Polaritons Research Articles

Room-Temperature Electrical Field-Enhanced Ultrafast Switch in Organic Microcavity Polariton Condensates.

Integrated electro-optical switches are essential as one of the fundamental elements in the development of modern optoelectronics. As an architecture for photonic systems exciton polaritons, hybrid bosonic quasiparticles that possess unique properties derived from both excitons and photons, have shown much promise. For this system, we demonstrate a significant improvement of emitted intensity and condensation threshold by applying an electric field to a microcavity filled with an organic microbelt. Our theoretical investigations indicate that the electric field makes the excitons dipolar and induces an enhancement of the exciton-polariton interaction and of the polariton lifetime. Based on these electric field-induced changes, a sub-nanosecond electrical field-enhanced polariton condensate switch is realized at room temperature, providing the basis for developing an on-chip integrated photonic device in the strong light-matter coupling regime.

Optical propagation of exciton polaritons in ultrathin van der Waals microcrystals down to few monolayers

The exciton polariton is a kind of quasiparticles and provides a promising opportunity to explore fundamental quantum phenomena for photonic applications. Transition-metal dichalcogenide (TMD) materials provide the platform of nanophotonics that supports the propagative exciton polaritons even at room-temperature. Previously, real space studies on thin flakes of TMDs by scattering-type scanning nearfield optical microscopy (s-SNOM) were limited to waveguide thickness down to 30 nm. In this work, we present the nano-optical imaging of ordinary transverse electric modes of exciton polaritons in MoS2 and WSe2 down to a few atomic layers, measured by atomic force microscope-based s-SNOM. Surprisingly, the interference fringe patterns can be observed clearly at the prepared ultrathin TMD flakes with thickness down to ~3 nm (4 ML) and ~8 nm (12 ML) for MoS2 and WSe2, respectively, which breaks greatly the previous measurement limitation. The wavevectors stay around 1.6k0−1.7k0 constantly when the thickness approaching to a few MLs, instead of 1k0 according to the theory. These modes are supported by the nearly-freestanding TMD microflakes in the form of three-layer symmetric waveguide to confine the exciton polaritons. Our results provide in-depth understanding and open new avenues to explore the polaritonic devices operating at the near infrared region based on ultrathin TMD materials.

Nonlocal Probing of Amplitude Mode Dynamics in Charge-Density-Wave Phase of EuTe 4

Amplitude mode is collective excitation emerging from frozen lattice distortions below the charge-density-wave (CDW) transition temperature T CDW and relates to the order parameter. Generally, the amplitude mode is non-polar (symmetry-even) and does not interact with incoming infrared photons. However, if the amplitude mode is polar (symmetry-odd), it can potentially couple with incoming photons, thus forming a coupled phonon–polariton quasiparticle that can travel with light-like speed beyond the optically excited region. Here, we present the amplitude mode dynamics far beyond the optically excited depth of ∼150 nm in the CDW phase of ∼10-μm-thick single-crystal EuTe 4 using time-resolved x-ray diffraction. The observed oscillations of the CDW peak, triggered by photoexcitation, occur at the amplitude mode frequency ω AM . However, the underdamped oscillations and their propagation beyond the optically excited depth are at odds with the observation of the overdamped nature of the amplitude mode measured using meV-resolution inelastic x-ray scattering and polarized Raman scattering. The ω AM is found to decrease with increasing fluence owing to a rise in the sample temperature, which is independently confirmed using polarized Raman scattering and ab-initio molecular dynamics simulations. We rationalize the above observations by explicitly calculating two coupled quasiparticles—phonon–polariton and exciton–polariton. Our data and simulations cannot conclusively confirm or rule out the one but point toward the likely origin from propagating phonon–polariton. The observed non-local behavior of amplitude mode thus provides an opportunity to engineer material properties at a substantially faster time scale with optical pulses.

Optical properties and polaritons of low symmetry 2D materials

Low symmetry 2D materials with intrinsic in-plane anisotropic optical properties and high tunability provide a promising platform to explore and manipulate light–matter interactions. To date, dozens of in-plane anisotropic 2D materials with diverse band structures have been discovered. They exhibit rich optical properties, indicating great potential for novel applications in optics, photonics, and optoelectronics. In this paper, we thoroughly review the anisotropic optical properties and polaritons in many kinds of low symmetry 2D materials, aiming to elicit more research interest in this field. First, the optical properties of anisotropic 2D semiconductors, including interband absorption, photoluminescence, excitons, and band structure engineering for tuning optical responses, are introduced. Then fundamentals and advances in experiments of hyperbolic polaritons in anisotropic 2D materials, including phonon, plasmon, and exciton polaritons, are discussed. Finally, a perspective on promising research directions is given.

