Polaritons Research Articles

Manipulating nonlinear exciton polaritons in an atomically-thin semiconductor with artificial potential landscapes

Exciton polaritons in atomically thin transition-metal dichalcogenide microcavities provide a versatile platform for advancing optoelectronic devices and studying the interacting Bosonic physics at ambient conditions. Rationally engineering the favorable properties of polaritons is critically required for the rapidly growing research. Here, we demonstrate the manipulation of nonlinear polaritons with the lithographically defined potential landscapes in monolayer WS2 microcavities. The discretization of photoluminescence dispersions and spatially confined patterns indicate the deterministic on-site localization of polaritons by the artificial mesa cavities. Varying the trapping sizes, the polariton-reservoir interaction strength is enhanced by about six times through managing the polariton–exciton spatial overlap. Meanwhile, the coherence of trapped polaritons is significantly improved due to the spectral narrowing and tailored in a picosecond range. Therefore, our work not only offers a convenient approach to manipulating the nonlinearity and coherence of polaritons but also opens up possibilities for exploring many-body phenomena and developing novel polaritonic devices based on 2D materials.

Charged biexciton polaritons sustaining strong nonlinearity in 2D semiconductor-based nanocavities

Controlling the interaction between light and matter at micro- and nano-scale can provide new opportunities for modern optics and optoelectronics. An archetypical example is polariton, a half-light-half-matter quasi particle inheriting simultaneously the robust coherence of light and the strong interaction of matter, which plays an important role in many exotic phenomena. Here, we open up a new kind of cooperative coupling between plasmon and different excitonic complexes in WS2-silver nanocavities, namely plasmon-exciton-trion-charged biexciton four coupling states. Thanks to the large Bohr radius of up to 5 nm, the charged biexciton polariton exhibits strong saturation nonlinearity, ~30 times higher than the neutral exciton polariton. Transient absorption dynamics further reveal the ultrafast many-body interaction nature, with a timescale of <100 fs. The demonstration of biexciton polariton here combines high nonlinearity, simple processing and strong scalability, permitting access for future energy-efficient optical switching and information processing.

Nonlinear self-action of ultrashort guided exciton–polariton pulses in dielectric slab coupled to 2D semiconductor

Recently reported large values of exciton–polariton nonlinearity of transition metal dichalcogenide (TMD) monolayers coupled to optically resonant structures approach the values characteristic for GaAs-based systems in the regime of strong light-matter coupling. Contrary to the latter, TMD-based polaritonic devices remain operational at ambient conditions and therefore have greater potential for practical nanophotonic applications. Here, we present the study of the nonlinear properties of Ta2O5 slab waveguide coupled to a WSe2 monolayer. We confirm that the hybridization between the waveguide mode and the exciton resonance in WSe2 gives rise to the formation of guided exciton–polaritons with Rabi splitting of 36 meV. By measuring transmission of ultrashort optical pulses through this TMD-based polaritonic waveguide, we demonstrate the strong nonlinear dependence of the output spectrum on the input pulse energy. We develop a theoretical model that shows agreement with the experimental results and gives insights into the dominating microscopic processes which determine the nonlinear pulse self-action: Coulomb exciton–exciton interaction and scattering to an incoherent excitonic reservoir. Based on the numerical simulation of nonlinear phenomena in our polariton system, we conclude that it may support a quasi-stationary solitonic regime of pulse propagation at intermediate pump energies. Our results provide an important step for the development of nonlinear on-chip polaritonic devices based on 2D semiconductors.

Efficient transfer of metallophosphor excitons via confined polaritons in organic nanocrystals

We investigate the transfer of phosphorescent energy between co-assembled metallophosphors in crystalline nanostructures [Angew. Chem. Int. Ed. 57 7820 (2018) and J. Am. Chem. Soc. 140 4269 (2018)]. Neither Dexter’s nor Förster’s mechanism of resonance energy transfer (RET) could account fully for the observed rates, which exceed 85% with significant temperature dependence. But there exists an alternative pathway on RET mediated by intermediate states of resonantly confined exciton–polaritons. Such a mechanism was used to analyze artificial photosynthesis in organic fluorescents [Phys. Rev. Lett. 122 257402 (2019)]. For metallophosphors, the confined modes act as extended states lying between the molecular S1 and T1 states, offering a bridge for the long-lived T1 excitons to migrate from donors to acceptors. Population dynamics with parameters taken entirely based on experiments fits the observed lifetimes of phosphorescence across a broad range of doping and temperature.

