Physics and applications of exciton-polariton lasers

Near-UV Tunable Polaritons from Magic-Size Clusters

Stronglight-matter coupling to form polaritons has gained significant attention for its applications in materials engineering, optoelectronics, and beyond. The combined properties of their underlying states allow for numerous advantages such as delocalization over long distances, room-temperature Bose-Einstein condensation, and tunability of energy states. Few exciton-polariton systems, however, reach into the UV, and identifying ideal materials that possess large oscillator strengths, large exciton binding energies, ease of processing, and that are stable for device integration has proven challenging. Here, we demonstrate that CdS magic-size clusters (MSCs) combine all these traits. Simple solution processing in metallic Fabry-Perot (FP) cavities enables the MSCs to exhibit room-temperature strong coupling, as demonstrated by the square root dependence of Rabi splitting on chromophore concentration. Rabi splitting as large as 390 meV can be achieved, with emission from polariton states spanning from 3.07 eV (403 nm) to 3.64 eV (340 nm). When Rabi splittings are normalized by the excitonic line width, this system is comparable with high-performing systems in the visible range and surpasses reported UV polariton systems. The strong UV absorption of these MSCs establishes a platform to develop stable polaritonic devices with tunability across the near-UV.

Exciton hybridization in a WS2/MoS2 heterobilayer mediated by a surface wave via strong photon-exciton coupling

The hybridization of multiple excitons in a heterobilayer composed of two transition metal dichalcogenides (TMDCs) based on strong light-matter interaction is interesting from the viewpoint of both fundamental research and practical application. Here, we investigate numerically and experimentally the hybridization of three excitons in a heterobilayer mediated by the surface plasmon polaritons (SPPs) excited on a thin Au film and the transverse-electric (TE) polarized waves excited on a Si3N4/Ag heterostructure via photon-exciton coupling. Relying on numerical simulation, we observe anticrossing behaviors in the angle-resolved reflection spectra calculated for MoS2/WS2/Au and WS2/MoS2/Si3N4/Ag heterostructures, which reveal the coupling between the surface wave (SPPs or TE waves) and the multiple excitons in the heterobilayer. In experiments, we employ the oligomers of polystyrene (PS) nanospheres as scatters to transfer the surface waves into far-field radiations. Similarly, we observe anticrossing behaviors in the angle-resolved scattering spectra measured for the oligomers of PS nanospheres. Relying on the coupled oscillator model, we observe Rabi splitting energies of ΩSPP ∼206.79 meV for the SPPs and ΩTE ∼237.60 meV for the TE waves. Based on the calculated current density distributions and Hopfield coefficients, we demonstrate the hybridization of the three excitons in the WS2/MoS2 heterobilayer mediated by the TE waves. Our findings open new horizons for manipulating light-matter interaction in TMDC heterobilayers and suggest the potential applications of exciton hybridization in energy transfer.

Complex control of polaritons based on optical Stark potential

Effectively controlling exciton-polaritons is crucial for advancing them in optical computation. In this work, we propose utilizing the valley-selective optical Stark effect (OSE) as an all-optical way to achieve the spatiotemporal control of polariton flow. We demonstrate the polarization-selective concentration of polaritons at pre-determined locations by wavefront shaping of the polaritons through an in-plane bar-code potential induced by the OSE, which helps overcome the intra-cavity disorder in potential distribution. In addition, a polariton decoder that converts binary inputs to decimal outputs is proposed by expanding the one-dimensional bar-code potential into a two-dimensional quick-response code potential offering enhanced control and encoding, whose robustness and valley selectivity are demonstrated.