Exciton-polariton light-emitting diode based on a single ZnO superlattice microwire heterojunction with performance enhanced by Rh nanostructures.

One-dimensional (1D) wirelike superlattice micro/nanostructures have received considerable attention for potential applications due to their versatility and capability for modulating optical and electrical characteristics. In this study, 1D superlattice microwires (MWs), which are made of undoped ZnO and Ga-doped ZnO with periodic and alternating crystalline layers (ZnO/ZnO:Ga), were synthesized individually. Under optical excitation, a series of resonance peaks in the photoluminescence spectrum can be ascribed to polariton emission, which originates from the coupling interaction of the 1D photonic crystal and confined excitons along the wire direction. Using a p-type GaN layer as the hole transport layer, a kind of waveguide light source based on an individual ZnO/ZnO:Ga superlattice MW was proposed and constructed. By analysing the spatially resolved electroluminescence spectra, the observed multipeak was ascribed to exciton-polariton emission with a vacuum Rabi splitting of about 275 meV. Cladding with Rh nanostructures gives rise to appropriate ultraviolet plasmons, and the Rabi splitting energy of our device was enhanced up to 413 meV. The exciton-polariton properties were further examined using angle-resolved electroluminescence measurements. Therefore, individual superlattice MWs can act as optical microresonators to achieve photon-exciton coupling with a large Rabi splitting energy. The experimental results indicate that an individual ZnO/ZnO:Ga superlattice MW can be generally used in developing exciton-polariton luminescence/lasing light sources, particularly for constructing low-threshold/thresholdless lasers toward pragmatic applications.

Leaky Integrate‐and‐Fire Mechanism in Exciton–Polariton Condensates for Photonic Spiking Neurons (Laser Photonics Rev. 17(1)/2023)

Leaky Integrate‐and‐Fire Mechanism in Exciton–Polariton Condensates for Photonic Spiking Neurons (Laser Photonics Rev. 17(1)/2023)

Kardar-Parisi-Zhang universality in discrete two-dimensional driven-dissipative exciton polariton condensates

The statistics of the fluctuations of quantum many-body systems are highly revealing of their nature. In driven-dissipative systems displaying macroscopic quantum coherence, as exciton polariton condensates under incoherent pumping, the phase dynamics can be mapped to the stochastic Kardar-Parisi-Zhang (KPZ) equation. However, in two dimensions (2D), it was theoretically argued that the KPZ regime may be hindered by the presence of vortices, and a non-equilibrium BKT behavior was reported close to condensation threshold. We demonstrate here that, when a discretized 2D polariton system is considered, universal KPZ properties can emerge. We support our analysis by extensive numerical simulations of the discrete stochastic generalized Gross-Pitaevskii equation. We show that the first-order correlation function of the condensate exhibits stretched exponential behaviors in space and time with critical exponents characteristic of the 2D KPZ universality class, and find that the related scaling function accurately matches the KPZ theoretical one, stemming from functional Renormalization Group. We also obtain the distribution of the phase fluctuations and find that it is non-Gaussian, as expected for a KPZ stochastic process.

Exciton–polaritons of hBN/WS2 heterostructure in cavity observed at room temperature

The layer-by-layer buildup of a two-dimensional transition metal dichalcogenide monolayer to form a stack is an important development for these materials, which performs many remarkable properties in the light–matter interaction. Herein, we find clear evidence of exciton–polaritons emerging from WS2/hBN/WS2 heterostructures embedded in a dielectric microcavity under optical pumping. A large Rabi splitting of 15 meV is observed at room temperature, which increases to 25 meV at 5 K. An all-dielectric Fabry–Pérot microcavity provides a simple but effective way to study the room temperature strong coupling between cavity photons and excitons. Our results pave the way for room-temperature polaritonic devices based on quantum-well van der Waals heterostructures.

