Lower Polariton Research Articles

Thermalization of Fluorescent Protein Exciton-Polaritons at Room Temperature.

Fluorescent proteins (FPs) have recently emerged as a serious contender for realizing ultralow threshold room temperature exciton-polariton condensation and lasing. This contribution investigates the thermalization of FP microcavity exciton-polaritons upon optical pumping under ambient conditions. Polariton cooling is realized using a new FP molecule, called mScarlet, coupled strongly to the optical modes in a Fabry-Pérot cavity. Interestingly, at the threshold excitation energy (fluence) of ≈9 nJ per pulse (15.6 mJ cm-2 ), an effective temperature is observed, Teff ≈ 350 ± 35 K close to the lattice temperature indicative of strongly thermalized exciton-polaritons at equilibrium. This efficient thermalization results from the interplay of radiative pumping facilitated by the energetics of the lower polariton branch and the cavity Q-factor. Direct evidence for dramatic switching from an equilibrium state into a metastable state is observed for the organic cavity polariton device at room temperature via deviation from the Maxwell-Boltzmann statistics at k‖ = 0 above the threshold. Thermalized polariton gases in organic systems at equilibrium hold substantial promise for designing room temperature polaritonic circuits, switches, and lattices for analog simulation.

Polariton Dynamics in Two-Dimensional Ruddlesden-Popper Perovskites Strongly Coupled with Plasmonic Lattices.

Strong coupling between light and matter can produce hybrid eigenstates known as exciton-polaritons. Although polariton dynamics are important photophysical properties, the relaxation pathways of polaritons in different coupling regimes have seen limited attention. This paper reports the dynamics of hybridized states from 2D Ruddlesden-Popper perovskites coupled to plasmonic nanoparticle lattices. The open cavity architecture of Al lattices enables the coupling strength to be modulated by varying either the lead halide perovskite film thickness or the superstrate refractive index. Both experiments and finite-difference time-domain simulations of the optical dispersion diagrams showed avoided crossings that are a signature of strong coupling. Our analytical model also elucidated the correlation between the exciton/plasmon mixing ratio and polariton coupling strength. Using fs-transient absorption spectroscopy, we found that both the upper and lower polaritons have shorter lifetimes than the excitons and that polaritons can show faster excited-state dynamics when they have access to additional energy transfer channels.

Highly efficient polariton emission of an ultrastrongly coupled MDMO-PPV OLED

We report on a highly efficient polariton emission of poly [2-methoxy-5-(3, 7-dimethyloctyloxy)-1, 4-phenylenevinylene] (MDMO-PPV) organic light-emitting diode (OLED) operating in the ultrastrong coupling regime. MDMO-PPV film with a broadband high absorbance and modest Stokes shift is shown to enable the generation of a large coupling strength and radiative pumping of the lower polariton (LP) mode. Further combining the optimized conditions of cavity design, electron injection, and emission side mirror, we demonstrate that the polariton OLED can produce a spectrally narrow and almost angle-independent emission from the LP mode, with a maximum external quantum efficiency of >0.1% and a Rabi splitting energy of >0.8 eV.

Analysis of luminescence decay dynamics in metal-dielectric photonic structures with organic layers

Metal-dielectric photonic structures with organic materials 4,4-bis(N-carbazolyl)-1,1-biphenyl and 4,4'-bis[4-(di-ptolylamino)styryl]biphenyl) as light emitting layers were investigated. The formation of the polaritonic modes in the investigated structures was experimentally demonstrated and a correlation between theoretical and experimental dispersion was shown. Was shown, that increase in interaction between organic exciton and optical mode leads to a significant decrease in the lower polariton emission bandwidth. Keywords: light-emitting layers, exciton, luminescence.

Self-Trapping Phenomenon in an Exciton-Polariton System When the Lower Polariton Branch is Pumped by Two Laser Pulses with Close Frequencies

The dynamics of exciton-polariton states in a semiconductor microcavity is studied under pumping of the state corresponding to the lower polariton branch, taking into account single-mode and intermode elastic polariton-polariton interactions. In this case, pumping is carried out by two laser pulses with close frequencies. It is shown that at the initial phase difference equal to theta0=π/2, the regime of quantum self-trapping of exciton-polaritons is observed. Periodic and aperiodic modes of evolution of a system of quasiparticles are obtained. Keywords: exciton-polariton states, quantum self-trapping, periodic and apeiodic regimes.

