quantitative agreement with the experimental observations, showing that the scattering from exciton reservoir to the lower polariton branches and the decay from these branches dominate the buildup process.

Identifying Vibrations that Control Non-adiabatic Relaxation of Polaritons in Strongly Coupled Molecule-Cavity Systems.

The strong light–matter coupling regime, in which excitations of materials hybridize with excitations of confined light modes into polaritons, holds great promise in various areas of science and technology. A key aspect for all applications of polaritonic chemistry is the relaxation into the lower polaritonic states. Polariton relaxation is speculated to involve two separate processes: vibrationally assisted scattering (VAS) and radiative pumping (RP), but the driving forces underlying these two mechanisms are not fully understood. To provide mechanistic insights, we performed multiscale molecular dynamics simulations of tetracene molecules strongly coupled to the confined light modes of an optical cavity. The results suggest that both mechanisms are driven by the same molecular vibrations that induce relaxation through nonadiabatic coupling between dark states and polaritonic states. Identifying these vibrational modes provides a rationale for enhanced relaxation into the lower polariton when the cavity detuning is resonant with specific vibrational transitions.

Intersubband Polariton-Polariton Scattering in a Dispersive Microcavity.

The ultrafast scattering dynamics of intersubband polaritons in dispersive cavities embedding GaAs/AlGaAs quantum wells are studied directly within their band structure using a noncollinear pump-probe geometry with phase-stable midinfrared pulses. Selective excitation of the lower polariton at a frequency of ∼25 THz and at a finite in-plane momentum k_{‖} leads to the emergence of a narrowband maximum in the probe reflectivity at k_{‖}=0. A quantum mechanical model identifies the underlying microscopic process as stimulated coherent polariton-polariton scattering. These results mark an important milestone toward quantum control and bosonic lasing in custom-tailored polaritonic systems in the mid and far infrared.

Brightening of a dark monolayer semiconductor via strong light-matter coupling in a cavity

Engineering the properties of quantum materials via strong light-matter coupling is a compelling research direction with a multiplicity of modern applications. Those range from modifying charge transport in organic molecules, steering particle correlation and interactions, and even controlling chemical reactions. Here, we study the modification of the material properties via strong coupling and demonstrate an effective inversion of the excitonic band-ordering in a monolayer of WSe2 with spin-forbidden, optically dark ground state. In our experiments, we harness the strong light-matter coupling between cavity photon and the high energy, spin-allowed bright exciton, and thus creating two bright polaritonic modes in the optical bandgap with the lower polariton mode pushed below the WSe2 dark state. We demonstrate that in this regime the commonly observed luminescence quenching stemming from the fast relaxation to the dark ground state is prevented, which results in the brightening of this intrinsically dark material. We probe this effective brightening by temperature-dependent photoluminescence, and we find an excellent agreement with a theoretical model accounting for the inversion of the band ordering and phonon-assisted polariton relaxation.

Vibration‐induced symmetry breaking in hybrid light‐matter dimer states

AbstractWe investigate the influence of vibronic coupling on a molecular dimer strongly coupled to a single cavity mode. In the framework of the Holstein‐Tavis‐Cummings model, the energy structure of the molecular dimer is analyzed by numerical exact diagonalization and perturbation theory. Under numerical exact diagonalization, we find that the degeneracy of lower polaritons vanishes in the presence of vibronic coupling. Under the second‐order degenerate perturbation theory, the degeneracy breaking of lower polaritons can be associated with asymmetric indirect interactions mediated by the upper polaritons and the dark states. The consistency of the two approaches confirms the robustness of our simulations, indicating that the vibration‐induced symmetry breaking should be experimentally observed.

Interplay between Polaritonic and Molecular Trap States.

Strong exciton–photon coupling exhibits the possibility to modify the photophysical properties of organic molecules. This is due to the introduction of hybrid light–matter states, called polaritons, which have unique physical and optical properties. Those strongly coupled systems provide altered excited-state dynamics in comparison to the bare molecule case. In this study, we investigate the interplay between polaritonic and molecular trap states, such as excimers. The molecules used in this study show either prompt or delayed emission from trap states. For both cases, a clear dependency on the exciton–photon energy tuning was observed. Polaritonic emission gradually increased with a concurrent removal of aggregation-induced emission when the systems were tuned toward lower energies. For prompt emission, it is not clear whether the experimental results are best explained by a predominant relaxation toward the lower polariton after excitation or by a direct excimer to polariton transition. However, for the delayed emission case, trap states are formed on the initially formed triplet manifold, making it evident that an excimer-to-polariton transition has occurred. These results unveil the possibility to control the trap state population by creating a strongly coupled system, which may form a mitigation strategy to counteract detrimental trap states in photonic applications.