Plasmonically-boosted exciton-photon coupling strength in a near-infrared LED based on a ZnO:Ga microwire/GaAs heterojunction with surface-coated Au&Ag alloy nanorods

The development of electrically-driven low-dimensional coherent light sources via highly-polarized polariton emission behavior has been extensively researched, but suffers from limited modulation of the exciton-photon coupling strengths. Herein, an electrically-biased near-infrared exciton-polariton light-emitting diode (LED), which includes a Ga-doped ZnO microwire (ZnO:Ga MW) and p-type GaAs substrate, is demonstrated. The well-designed LED structure is conducive to producing strong coupling between excitons and cavity photons, thus yielding highly-polarized light-emissions due to the optical birefringence in the ZnO:Ga MW microcavity. In particular, when the LED device is modified using Au&Ag alloy nanorods (AuAgNRs) with desired plasmonic properties, the electroluminescence (EL) performance is significantly boosted, especially the Rabi-splitting energy, which increases from 96 to 285 meV. The current-injection exciton-polariton emission from the LED undergoing a strong coupling regime is confirmed through angle-resolved EL measurements. This study exhibits a performance-boosted near-infrared exciton-polariton LED at room temperature, which provides a new scheme toward the realization of highly energy-efficient polariton coherent light sources. Further, the significantly lower density of polariton states induced by the incorporated metal nanostructures highlights a bright future of realizing ultralow-threshold polariton lasers much more feasibly, in comparison to conventional lasers based on narrow bandgap semiconductors.

Active strong coupling of exciton and nanocavity based on GSST-WSe2 hybrid nanostructures

The strong coupling between optical resonance microcavity and matter excitations provides a practical path for controlling light-matter interactions. However, conventional microcavity, whose functions are fixed at the fabrication stage, dramatically limits the modulation of light-matter interactions. Here, we investigate the active strong coupling of resonance mode and exciton in GSST-WSe2 hybrid nanostructures. It is demonstrated that significant spectral splitting is observed in single nanostructures, tetramers, and metasurfaces. We further confirm the strong coupling by calculating the enhanced fluorescence spectra. The coupling effect between the excited resonance and exciton is dramatically modulated during the change of GSST from amorphous to crystalline, thus realizing the strong coupling switching. This switching property has been fully demonstrated in several systems mentioned earlier. Our work is significant in guiding the study of actively tunable strong light-matter interactions at the nanoscale.

Polaritonic Bottleneck in Colloidal Quantum Dots

Controlling the relaxation dynamics of excitons is key to improving the efficiencies of semiconductor-based applications. Confined semiconductor nanocrystals (NCs) offer additional handles to control the properties of excitons, for example, by changing their size or shape, resulting in a mismatch between excitonic gaps and phonon frequencies. This has led to the hypothesis of a significant slowing-down of exciton relaxation in strongly confined NCs, but in practice due to increasing exciton-phonon coupling and rapid multiphonon relaxation channels, the exciton relaxation depends only weakly on the size or shape. Here, we focus on elucidating the nonradiative relaxation of excitons in NCs placed in an optical cavity. We find that multiphonon emission of carrier governs the decay, resulting in a polariton-induced phonon bottleneck with relaxation time scales that are slower by orders of magnitude compared to the cavity-free case, while the photon fraction plays a secondary role.

Near-Unity Emitting, Widely Tailorable, and Stable Exciton Concentrators Built from Doubly Gradient 2D Semiconductor Nanoplatelets

The strength of electrostatic interactions (EIs) between electrons and holes within semiconductor nanocrystals profoundly affects the performance of their optoelectronic systems, and different optoelectronic devices demand distinct EI strength of the active medium. However, achieving a broad range and fine-tuning of the EI strength for specific optoelectronic applications is a daunting challenge, especially in quasi two-dimensional core-shell semiconductor nanoplatelets (NPLs), as the epitaxial growth of the inorganic shell along the direction of the thickness that solely contributes to the quantum confined effect significantly undermines the strength of the EI. Herein we propose and demonstrate a doubly gradient (DG) core-shell architecture of semiconductor NPLs for on-demand tailoring of the EI strength by controlling the localized exciton concentration via in-plane architectural modulation, demonstrated by a wide tuning of radiative recombination rate and exciton binding energy. Moreover, these exciton-concentration-engineered DG NPLs also exhibit a near-unity quantum yield, high photo- and thermal stability, and considerably suppressed self-absorption. As proof-of-concept demonstrations, highly efficient color converters and high-performance light-emitting diodes (external quantum efficiency: 16.9%, maximum luminance: 43,000 cd/m2) have been achieved based on the DG NPLs. This work thus provides insights into the development of high-performance colloidal optoelectronic device applications.