Two-photon pumped exciton-polariton condensation

Two-photon absorption (TPA) allows accessing “dark” states of matter that are otherwise inaccessible to light, which serve as important building blocks for quantum information processing. In a semiconductor microcavity, TPA-driven condensation of strongly coupled light-matter exciton–polaritons can enable new solid-state quantum simulations of “dark” state-condensate interactions and was predicted to stimulate THz emission. Here, we report the first observation of two-photon-pumped polariton condensation, demonstrated by angle-resolved photoluminescence in a GaAs-based microcavity. TPA is evidenced in the quadratic emission dependence on pump power below and above the condensation threshold, and second-harmonic generation is ruled out by both this threshold behavior and by the emission peak energy showing no dependence on pump photon energy. Our results pave the way toward novel polariton-based sources and solid-state coherent control of collective quantum states with individual two-level systems.

Microscopic theory of excitons bound by light

We theoretically investigate the scenario of a semiconductor quantum well in a microcavity, where the band structure is arranged such that optically excited electron-hole pairs cannot form Coulomb-bound excitonic states. However, it is still possible to form exciton polaritons (part-light, part-matter quasiparticles), where the excitons are bound via the exchange of microcavity photons rather than via Coulomb interactions. Using a diagrammatic theory, we determine the spectral response of the semiconductor microcavity, which includes exciton-polariton resonances as well as a continuum of unbound electron-hole pairs. Our method also gives us access to the photon fraction and the electron-hole wave function of the exciton polariton. In particular, we obtain the conditions under which an exciton is bound by photon exchange and we show that Coulomb interactions can enhance binding at large cavity photon frequencies. Our results for the spectral response are in good agreement with a recent experiment on doped quantum wells [E. Cortese et al., Nat. Phys. 17, 31 (2021)].

Signatures of dark excitons in exciton–polariton optics of transition metal dichalcogenides

Integrating 2D materials into high-quality optical microcavities opens the door to fascinating many-particle phenomena including the formation of exciton–polaritons. These are hybrid quasi-particles inheriting properties of both the constituent photons and excitons. In this work, we investigate the so-far overlooked impact of dark excitons on the momentum-resolved absorption spectra of hBN-encapsulated WSe2 and MoSe2 monolayers in the strong-coupling regime. In particular, thanks to the efficient phonon-mediated scattering of polaritons into energetically lower dark exciton states, the absorption of the lower polariton branch in WSe2 is much higher than in MoSe2. It shows unique step-like increases in the momentum-resolved profile indicating opening of specific scattering channels. We study how different externally accessible quantities, such as temperature or mirror reflectance, change the optical response of polaritons. Our study contributes to an improved microscopic understanding of exciton–polaritons and their interaction with phonons, potentially suggesting experiments that could determine the energy of dark exciton states via momentum-resolved polariton absorption.

Double-Cavity Modulation of Exciton Polaritons in CsPbBr3 Microwire.

The lead halide perovskite has become a promising candidate for the study of exciton polaritons due to their excellent optical properties. Here, both experimental and simulated results confirm the existence of two kinds of Fabry-Pérot microcavities in a single CsPbBr3 microwire with an isosceles right triangle cross section, and we experimentally demonstrate that confined photons in a straight and a folded Fabry-Pérot microcavity are strongly coupled with excitons to form exciton polaritons. Furthermore, we reveal the polarization characteristic and double-cavity modulation of exciton polaritons emission by polarization-resolved fluorescence spectroscopy. Our results not only prove that the modulation of exciton polaritons emission can occur in this simple double-cavity system but also provide a possibility to develop related polariton devices.

Valley-polarized hyperbolic exciton polaritons in few-layer two-dimensional semiconductors at visible frequencies

Polaritons are quasiparticles describing the coupling between light and matter. In two-dimensional materials, polaritonic phenomena are abundant and unique, owing to their low dimensionality and the extraordinary properties of the supported quasiparticles. In this work, we predict the existence of hyperbolic exciton polaritons (HEPs) in few-layer transition-metal dichalcogenides (TMDs) at visible frequencies. We show that hyperbolicity can be induced in the TMD under certain conditions owing to the resonant behavior of the supported excitons, leading to the existence of HEPs. We derive the HEPs' dispersion relation under these conditions and analyze their confinement and loss properties, incorporating nonlocal corrections stemming from the high momentum of the modes. In addition, we show that owing to the valley properties of TMDs, the HEPs are coupled to the valley degree of freedom, leading to a hyperbolic spin-valley Hall effect. Such highly confined and valley-polarized HEPs provide opportunities to control strong light-matter interaction at the atomic scale.