Generalized Fresnel-Floquet equations for driven quantum materials

Optical drives at terahertz and midinfrared frequencies in quantum materials are increasingly used to reveal the nonlinear dynamics of collective modes in correlated many-body systems and their interplay with electromagnetic waves. Recent experiments demonstrated several surprising optical properties of transient states induced by driving, including the appearance of photo-induced edges in the reflectivity in cuprate superconductors (SCs), observed both below and above the equilibrium transition temperature. Furthermore, in other driven materials, reflection coefficients larger than unity have been observed. In this paper we demonstrate that unusual optical properties of photoexcited systems can be understood from the perspective of a Floquet system, a system with periodically modulated parameters originating from pump-induced oscillations of a collective mode. These oscillations lead to an effective Floquet system with periodically modulated parameters. We present a general phenomenological model of reflectivity from Floquet materials, which takes into account parametric generation of excitation pairs. We find a universal phase diagram of drive-induced features in reflectivity which evidence a competition between driving and dissipation. To illustrate our general analysis, we apply our formalism to two concrete examples motivated by recent experiments: A single plasmon band, which describes Josephson plasmons (JPs) in layered SCs, and a phonon-polariton system, which describes upper and lower polaritons in materials such as insulating SiC. Finally, we demonstrate that our model can be used to provide an accurate fit to results of phonon-pump--terahertz-probe experiments in the high-temperature SC ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.5}$. Our model explains the appearance of a pump-induced edge, which is higher in energy than the equilibrium JP edge, even if the interlayer Josephson coupling is suppressed by the pump pulse.

Polariton relaxation under vibrational strong coupling: Comparing cavity molecular dynamics simulations against Fermi's golden rule rate.

Under vibrational strong coupling (VSC), the formation of molecular polaritons may significantly modify the photo-induced or thermal properties of molecules. In an effort to understand these intriguing modifications, both experimental and theoretical studies have focused on the ultrafast dynamics of vibrational polaritons. Here, following our recent work [Li et al., J. Chem. Phys. 154, 094124 (2021)], we systematically study the mechanism of polariton relaxation for liquid CO2 under a weak external pumping. Classical cavity molecular dynamics (CavMD) simulations confirm that polariton relaxation results from the combined effects of (i) cavity loss through the photonic component and (ii) dephasing of the bright-mode component to vibrational dark modes as mediated by intermolecular interactions. The latter polaritonic dephasing rate is proportional to the product of the weight of the bright mode in the polariton wave function and the spectral overlap between the polariton and dark modes. Both these factors are sensitive to parameters such as the Rabi splitting and cavity mode detuning. Compared to a Fermi's golden rule calculation based on a tight-binding harmonic model, CavMD yields a similar parameter dependence for the upper polariton relaxation lifetime but sometimes a modest disagreement for the lower polariton. We suggest that this disagreement results from polariton-enhanced molecular nonlinear absorption due to molecular anharmonicity, which is not included in our analytical model. We also summarize recent progress on probing nonreactive VSC dynamics with CavMD.

Not dark yet for strong light-matter coupling to accelerate singlet fission dynamics

SummaryPolaritons are unique hybrid light-matter states that offer an alternative way to manipulate chemical processes. In this work, we show that singlet fission dynamics can be accelerated under strong light-matter coupling. For superexchange-mediated singlet fission, state mixing speeds up the dynamics in cavities when the lower polariton is close in energy to the multiexcitonic state. This effect is more pronounced in non-conventional singlet fission materials in which the energy gap between the bright singlet exciton and the multiexcitonic state is large ( eV). In this case, the dynamics is dominated by the polaritonic modes and not by the bare-molecule-like dark states, and, additionally, the resonant enhancement due to strong coupling is robust even for energetically broad molecular states. The present results provide a new strategy to expand the range of suitable materials for efficient singlet fission by making use of strong light-matter coupling.