Natural exceptional points in the excitation spectrum of a light–matter system

In this work, we observe natural exceptional points in the excitation spectrum of an exciton–polariton system by optically tuning the light–matter interactions. The observed exceptional points do not require any spatial or polarization degrees of freedom and result solely from the transition from weak to strong light–matter coupling. It was demonstrated that they do not coincide with the threshold for photon lasing, confirming previous theoretical predictions [Phys. Rev. Lett. 122, 185301 (2019)PRLTAO0031-900710.1103/PhysRevLett.122.185301, Optica 7, 1015 (2020)OPTIC82334-253610.1364/OPTICA.397378]. Using a technique where a strong coherent laser pump induces up-converted excitations, we encircle the exceptional point in the parameter space of coupling strength and particle momentum. Our method of local optical control of light–matter coupling paves the way to the investigation of fundamental phenomena, including dissipative phase transitions and non-Hermitian topological states.

High sensitivity gas sensor based on surface exciton polariton enhanced photonic spin Hall effect.

In this paper, the sub-wavelength transverse displacement of photonic spin Hall effect (PSHE) is significantly enhanced by the surface exciton polariton (SEP) for application in gas sensing. The transverse displacement of 14.4 times the wavelength of incident light is achieved with the SEP enhanced PSHE, which is about 3 times that of surface plasmon resonance enhanced PSHE. A gas sensor based on SEP enhanced PSHE is proposed for the detection of SO2, and the refractive index sensitivity of 6320.4 µm/RIU is obtained in the refractive index range from 1.00027281 to 1.00095981. These results undoubtedly demonstrate SEP to be a promising mechanism for PSHE enhancement, and open up new opportunities for highly sensitive gas sensing, biosensing, and chemical sensing.

Vibrational polariton transport in disordered media.

Chemical reactions and energy transport phenomena have been experimentally reported to be significantly affected by strong light-matter interactions and vibrational polariton formation. These quasiparticles exhibit nontrivial transport phenomena due to the long-range correlations induced by the photonic system and elastic and inelastic scattering processes driven by matter disorder. In this article, we employ the Ioffe-Regel criterion to obtain vibrational polariton mobility edges and to identify distinct regimes of delocalization and transport under variable experimental conditions of light-matter detuning, disorder, and interaction strength. Correlations between the obtained trends and recent observations of polariton effects on reactivity are discussed, and essential differences between transport phenomena in organic electronic exciton and vibrational polaritons are highlighted. Our transport diagrams show the rich diversity of transport phenomena under vibrational strong coupling and indicate that macroscopic delocalization is favored at negative detuning and large light-matter interaction strength. We also find the surprising feature that, despite the presence of dephasing-induced inelastic scattering processes, macroscopic lower polariton delocalization and wave transport are expected to persist experimentally, even in modes with small photonic weight.

Ultrafast imaging of polariton propagation and interactions

Semiconductor excitations can hybridize with cavity photons to form exciton-polaritons (EPs) with remarkable properties, including light-like energy flow combined with matter-like interactions. To fully harness these properties, EPs must retain ballistic, coherent transport despite matter-mediated interactions with lattice phonons. Here we develop a nonlinear momentum-resolved optical approach that directly images EPs in real space on femtosecond scales in a range of polaritonic architectures. We focus our analysis on EP propagation in layered halide perovskite microcavities. We reveal that EP–phonon interactions lead to a large renormalization of EP velocities at high excitonic fractions at room temperature. Despite these strong EP–phonon interactions, ballistic transport is maintained for up to half-exciton EPs, in agreement with quantum simulations of dynamic disorder shielding through light-matter hybridization. Above 50% excitonic character, rapid decoherence leads to diffusive transport. Our work provides a general framework to precisely balance EP coherence, velocity, and nonlinear interactions.