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.

Polaritons in Van der Waals Heterostructures

2D monolayers supporting a wide variety of highly confined plasmons, phonon polaritons, and exciton polaritons can be vertically stacked in van der Waals heterostructures (vdWHs) with controlled constituent layers, stacking sequence, and even twist angles. vdWHs combine advantages of 2D material polaritons, rich optical structure design, and atomic scale integration, which have greatly extended the performance and functions of polaritons, such as wide frequency range, long lifetime, ultrafast all-optical modulation, and photonic crystals for nanoscale light. Here, the state of the art of 2D material polaritons in vdWHs from the perspective of design principles and potential applications is reviewed. Some fundamental properties of polaritons in vdWHs are initially discussed, followed by recent discoveries of plasmons, phonon polaritons, exciton polaritons, and their hybrid modes in vdWHs. The review concludes with a perspective discussion on potential applications of these polaritons such as nanophotonic integrated circuits, which will benefit from the intersection between nanophotonics and materials science.

Strong light-matter interactions of exciton in bulk WS2 and a toroidal dipole resonance

Exciton-polaritonic states are generated by strong interactions between photons and excitons in nanocavities. Bulk transition metal dichalcogenides (TMDCs) host excitons with a large binding energy at room temperature, and thus are regarded as an ideal platform for realizing exciton-polaritons. In this work, we investigate the strong coupling properties between high-Q toroidal dipole (TD) resonance and bulk WS₂ excitons in a hybrid metasurface, consisting of Si₃N₄ nanodisk arrays with embedded WS₂. Multipole decomposition and near-field distribution confirm that Si₃N₄ nanodisk arrays support strong TD resonance. The TD resonance wavelength is easily tuned to overlap with the bulk WS₂ exciton wavelength, and strong coupling is observed when the bulk WS₂ is integrated with the hollow nanodisk and the oscillator strength of the WS₂ material is adjusted to be greater than 0.6. The Rabi splitting of the hybrid device is up to 65 meV. In addition, strong coupling is confirmed by the anticrossing of fluorescence enhancement in the hybrid Si₃N₄-WS₂ metastructure. Our findings are expected to be of importance for both fundamental research in TMDC-based light-matter interactions and practical applications in the design of high-performance exciton-polariton devices.

Unusually large exciton binding energy in multilayered 2H-MoTe2

Although large exciton binding energies of typically 0.6–1.0 eV are observed for monolayer transition metal dichalcogenides (TMDs) owing to strong Coulomb interaction, multilayered TMDs yield relatively low exciton binding energies owing to increased dielectric screening. Recently, the ideal carrier-multiplication threshold energy of twice the bandgap has been realized in multilayered semiconducting 2H-MoTe₂ with a conversion efficiency of 99%, which suggests strong Coulomb interaction. However, the origin of strong Coulomb interaction in multilayered 2H-MoTe₂, including the exciton binding energy, has not been elucidated to date. In this study, unusually large exciton binding energy is observed through optical spectroscopy conducted on CVD-grown 2H-MoTe₂. To extract exciton binding energy, the optical conductivity is fitted using the Lorentz model to describe the exciton peaks and the Tauc–Lorentz model to describe the indirect and direct bandgaps. The exciton binding energy of 4 nm thick multilayered 2H-MoTe₂ is approximately 300 meV, which is unusually large by one order of magnitude when compared with other multilayered TMD semiconductors such as 2H-MoS₂ or 2H-MoSe₂. This finding is interpreted in terms of small exciton radius based on the 2D Rydberg model. The exciton radius of multilayered 2H-MoTe₂ resembles that of monolayer 2H-MoTe₂, whereas those of multilayered 2H-MoS₂ and 2H-MoSe₂ are large when compared with monolayer 2H-MoS₂ and 2H-MoSe₂. From the large exciton binding energy in multilayered 2H-MoTe₂, it is expected to realize the future applications such as room-temperature and high-temperature polariton lasing.