Анализ динамики затухания люминесценции в металл-диэлектрических фотонных структурах с органическими слоями

Metal-dielectric photonic structures with organic materials 4,4-bis(N-carbazolyl)-1,1-biphenyl and 4,4′-bis[4-(di-ptolylamino)styryl] biphenyl) as light emitting layers were investigated. The formation of the polaritonic modes in the investigated structures was experimentally demonstrated and a correlation between theoretical and experimental dispersion was shown. Was shown, that increase in interaction between organic exciton and optical mode leads to a significant decrease in the lower polariton emission bandwidth.

Явление самозахвата в системе экситон-поляритонов при накачке нижней поляритонной ветви двумя лазерными импульсами с близкими частотами

The dynamics of exciton-polariton states in a semiconductor microcavity is studied under pumping of the state corresponding to the lower polariton branch, taking into account single-mode and intermode elastic polariton-polariton interactions. In this case, pumping is carried out by two laser pulses with close frequencies. It is shown that at the initial phase difference equal to θ_0=π⁄2, the regime of quantum self-trapping of exciton-polaritons is observed. Periodic and aperiodic modes of evolution of a system of quasiparticles are obtained.

Snapshot Angle-Resolved Spectroscopy and Its Application for Study of Highly Efficient Polariton OLEDs

The multifunctional snapshot angle-resolved spectroscopy (ARS) system capable of electroluminescence, photoluminescence, and reflectance measurements for thin film devices is developed based on the k-space imaging technique. Compared with the conventional goniometric ARS system, this snapshot spectroscopy system offers great advantages of rapid and simple measurement, suitable for characterizing thin film devices that are unstable or degraded under long-time or high-power driving conditions, such as OLEDs. We perform a detailed calibration of the snapshot system and show that the measured results closely match with those obtained using a goniometric system. Furthermore, we show the capabilities of the system with application in studying polariton OLEDs. The result provides comprehensive information on the polariton mode dispersion and emission distribution, and shows an effective radiative pumping of the lower polariton branch for high emission efficiency.

Phonon signatures in spectra of exciton polaritons in transition metal dichalcogenides

Embedding a monolayer of a transition metal dichalcogenide in a high-$Q$ optical cavity results in the formation of distinct exciton polariton modes. The polaritons are affected by the strong exciton-phonon interaction in the monolayer. We use a time convolutionless master equation to calculate the phonon influence on the spectra of the polaritons. We discuss the nontrivial dependence of the line shapes of both branches on temperature and detuning. The peculiar polariton dispersion relation results in a linewidth of the lower polariton being largely independent of the coupling to acoustic phonons. For the upper polariton, acoustic phonons lead to a low-energy shoulder of the resonance in the linear response. Furthermore, we analyze the influence of inhomogeneous broadening being the dominant contribution to the lower polariton linewidth at low temperatures. Our results point towards interesting phonon features in polariton spectra in transition metal dichalcogenides.

Low‐Threshold Exciton‐Polariton Condensation via Fast Polariton Relaxation in Organic Microcavities

AbstractIn organic microcavities, a macroscopic condensate of exciton‐polaritons can be formed at high‐exciton polariton densities. The threshold for forming this condensate is proportional to the relaxation rate from initially excited excitons to these polaritons and the lifetime of the lowest energy polariton states. Although the influence of the lower polariton (LP) lifetime on the threshold has been studied, the relationship between the polariton relaxation rate and the threshold has not been fully explored. In this study, a room‐temperature polariton condensate is demonstrated at a threshold pump fluence of 9.7 ± 0.1 µJ cm−2, in a microcavity containing 4,4″‐bis((E)‐4‐(3,6‐bis(2‐ethylhexyl)‐(9H‐carbazol‐9‐yl))styryl)‐1,1″‐biphenyl (BSBCz‐EH). By using a semiclassical model to describe the polariton kinetics, it is revealed that this low threshold results from the rapid relaxation rate from the dark exciton reservoir to the set of the LP states forming the condensate, with an effective rateWep ≈ 2.0 × 10−5cm3s−1. These results show that accelerating polariton relaxation is possible and is an important factor for realizing low‐threshold polariton condensates.

Topological Control of 2D Perovskite Emission in the Strong Coupling Regime.