Ultrafast Optical Control of Polariton Energy in an Organic Semiconductor Microcavity

AbstractThe manipulation of exciton–polaritons and their condensates is of great interest due to their applications in polariton simulators and high‐speed, all‐optical logic devices. Until now, methods of trapping and manipulating such condensates are not dynamically reconfigurable or resulted in an undesirable reduction in the exciton–photon coupling strength. Here, a new strategy for the ultrafast control of polariton resonances via transient modification of an optical cavity mode is presented. Multilayer organic semiconductor microcavities that contain two absorbers are constructed: one strongly‐coupled to the cavity photon mode and one that is out‐of‐resonance. By selectively exciting the out‐of‐resonance absorber using ultrashort laser pulses, the cavity refractive index is modulated, and fully‐reversible blueshifts of the lower polariton branch by up to 8 meV in sub‐ps timescales with no corresponding reduction in the exciton–photon coupling strength are generated. This work demonstrates the ability to manipulate polariton energy landscapes over ultrafast timescales with important applications in emerging computing technologies.

Investigating Molecular Exciton Polaritons Using Ab Initio Cavity Quantum Electrodynamics.

Coupling molecules to the quantized radiation field inside an optical cavity creates a set of new photon-matter hybrid states called polariton states. We combine electronic structure theory with quantum electrodynamics (QED) to investigate molecular polaritons using ab initio simulations. This framework joins unperturbed electronic adiabatic states with the Fock state basis to compute the eigenstates of the QED Hamiltonian. The key feature of this "parametrized QED" approach is that it provides the exact molecule-cavity interactions, limited by only approximations made in the electronic structure. Using time-dependent density functional theory, we demonstrated comparable accuracy with QED coupled cluster benchmark results for predicting potential energy surfaces in the ground and excited states and showed selected applications to light-harvesting and light-emitting materials. We anticipate that this framework will provide a set of general and powerful tools that enable direct ab initio simulation of exciton polaritons in molecule-cavity hybrid systems.

Polaromechanics: polaritonics meets optomechanics

Cavity exciton polariton physics and cavity optomechanics have evolved into mature and active domains with, so far, very little connections between them. We argue here that there are strong reasons to bridge the two fields, opening interesting opportunities. Polaritons are entities sharing the properties of photons and excitons in a controllable way. They can lead to tunable and strongly enhanced optomechanical couplings and, through them, to single-particle cooperativies C0 &gt; 1 as well as ultra-strong optomechanical coupling in the many-particle regime. Besides, exciton-exciton Coulomb interactions define a new regime of non-linear many-body optomechanics with notable and largely unexplored consequences. Conversely, coherent vibrations can add a qualitatively distinct ingredient to the field of polaritonics by introducing the variable of time. Indeed, the mechanics built-in in polariton resonators allows for controllable time-modulation up to frequencies of tens of GHz with important consequences for the control of quantum emitters and bidirectional optical-to-microwave conversion. Most interestingly, it also enables polaritons to access Floquet physics, Landau-Zenner-Stückelberg state preparation, spinor pseudo-magnetic resonance, as well as optomechanically induced non-reciprocal phenomena. This guest-editorial addresses the opportunities and challenges in these emerging field.

Symmetry breaking and superfluid currents in a split-ring spinor polariton condensate

Bosonic condensates of spinless noninteracting particles confined on a ring cannot propagate circular periodic currents once rotation symmetry of the system is broken. However, a persistent current may appear due to interparticle interactions exceeding some critical strength. In this up-critical regime breaking of the symmetry between the clockwise and anticlockwise rotations takes place. We consider this symmetry-breaking scenario in the case of a spinor condensate of exciton polaritons trapped on a ring split by a potential barrier. Due to the intrinsic symmetry of the effective spin-orbit interaction, which stems from the linear splitting between transverse-electric and transverse-magnetic microcavity modes, the potential barrier blocks the circulating current and imposes linear polarization patterns. On the other hand, circularly polarized polaritons form circular currents propagating in opposite directions with equal absolute values of angular momentum. In the presence of interparticle interactions, the symmetry of clockwise and anticlockwise currents can be broken spontaneously. We describe several symmetry-breaking scenarios, which imply either restoration of the global condensate rotation or the onset of the circular polarization in the symmetry-broken state.