Microcavity phonon polaritons from the weak to the ultrastrong phonon-photon coupling regime

Strong coupling between molecular vibrations and microcavity modes has been demonstrated to modify physical and chemical properties of the molecular material. Here, we study the less explored coupling between lattice vibrations (phonons) and microcavity modes. Embedding thin layers of hexagonal boron nitride (hBN) into classical microcavities, we demonstrate the evolution from weak to ultrastrong phonon-photon coupling when the hBN thickness is increased from a few nanometers to a fully filled cavity. Remarkably, strong coupling is achieved for hBN layers as thin as 10 nm. Further, the ultrastrong coupling in fully filled cavities yields a polariton dispersion matching that of phonon polaritons in bulk hBN, highlighting that the maximum light-matter coupling in microcavities is limited to the coupling strength between photons and the bulk material. Tunable cavity phonon polaritons could become a versatile platform for studying how the coupling strength between photons and phonons may modify the properties of polar crystals.

Strong coupling between light and matter can be engineered to influence their properties and behaviour. Here, the authors demonstrate the evolution from weak to ultrastrong coupling of microcavity modes and optical phonons with hexagonal boron nitride layers in a Fabry-Perot resonator.

Room temperature exciton-polariton Bose-Einstein condensation in organic single-crystal microribbon cavities

Exciton–polariton Bose–Einstein condensation (EP BEC) is of crucial importance for the development of coherent light sources and optical logic elements, as it creates a new state of matter with coherent nature and nonlinear behaviors. The demand for room temperature EP BEC has driven the development of organic polaritons because of the large binding energies of Frenkel excitons in organic materials. However, the reliance on external high-finesse microcavities for organic EP BEC results in poor compactness and integrability of devices, which restricts their practical applications in on-chip integration. Here, we demonstrate room temperature EP BEC in organic single-crystal microribbon natural cavities. The regularly shaped microribbons serve as waveguide Fabry–Pérot microcavities, in which efficient strong coupling between Frenkel excitons and photons leads to the generation of EPs at room temperature. The large exciton–photon coupling strength due to high exciton densities facilitates the achievement of EP BEC. Taking advantages of interactions in EP condensates and dimension confinement effects, we demonstrate the realization of controllable output of coherent light from the microribbons. We hope that the results will provide a useful enlightenment for using organic single crystals to construct miniaturized polaritonic devices.

The use of room temperature exciton–polariton Bose–Einstein condensation is limited by the need for external high-finesse microcavities. The authors generate room temperature EPs with single-crystal microribbons as waveguide Fabry–Pérot microcavities, and demonstrate controllable output of coherent light.

Ultralow Threshold Polariton Condensate in a Monolayer Semiconductor Microcavity at Room Temperature

Exciton-polaritons, hybrid light-matter bosonic quasiparticles, can condense into a single quantum state, i.e., forming a polariton Bose-Einstein condensate (BEC), which represents a crucial step for the development of nanophotonic technology. Recently, atomically thin transition-metal dichalcogenides (TMDs) emerged as promising candidates for novel polaritonic devices. Although the formation of robust valley-polaritons has been realized up to room temperature, the demonstration of polariton lasing remains elusive. Herein, we report for the first time the realization of this important milestone in a TMD microcavity at room temperature. Continuous wave pumped polariton lasing is evidenced by the macroscopic occupation of the ground state, which undergoes a nonlinear increase of the emission along with the emergence of temporal coherence, the presence of an exciton fraction-controlled threshold and the buildup of linear polarization. Our work presents a critically important step toward exploiting nonlinear polariton-polariton interactions, as well as offering a new platform for thresholdless lasing.