Formation of matter-wave polaritons in an optical lattice

The polariton, a quasiparticle formed by strong coupling of a photon to a matter excitation, is a fundamental ingredient of emergent photonic quantum systems ranging from semiconductor nanophotonics to circuit quantum electrodynamics. Exploiting the interaction between polaritons has led to the realization of superfluids of light as well as of strongly correlated phases in the microwave domain, with similar efforts underway for microcavity exciton-polaritons. Here, we develop an ultracold-atom analogue of an exciton-polariton system in which interacting polaritonic phases can be studied with full tunability and without dissipation. In our optical-lattice system, the exciton is replaced by an atomic excitation, while an atomic matter wave is substituted for the photon under a strong dynamical coupling. We access the band structure of the matter-wave polariton spectroscopically by coupling the upper and lower polariton branches, and explore polaritonic many-body transport in the superfluid and Mott-insulating regimes, finding quantitative agreement with our theoretical expectations. Our work opens up novel possibilities for studies of polaritonic quantum matter.

Driving chemical reactions with polariton condensates

When molecular transitions strongly couple to photon modes, they form hybrid light-matter modes called polaritons. Collective vibrational strong coupling is a promising avenue for control of chemistry, but this can be deterred by the large number of quasi-degenerate dark modes. The macroscopic occupation of a single polariton mode by excitations, as observed in Bose-Einstein condensation, offers promise for overcoming this issue. Here we theoretically investigate the effect of vibrational polariton condensation on the kinetics of electron transfer processes. Compared with excitation with infrared laser sources, the vibrational polariton condensate changes the reaction yield significantly at room temperature due to additional channels with reduced activation barriers resulting from the large accumulation of energy in the lower polariton, and the many modes available for energy redistribution during the reaction. Our results offer tantalizing opportunities to use condensates for driving chemical reactions, kinetically bypassing usual constraints of fast intramolecular vibrational redistribution in condensed phase.

Slow-light dispersion of a cavity polariton in an organic crystal microcavity

Herein, we have measured the phase dispersion of a cavity polariton in a single-crystalline anthracene microcavity via interference spectroscopy. Results show slow-light dispersion at the cavity polariton's lower polariton resonance, which agrees well with the transfer matrix method calculation results. According to the theoretical calculations, the slow-light dispersion's group refractive index is 24. We found that the structural dispersion due to a phenomenological photon cavity and material dispersion caused by the exciton contributed to the large group refractive index. These efforts will aid in realizing optical buffer memories for organic microcavities.

Ultrafast vibrational excitation transfer on resonant antenna lattices revealed by two-dimensional infrared spectroscopy.

High-quality lattice resonances in arrays of infrared antennas operating in an open-cavity regime form polariton states by means of strong coupling to molecular vibrations. We studied polaritons formed by carbonyl stretching modes of (poly)methyl methacrylate on resonant antenna arrays using femtosecond 2DIR spectroscopy. At a normal incidence of excitation light, doubly degenerate antenna-lattice resonances (ALRs) form two polariton states: a lower polariton and an upper polariton. At an off-normal incidence geometry of 2DIR experiments, the ALR degeneracy is lifted and, consequently, the polariton energies are split. We spectrally resolved and tracked the time-dependent evolution of a cross-peak signal associated with the excitation of reservoir states and the unidirectional transfer of the excess energy to lower polaritons. Bi-exponential decay of the cross-peak suggests that a reversible energy exchange between the bright and dark lower polaritons occurs with a characteristic transfer time of ∼200fs. The cross-peak signal further decays within ∼800fs, which is consistent with the relaxation time of the carbonyl stretching vibration and with the dephasing time of the ALR. An increase in the excitation pulse intensity leads to saturation of the cross-peak amplitude and a modification of the relaxation dynamics. Using quantum-mechanical modeling, we found that the kinetic scheme that captures all the experimental observations implies that only the bright lower polariton accepts the energy from the reservoir, suggesting that transfer occurs via a mechanism involving dipole-dipole interaction. An efficient reservoir-to-polariton transfer can play an important role in developing novel room-temperature quantum optical devices in the mid-infrared wavelength region.

Strong coupling between colloidal quantum dots and a microcavity with hybrid structure at room temperature

The interaction between light and matter has always been the focus of quantum science, and the realization of truly strong coupling between an exciton and the optical cavity is a basis of quantum information systems. As a special semiconductor material, colloidal quantum dots have fascinating optical properties. In this study, the photoluminescence spectra of colloidal quantum dots are measured at different collection angles in microcavities based on hybrid refractive-index waveguides. The photon bound states in the continuum are found in the low–high–low refractive-index hybrid waveguides in the appropriate waveguide width region, where the photoluminescence spectra of colloidal quantum dots split into two or more peaks. The upper polaritons and lower polaritons avoid resonance crossings in the systems. The Rabi splitting energy of 96.0 meV can be obtained. The observed phenomenon of vacuum Rabi splitting at room temperature is attributed to the strong coupling between quantum dots and the bound states in the continuum.