Room Temperature Exciton-Polariton Condensation in Silicon Metasurfaces Emerging from Bound States in the Continuum.

We show the first experimental demonstration of room-temperature exciton-polariton (EP) condensation from a bound state in the continuum (BIC). This demonstration is achieved by strongly coupling stable excitons in an organic perylene dye with the extremely long-lived BIC in a dielectric metasurface of silicon nanoparticles. The long lifetime of the BIC, mainly due to the suppression of radiation leakage, allows for EP thermalization to the ground state before decaying. This property results in a condensation threshold of less than 5 μJ cm-2, 1 order of magnitude lower than the lasing threshold reported in similar systems in the weak coupling limit.

Molecular and solid-state topological polaritons induced by population imbalance

Abstract Strong coupling between electronic excitations in materials and photon modes results in the formation of polaritons, which display larger nonlinearities than their photonic counterparts due to their material component. We theoretically investigate how to optically control the topological properties of molecular and solid-state exciton–polariton systems by exploiting one such nonlinearity: saturation of electronic transitions. We demonstrate modification of the Berry curvature of three different materials when placed within a Fabry–Perot cavity and pumped with circularly polarized light, illustrating the broad applicability of our scheme. Importantly, while optical pumping leads to nonzero Chern invariants, unidirectional edge states do not emerge in our system as the bulk-boundary correspondence is not applicable. This work demonstrates a versatile approach to control topological properties of novel optoelectronic materials.

Excitonic processes and lasing in ZnO thin films and micro/nanostructures

Low dimensional ZnO-based materials have drawn much attention for the past few decades due to their unique electronic and optical properties and potential applications in optoelectronic devices. In this Tutorial, we will cover the past and the latest developments in ZnO thin films and micro/nanostructures in terms of excitonic and related lasing processes. First, we give a brief overview of structural and band properties of ZnO along with the linear optical and excitonic properties. Second, we introduce a feedback mechanism for lasing in various forms of ZnO, ranging from nanoparticles to nanowires, nanodisks, and thin films. As for the feedback mechanism, detailed descriptions are given to random lasing, Fabry–Pérot lasing, and whispering gallery mode lasing. Third, we discuss possible gain mechanisms, i.e., excitonic gain and electron–hole plasma (EHP) gain, in ZnO. A special interest is also devoted to the Mott carrier density, which is a crucial parameter to distinguish between excitonic and EHP contributions to lasing. Lastly, recent developments on exciton–polariton lasers based on ZnO microcavities are introduced.

Higher-order topological polariton corner state lasing.

Unlike conventional laser, the topological laser is able to emit coherent light robustly against disorders and defects because of its nontrivial band topology. As a promising platform for low-power consumption, exciton polariton topological lasers require no population inversion, a unique property that can be attributed to the part-light-part-matter bosonic nature and strong nonlinearity of exciton polaritons. Recently, the discovery of higher-order topology has shifted the paradigm of topological physics to topological states at boundaries of boundaries, such as corners. However, such topological corner states have never been realized in the exciton polariton system yet. Here, on the basis of an extended two-dimensional Su-Schrieffer-Heeger lattice model, we experimentally demonstrate the topological corner states of perovskite polaritons and achieved polariton corner state lasing with a low threshold (approximately microjoule per square centimeter) at room temperature. The realization of such polariton corner states also provides a mechanism of polariton localization under topological protection, paving the way toward on-chip active polaritonics using higher-order topology.

Valley-dependent vortex emission from exciton-polariton in non-centrosymmetric transition metal dichalcogenide metasurfaces.