Plasmon-enhanced strong exciton-polariton coupling in single microwire-based heterojunction light-emitting diodes

Manipulating the strong light-matter coupling interaction in optical microresonators that are naturally formed by semiconductor micro- or nanostructures is crucial for fabricating high-performance exciton-polariton devices. Such devices can function as coherent light sources having considerably lower emission threshold. In this study, an exciton-polariton light-emitting diode (LED), made of a single ZnO microwire (MW) and a p-GaN substrate, serving as the hole injector, was fabricated, and its working characteristics, in the near-ultraviolet region, were demonstrated. To further improve the quality of the single ZnO MW-based optical microresonator, Ag nanowires (AgNWs) with ultraviolet plasmonic response were deposited on the MW. Apart from the improvement of the electrical and optical properties of the hexagonal ZnO MW, the optically pumped whispering-gallery-mode lasing characteristics were significantly enhanced. Furthermore, a single ZnO MW not covered, and covered by AgNWs, was used to construct a heterojunction LED. Compared with single bare ZnO MW-based LED, significant enhancement of the device performance was achieved, including a significant enhancement in the light output and a small emission band blueshift. Specifically, the exciton-polariton emission was observably enhanced, and the corresponding Rabi splitting energy (∼ 495 meV) was significantly higher than that of the bare ZnO MW-based LED (∼ 370 meV). That ultraviolet plasmons of AgNWs enhanced the exciton-polariton coupling strength was further confirmed via angle-resolved electroluminescence measurements of the single MW-based polaritonic devices, which clearly illustrated the presence of Rabi splitting and subband anti-crossing characteristics. The experimental results provide new avenues to achieve extremely high coupling strengths, which can accelerate the advancements in electrically driven high-efficiency polaritonic coherent emitters and nonlinear devices.

Engineering Giant Rabi Splitting via Strong Coupling between Localized and Propagating Plasmon Modes on Metal Surface Lattices: Observation of √N Scaling Rule

We present a strong coupling system realized by coupling the localized surface plasmon mode in individual silver nanogrooves and propagating surface plasmon modes launched by periodic nanogroove arrays with varied periodicities on a continuous silver medium. When the propagating modes are in resonance with the localized mode, we observe a √N scaling of Rabi splitting energy, where N is the number of propagating modes coupled to the localized mode. Here, we confirm a giant Rabi splitting on the order of 450-660 meV (N = 2) in the visible spectral range, and the corresponding coupling strength is 160-235 meV. In some of the strong coupling cases studied by us, the coupling strength is about 10% of the mode energy, reaching the ultrastrong coupling regime.

Polariton hyperspectral imaging of two-dimensional semiconductor crystals

Atomically thin crystals of transition metal dichalcogenides (TMDs) host excitons with strong binding energies and sizable light-matter interactions. Coupled to optical cavities, monolayer TMDs routinely reach the regime of strong light-matter coupling, where excitons and photons admix coherently to form polaritons up to room temperature. Here, we explore the two-dimensional nature of TMD polaritons with scanning-cavity hyperspectral imaging. We record a spatial map of polariton properties of extended WS₂ monolayers coupled to a tunable micro cavity in the strong coupling regime, and correlate it with maps of exciton extinction and fluorescence taken from the same flake with the cavity. We find a high level of homogeneity, and show that polariton splitting variations are correlated with intrinsic exciton properties such as oscillator strength and linewidth. Moreover, we observe a deviation from thermal equilibrium in the resonant polariton population, which we ascribe to non-Markovian polariton-phonon coupling. Our measurements reveal a promisingly consistent polariton landscape, and highlight the importance of phonons for future polaritonic devices.

Photonic crystals for controlling strong coupling in van der Waals materials

The design of photonic structures plays a crucial role in the engineering of light-matter interactions. Planar microcavities have been widely used to establish strong light-matter coupling in semiconductor quantum wells, leading to intense research on exciton-polariton systems in the past few decades. However, planar cavities are limited in material compatibility, inflexible for mode engineering, and bulky for integration. Here we demonstrate dielectric slab photonic crystals as a flexible and compact platform for polaritons, where excitons are strongly coupled to photons confined in the leaky modes of the slab. We show our structure is well-suited for van der Waals materials, features unusual adjustable dispersions, and allows for multi-wavelength operation on a single chip.