Momentum space topology can be exploited to manipulate radiation in real space. Here we demonstrate topological control of 2D perovskite emission in the strong coupling regime via polaritonic bound states in the continuum (BICs). Topological polarization singularities (polarization vortices and circularly polarized eigenstates) are observed at room temperature by measuring the Stokes parameters of photoluminescence in momentum space. Particularly, in symmetry-broken structures, a very large degree of circular polarization (DCP) of ∼0.835 is achieved in the perovskite emission, which is the largest in perovskite materials to our knowledge. In the strong coupling regime, lower polariton modes shift to the low-loss spectral region, resulting in strong emission enhancement and large DCP. Our reciprocity analysis reveals that DCP is limited by material absorption at the emission wavelength. Polaritonic BICs based on 2D perovskite materials combine unique topological features with exceptional material properties and may become a promising platform for active nanophotonic devices.

Strong Coupling in Semiconductor Hyperbolic Metamaterials.

Semiconductor-based layered hyperbolic metamaterials (HMMs) house high-wavevector volume plasmon polariton (VPP) modes in the infrared spectral range. VPP modes have successfully been exploited in the weak-coupling regime through the enhanced Purcell effect. In this paper, we experimentally demonstrate strong coupling between the VPP modes in a semiconductor HMM and the intersubband transition of epitaxially embedded quantum wells. We observe clear anticrossings in the dispersion curves for the zeroth-, first-, second-, and third-order VPP modes, resulting in upper and lower polariton branches for each mode. This demonstration sets the stage for the creation of novel infrared optoelectronic structures combining HMMs with embedded epitaxial emitter or detector structures.

Enhanced Reverse Intersystem Crossing Promoted by Triplet Exciton-Photon Coupling.

Polaritons are hybrid light-matter states formed via strong coupling between excitons and photons inside a microcavity, leading to upper and lower polariton (LP) bands splitting from the exciton. The LP has been applied to reduce the energy barrier of the reverse intersystem crossing (rISC) process from T1, harvesting triplet energy for fluorescence through thermally activated delayed fluorescence. The spin-orbit coupling between T1 and the excitonic part of the LP was considered as the origin for such an rISC transition. Here we propose a mechanism, namely, rISC promoted by the light-matter coupling (LMC) between T1 and the photonic part of LP, which is originated from the ISC-induced transition dipole moment of T1. This mechanism was excluded in previous studies. Our calculations demonstrate that the experimentally observed enhancement to the rISC process of the erythrosine B molecule can be effectively promoted by the LMC between T1 and a photon. The proposed mechanism would substantially broaden the scope of the molecular design toward highly efficient cavity-promoted light-emitting materials and immediately benefit the illumination of related experimental phenomena.

Polariton Decay in Donor-Acceptor Cavity Systems.

Enhanced delocalization is beneficial for absorbing molecules in organic solar cells, and in particular bilayer devices, where excitons face small diffusion lengths as a barrier to reaching the charge-generating donor-acceptor interface. As hybrid light-matter states, polaritons offer exceptional delocalization which could be used to improve the efficiency of bilayer organic photovoltaics. Polariton delocalization can aid in delivering excitons to the donor-acceptor interface, but the subsequent charge transfer event must compete with the fast decay of the polariton. To evaluate the viability of polaritons as tools to improve bilayer organic solar cells, we studied the decay of the lower polariton in three cavity systems: a donor only, a donor-acceptor bilayer, and a donor-acceptor blend. Using several spectroscopic techniques, we identified an additional decay pathway through charge transfer for the polariton in the bilayer cavity, demonstrating charge transfer from the polariton is fast enough to outcompete the decay to the ground state.

Enhanced Light–Matter Interaction and Polariton Relaxation by the Control of Molecular Orientation

AbstractExciton‐polaritons, in which the electronic state of an excited organic molecule and a photonic state are strongly coupled, can form a Bose–Einstein condensate (BEC) at room temperature. However, so far, the reported thresholds of organic polariton BECs under optical excitation are as high asPth = 11–500 μJ cm–2. One route toward lowering the condensation threshold is to increase the Rabi energy by aligning the molecular transition dipole moments. In this report, it is demonstrated that control of the orientation of a perylene‐based discotic dye, which is able to self‐organize in mesogenic columnar structures, can significantly enhance exciton–photon interaction and polariton relaxation rate in optical cavities. These results show the importance of the molecular orientation for strong light–matter interactions and provide a promising strategy toward the realization of an organic low threshold polariton BEC system and electrically driven organic polariton BEC.