Transition metal dichalcogenides (TMDs) have attracted great attention in valleytronics. Owing to the giant valley coherence at room temperature, valley pseudospin of TMDs open a new degree of freedom to encode and process binary information. The valley pseudospin only exists in non-centrosymmetric TMDs (e.g., monolayer or 3R-stacked multilayer), which is prohibited in conventional centrosymmetric 2H-stacked crystals. Here, we propose a general recipe to generate valley-dependent vortex beams by using a mix-dimensional TMD metasurface composed of nanostructured 2H-stacked TMD crystals and monolayer TMDs. Such an ultrathin TMD metasurface involves a momentum-space polarization vortex around bound states in the continuum (BICs), which can simultaneously achieve strong coupling (i.e., form exciton polaritons) and valley-locked vortex emission. Moreover, we report that a full 3R-stacked TMD metasurface can also reveal the strong-coupling regime with an anti-crossing pattern and a Rabi splitting of 95 meV. The Rabi splitting can be precisely controlled by geometrically shaping the TMD metasurface. Our results provide an ultra-compact TMD platform for controlling and structuring valley exciton polariton, in which the valley information is linked with the topological charge of vortex emission, which may advance valleytronic, polaritonic, and optoelectronic applications.

Controllability of shock waves in one-dimensional polariton condensates

In one-dimensional incoherent pumped exciton–polariton condensates, we realize the generation and control of supersonic shock waves. By choosing a suitable initial input wave, we obtain the region of existence of various shock waves as a function of the phase of the initial wave, the coefficient of polariton interaction, the coefficient of the interaction between polariton and reservoir and the condensation rate and intensity of pumping. Using these results, we discuss the effect of different parameters on shock waves composed of two shock waves, and find that the gain and loss will result in a sloping platform between two waves. In addition, we also consider the effect of the non-adiabatic approximation and the diffusion coefficient of the reservoir on the profiles of the shock waves. We find that the non-adiabatic approximation will result in some small oscillations on the uniform background, and the platform between two waves becomes flat; however, the diffusion effect of the reservoir will clear these small oscillations and tilt the platform. Our results may pave the way to control shock waves in an incoherent pumped exciton–polariton system.

Ultrathin WS2 Polariton Waveguide for Efficient Light Guiding

AbstractLight confinement to sub‐wavelength scales is actively studied for nanophotonic applications because it gives rise to intriguing optical properties with enhanced light‐matter interaction. Herein, an ultrathin exciton–polariton waveguide based on a WS2 multilayer is proposed for sub‐wavelength light guiding. A WS2 waveguide supports guided exciton–polariton modes even with a few tens of nanometers of thickness. It is theoretically confirmed that the WS2 waveguide has significantly lower propagation loss than a plasmonic mode, although its thickness may be similar to that of a plasmonic waveguide. By injecting white light into a fabricated WS2 waveguide, light guiding is experimentally demonstrated through a 20 nm thick WS2 waveguide. Notably, this study integrated the WS2 waveguide with a WSe2 monolayer to demonstrate the coupling of monolayer emission to the polariton waveguide. The results suggest that WS2 multilayers provide a novel material platform with potential for nanophotonic applications, including ultrathin optical integrated circuits.

Enhanced Strong Coupling in the Hybrid Dielectric-Metallic Nanoresonator and WS2 Monolayer

Exciton polaritons in metallic nanocavities and transition-metal dichalcogenide monolayers has led to striking discoveries, ranging from Bose-Einstein condensation to slowing light. Although plasmonic nanocavities offer small mode volumes, the intrinsic losses of plasmonic nanocavities remain an open challenge in exciton-polariton devices. Consequently, dielectric nanocavities are used as an alternative candidate due to their low intrinsic losses. However, large mode volumes are a central bottleneck in dielectric nanocavities. Here, we theoretically propose to use a hybrid dielectric-metallic nanocavity to enhance light-matter interactions between the excitons of the ${\mathrm{WS}}_{2}$ monolayer and the hybrid nanocavity. Such a hybrid nanoresonator inherits the advantages of both dielectric and metallic nanocavities, including ultrasmall mode volume, ultralow losses, and reasonably-high-Q factor. It is demonstrated that the thickness and material of the central gap film together with the thickness of the metallic substrate play vital roles in governing the coupling strength between 1L-${\mathrm{WS}}_{2}$ excitons and the cavity. After optimizing the geometry and material parameters, the Rabi splitting is increased to 113 meV, almost twice that in dielectric metasystems. The significant improvement can be attributed to the greatly enhanced near field and the ultrasmall mode volume. Furthermore, we show that Rabi splitting can be further boosted to 151 meV by increasing the number of layers of ${\mathrm{WS}}_{2}$ and h-$\mathrm{BN}$ film in the nanocavity.