Inorganic and Hybrid Perovskite Based Laser Devices: A Review

Inorganic and organic-inorganic (hybrid) perovskite semiconductor materials have attracted worldwide scientific attention and research effort as the new wonder semiconductor material in optoelectronics. Their excellent physical and electronic properties have been exploited to boost the solar cells efficiency beyond 23% and captivate their potential as competitors to the dominant silicon solar cells technology. However, the fundamental principles in Physics, dictate that an excellent direct band gap material for photovoltaic applications must be also an excellent light emitter candidate. This has been realized for the case of perovskite-based light emitting diodes (LEDs) but much less for the case of the respective laser devices. Here, the strides, exclusively in lasing, made since 2014 are presented for the first time. The solution processability, low temperature crystallization, formation of nearly defect free, nanostructures, the long range ambipolar transport, the direct energy band gap, the high spectral emission tunability over the entire visible spectrum and the almost 100% external luminescence efficiency show perovskite semiconductors’ potential to transform the nanophotonics sector. The operational principles, the various adopted material and laser configurations along the future challenges are reviewed and presented in this paper.

High-Quality In-Plane Aligned CsPbX3 Perovskite Nanowire Lasers with Composition-Dependent Strong Exciton-Photon Coupling

Cesium lead halide perovskite nanowires have emerged as promising low-dimensional semiconductor structures for integrated photonic applications. Understanding light-matter interactions in a nanowire cavity is of both fundamental and practical interest in designing low-power-consumption nanoscale light sources. In this work, high-quality in-plane aligned halide perovskite CsPbX₃ (X = Cl, Br, I) nanowires are synthesized by a vapor growth method on an annealed M-plane sapphire substrate. Large-area nanowire laser arrays have been achieved based on the as-grown aligned CsPbX₃ nanowires at room temperature with quite low pumping thresholds, very high quality factors, and a high degree of linear polarization. More importantly, it is found that exciton-polaritons are formed in the nanowires under the excitation of a pulsed laser, indicating a strong exciton-photon coupling in the optical microcavities made of cesium lead halide perovskites. The coupling strength in these CsPbX₃ nanowires is dependent on the atomic composition, where the obtained room-temperature Rabi splitting energy is ∼210 ± 13, 146 ± 9, and 103 ± 5 meV for the CsPbCl₃, CsPbBr₃, and CsPbI₃ nanowires, respectively. This work provides fundamental insights for the practical applications of all-inorganic perovskite CsPbX₃ nanowires in designing light-emitting devices and integrated nanophotonic systems.

Photonic-crystal exciton-polaritons in monolayer semiconductors

Semiconductor microcavity polaritons, formed via strong exciton-photon coupling, provide a quantum many-body system on a chip, featuring rich physics phenomena for better photonic technology. However, conventional polariton cavities are bulky, difficult to integrate, and inflexible for mode control, especially for room-temperature materials. Here we demonstrate sub-wavelength-thick, one-dimensional photonic crystals as a designable, compact, and practical platform for strong coupling with atomically thin van der Waals crystals. Polariton dispersions and mode anti-crossings are measured up to room temperature. Non-radiative decay to dark excitons is suppressed due to polariton enhancement of the radiative decay. Unusual features, including highly anisotropic dispersions and adjustable Fano resonances in reflectance, may facilitate high temperature polariton condensation in variable dimensions. Combining slab photonic crystals and van der Waals crystals in the strong coupling regime allows unprecedented engineering flexibility for exploring novel polariton phenomena and device concepts.

Semiconductor microcavities can host polaritons formed by strong exciton-photon coupling, yet they may be plagued by scalability issues. Here, the authors demonstrate a sub-wavelength-thick, one-dimensional photonic crystal platform for strong coupling with atomically thin van der Waals crystals.