Non-polar GaN/AlGaN quantum-well polariton laser at room temperature

We demonstrate room temperature (RT) polariton lasing in an all-dielectric microcavity containing non-polar III-nitride quantum wells (QWs) as active media. The microcavity is fabricated using the photo-electrochemical etching method, by which an optimally grown $m$-plane III-nitride active region is detached from the substrate in the form of a membrane, which is subsequently inserted between two $\mathrm{Si}{\mathrm{O}}_{2}/{\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$ distributed Bragg reflectors, with 4 and 10 pairs for the top and bottom mirrors, respectively. The active region consists of 25 $\mathrm{GaN}/{\mathrm{Al}}_{0.1}{\mathrm{Ga}}_{0.9}\mathrm{N}$ (5 nm/3 nm) QWs. The produced microcavities exhibit two closely spaced polarization-resolved lower polariton branches at RT, in line with the selection rules of the non-polar orientation, having a Rabi splitting of 62 and 72 meV in the $E\ensuremath{\parallel}a$ and $E\ensuremath{\parallel}c$ polarizations, respectively. In a positively detuned 3\ensuremath{\lambda}/2-thick microcavity, polariton lasing is observed at ambient conditions in the $E\ensuremath{\parallel}a$ polarization, with a threshold \ensuremath{\sim} 3 times lower than previous state of the art, despite the use of a relatively weak top reflector.

Strong Coupling Between Plasmons and Molecular Excitons in Metal–Organic Frameworks

This Letter describes strong coupling of densely packed molecular emitters in metal-organic frameworks (MOFs) and plasmonic nanoparticle (NP) lattices. Porphyrin-derived ligands with small transition dipole moments in an ordered MOF film were grown on Ag NP arrays. Angle-resolved optical measurements of the MOF-NP lattice system showed the formation of a polariton that is lower in energy and does not cross the uncoupled MOF Q1 band. Modeling predicted the upper polariton energy and a calculated Rabi splitting of 110 meV. The coupling strength was systematically controlled by detuning the plasmon energy by changing the refractive index of the solvents infiltrating the MOF pores. Through transient absorption spectroscopy, we found that the lower polariton decays quickly at shorter time scales (<500 ps) and slowly at longer times because of energy transfer from the upper polariton. This hybrid system demonstrates how MOFs can function as an accessible excitonic material for polariton chemistry.

Strong coupling of Bloch surface waves and excitons in ZnO up to 430 K

We report on the investigation and observation of Bloch surface wave polaritons, resulting from the interaction between excitons in ZnO and a Bloch surface wave supported by a distributed Bragg reflector (DBR), for temperatures up to 430 K. The samples were fabricated using pulsed laser deposition and consist of a DBR made of 6.5 layer pairs of yttrica-stabilized zirconia and Al2O3 with a ZnO surface layer. We measured the reflectivity of transverse electric modes using a SiO2 prism in Kretschmann–Raether configuration, giving access to high in-plane momenta. Whereas the lower polariton branch was clearly observable, the upper polariton branch was not visible, due to the strong absorption in ZnO above the excitonic resonance. By employing a coupled oscillator model for the interaction between the bare surface mode and exciton, we derived a corresponding Rabi splitting between 100–192 meV at 294 K, which decreases with increasing temperature.

Defect Polaritons from First Principles.

Control over the optical properties of defects in solid-state materials is necessary for their application in quantum technologies. In this study, we demonstrate, from first principles, how to tune these properties via the formation of defect polaritons in an optical cavity. We show that the polaritonic splitting that shifts the absorption energy of the lower polariton is much higher than can be expected from a Jaynes-Cummings interaction. We also find that the absorption intensity of the lower polariton increases by several orders of magnitude, suggesting a possible route toward overcoming phonon-limited single-photon emission from defect centers. These findings are a result of an effective continuum of electronic transitions near the lowest-lying electronic transition that dramatically enhances the strength of the light-matter interaction. We expect our findings to spur experimental investigations of strong light-matter coupling between defect centers and cavity photons for applications in quantum technologies.

Mechanisms for nonlinear refractive index in organic cavity polaritons

The nonlinear optical response of organic polaritonic matter has received increasing attention due to their enhanced and controllable nonlinear response and their potential for novel optical devices such as compact photon sources and optical and quantum information devices. Using $z$ scans at different wavelengths and incident powers we have studied the nonlinear optical dispersion of ultrastrongly coupled organic cavity polaritons near the lower polariton band. We show that the up to 150-fold enhancement of the nonlinear response compared to a cavityless organic film arises from an intensity-dependent polaritonic resonant frequency shift (``blueshift''). Consequently, we find that these $z$-scan data can only be described by several terms of a power series expansion in intensity whose respective contributions depend on power broadening and detuning from the lower polariton band. We further show that the nonlinear response can be quantitatively described by a semiclassical three-level molecular model coupled to the cavity in which saturation reduces the Rabi splitting, thus accounting for the lower polariton band's observed blueshift.