Topological order and thermal equilibrium in polariton condensates

The Berezinskii-Kosterlitz-Thouless phase transition from a disordered to a quasi-ordered state, mediated by the proliferation of topological defects in two dimensions, governs seemingly remote physical systems ranging from liquid helium, ultracold atoms and superconducting thin films to ensembles of spins. Here we observe such a transition in a short-lived gas of exciton-polaritons, bosonic light-matter particles in semiconductor microcavities. The observed quasi-ordered phase, characteristic for an equilibrium two-dimensional bosonic gas, with a decay of coherence in both spatial and temporal domains with the same algebraic exponent, is reproduced with numerical solutions of stochastic dynamics, proving that the mechanism of pairing of the topological defects (vortices) is responsible for the transition to the algebraic order. This is made possible thanks to long polariton lifetimes in high-quality samples and in a reservoir-free region. Our results show that the joint measurement of coherence both in space and time is required to characterize driven-dissipative phase transitions and enable the investigation of topological ordering in open systems.

Charged Polaron Polaritons in an Organic Semiconductor Microcavity

We report strong coupling between light and polaron optical excitations in a doped organic semiconductor microcavity at room temperature. Codepositing MoO_{3} and the hole transport material 4, 4^{'}-cyclohexylidenebis[N, N-bis(4-methylphenyl)benzenamine] introduces a large hole density with a narrow linewidth optical transition centered at 1.8 eV and an absorption coefficient exceeding 10^{4}  cm^{-1}. Coupling this transition to a Fabry-Pérot cavity mode yields upper and lower polaron polariton branches that are clearly resolved in angle-dependent reflectivity with a vacuum Rabi splitting ℏΩ_{R}>0.3  eV. This result establishes a path to electrically control polaritons in organic semiconductors and may lead to increased polariton-polariton Coulombic interactions that lower the threshold for nonlinear phenomena such as polariton condensation and lasing.

Electronic properties of single-layer tungsten disulfide on epitaxial graphene on silicon carbide

This work reports an electronic and micro-structural study of an appealing system for optoelectronics: tungsten disulfide (WS₂) on epitaxial graphene (EG) on SiC(0001). The WS₂ is grown via chemical vapor deposition (CVD) onto the EG. Low-energy electron diffraction (LEED) measurements assign the zero-degree orientation as the preferential azimuthal alignment for WS₂/EG. The valence-band (VB) structure emerging from this alignment is investigated by means of photoelectron spectroscopy measurements, with both high space and energy resolution. We find that the spin-orbit splitting of monolayer WS₂ on graphene is of 462 meV, larger than what is reported to date for other substrates. We determine the value of the work function for the WS₂/EG to be 4.5 ± 0.1 eV. A large shift of the WS₂ VB maximum is observed as well, due to the lowering of the WS₂ work function caused by the donor-like interfacial states of EG. Density functional theory (DFT) calculations carried out on a coincidence supercell confirm the experimental band structure to an excellent degree. X-ray photoemission electron microscopy (XPEEM) measurements performed on single WS₂ crystals confirm the van der Waals nature of the interface coupling between the two layers. In virtue of its band alignment and large spin-orbit splitting, this system gains strong appeal for optical spin-injection experiments and opto-spintronic applications in general.

Electrical pumping and tuning of exciton-polaritons in carbon nanotube microcavities

Exciton-polaritons are hybrid light-matter particles that form upon strong coupling of an excitonic transition to a cavity mode. As bosons, polaritons can form condensates with coherent laser-like emission. For organic materials, optically pumped condensation was achieved at room temperature but electrically pumped condensation remains elusive due to insufficient polariton densities. Here we combine the outstanding optical and electronic properties of purified, solution-processed semiconducting (6,5) single-walled carbon nanotubes (SWCNTs) in a microcavity-integrated light-emitting field-effect transistor to realize efficient electrical pumping of exciton-polaritons at room temperature with high current densities (>10 kA cm^-2) and tunability in the near-infrared (1,060 nm to 1,530 nm). We demonstrate thermalization of SWCNT polaritons, exciton-polariton pumping rates ∼10⁴ times higher than in current organic polariton devices, direct control over the coupling strength (Rabi splitting) via the applied gate voltage, and a tenfold enhancement of polaritonic over excitonic emission. This powerful material-device combination paves the way to carbon-based polariton emitters and possibly lasers.

High quality factor confined Tamm modes

We demonstrate that quality factors up to 5000 can be obtained in Tamm-like hybrid metal/semiconductor structures. To do this, a Bragg mirror is covered by a thin transparent layer and a metallic film. The reduced losses of these modes are related to an intermediate behavior between conventional Tamm plasmon and Bragg modes lying deeper in the semiconductor medium. One of the most striking features of this approach is that these super Tamm modes can still be spatially confined with the metal. Confinement on micrometric scale is experimentally demonstrated. The simplicity and versatility of high-Q mode control by metal structuration open perspectives for lasing and polaritonic applications.

Tunable Bragg polaritons and nonlinear emission from a hybrid metal-unfolded ZnSe-based microcavity

Strong light-matter interaction in Bragg structures possesses several advantages over conventional microcavity system. These structures provide an opportunity to incorporate a large number of quantum wells without increasing the mode volume. Further, it is expected that the strong coupling could occur over the entire thickness of the Bragg structure, and the system offers an improved overlap between exciton wave function and light mode. However, advanced experiments in Bragg structures require a precise control and manipulation of quantum states of Bragg polaritons. Here, we propose and experimentally demonstrate novel methods for the modulation of Bragg polariton eigenstates. The modulation will be shown to even exceed 10 meV if the thickness of the top layer of the ZnSe-based Bragg structure is changed or if a thin silver layer is deposited on top of the structure. The Q value of the Bragg mode will be enhanced by a factor of 2.3 for a 30 nm silver layer. In addition, we report on the observation of nonlinear emission of the lower Bragg polariton mode in the hybrid structure being achieved when excitation dependent measurements are performed. Our results open the door to create a confined Bragg polariton system similar to conventional microcavities.

Enabling valley selective exciton scattering in monolayer WSe2 through upconversion

Excitons, Coulomb bound electron–hole pairs, are composite bosons and their interactions in traditional semiconductors lead to condensation and light amplification. The much stronger Coulomb interaction in transition metal dichalcogenides such as WSe₂ monolayers combined with the presence of the valley degree of freedom is expected to provide new opportunities for controlling excitonic effects. But so far the bosonic character of exciton scattering processes remains largely unexplored in these two-dimensional materials. Here we show that scattering between B-excitons and A-excitons preferably happens within the same valley in momentum space. This leads to power dependent, negative polarization of the hot B-exciton emission. We use a selective upconversion technique for efficient generation of B-excitons in the presence of resonantly excited A-excitons at lower energy; we also observe the excited A-excitons state 2s. Detuning of the continuous wave, low-power laser excitation outside the A-exciton resonance (with a full width at half maximum of 4 meV) results in vanishing upconversion signal.

Monolayer transition metal dichalcogenides host excitons, bound electron-hole pairs that play a pivotal role in optoelectronic applications relying on strong light-matter interaction. Here, the authors unveil the spectroscopic signature of boson scattering of two-dimensional excitons in monolayer WSe₂.

Electrically tunable organic-inorganic hybrid polaritons with monolayer WS2

Exciton-polaritons are quasiparticles consisting of a linear superposition of photonic and excitonic states, offering potential for nonlinear optical devices. The excitonic component of the polariton provides a finite Coulomb scattering cross section, such that the different types of exciton found in organic materials (Frenkel) and inorganic materials (Wannier-Mott) produce polaritons with different interparticle interaction strength. A hybrid polariton state with distinct excitons provides a potential technological route towards in situ control of nonlinear behaviour. Here we demonstrate a device in which hybrid polaritons are displayed at ambient temperatures, the excitonic component of which is part Frenkel and part Wannier-Mott, and in which the dominant exciton type can be switched with an applied voltage. The device consists of an open microcavity containing both organic dye and a monolayer of the transition metal dichalcogenide WS₂. Our findings offer a perspective for electrically controlled nonlinear polariton devices at room temperature.