Independent Tuning of Exciton and Photon Energies in an Exciton–Polariton Condensate by Proton Implantation‐Induced Interdiffusion

AbstractThe use of high energy proton implantation is demonstrated to precisely and independently shift the energies of both exciton and photon components of GaAs microcavity exciton–polaritons. The technique applies post‐growth proton implantation and annealing steps in order to create a small local interdiffusion across either the quantum well–barrier material interfaces, or between the layers of the cavity distributed Bragg reflector mirrors to induce energy shifts to the exciton or photon components, respectively. The polariton mode is tunable by an energy exceeding 10 meV with a corresponding increase (decrease) in effective mass for photon (exciton) energy shifts, while maintaining narrow‐linewidth polariton photoemission and condensation for both photonic and excitonic polaritons. This technique uniquely enables new opportunities to explore coherent polariton matter with narrow‐linewidth and heavy masses in tight‐binding, non‐Hermitian, and topological landscapes with sub‐µm feature‐sizes, while also being a simple post‐growth process.

Photophysics of β-Ga2O3: Phonon polaritons, exciton polaritons, free-carrier absorption, and band-edge absorption

Monoclinic gallium oxide (β-Ga2O3) has attracted much attention from the fields of optoelectronic and electronic devices owing to the properties of wide bandgap, great breakdown field strength, as well as the economic advantages of low-cost growth of large-size single crystals. Here, the basic photophysical properties including absorption (free-carrier absorption and band-edge absorption) and reflection (phonon polaritons and exciton polaritons) of differently doped β-Ga2O3 with diverse carrier concentrations are studied in detail. The unpolarized reflection spectra of differently doped β-Ga2O3 crystals are well fitted based on the non-polarized reflection model. Besides, according to analysis, the longitudinal–transverse splitting energy of β-Ga2O3 direct excitons is estimated to be as high as 100 meV, reflecting the strong interaction between light and excitons. It is hoped that this work can provide beneficial reference for a comprehensive understanding on the spectral physical characteristics of β-Ga2O3, so as to deepen and expand the basic recognition of this material in the aspect of photophysical properties.

Scattering solution to the problem of additional boundary conditions

Maxwell's boundary conditions (MBCs) were long known insufficient to determine the optical responses of spatially dispersive medium. Supplementing MBCs with additional boundary conditions (ABCs) has become a normal yet controversial practice. Here the problem of ABCs is solved by analyzing some subtle aspects of a physical surface. A generic theory is presented for handling the interaction of light with the surfaces of an arbitrary medium and applied to study the traditional problem of exciton polaritons. We show that ABCs can always be adjusted to fit the theory but they can by no means be construed as intrinsic surface characteristics, which are instead captured by a \textit{surface response function} (SRF). Unlike any ABCs, a SRF describes essentially non-local boundary effects. Methods for experimentally extracting the spatial profile of this function are proposed.

Direct Observation of Self-Hybridized Exciton-Polaritons and Their Valley Polarizations in a Bare WS2 Layer.

The strong excitonic properties of transition metal dichalcogenides (TMD) have led to the successful demonstration of exciton-polaritons (EPs) in various optical cavity structures. Recently, self-hybridized EPs have been discovered in a bare TMD layer, but experimental investigation is still lacking because of their nonradiative nature. Herein, the direct observation of self-hybridized EPs in a bare multilayer WS2 via the evanescent field coupling technique is reported. Because of the thickness-dependent Rabi splitting energy, the dispersion curves of the EPs change sensitively with sample thickness. Moreover, continuous tuning of EP dispersion curves is demonstrated by controlling the excitation laser power. Lastly, it is observed that guided EPs retain valley polarization up to 0.2 at room temperature, representing a valley-preserved strong coupling regime. It is believed that the high tunability and valley polarization properties of the guided EPs in bare TMD layers can facilitate new nanophotonic and valleytronic applications.

Highly nonlinear dipolar exciton-polaritons in bilayer MoS2

Realizing nonlinear optical response in the low photon density limit in solid-state systems has been a long-standing challenge. Semiconductor microcavities in the strong coupling regime hosting exciton-polaritons have emerged as attractive candidates in this context. However, the weak interaction between these quasiparticles has been a hurdle in this quest. Dipolar excitons provide an attractive strategy to overcome this limitation but are often hindered by their weak oscillator strength. The interlayer dipolar excitons in naturally occurring homobilayer MoS2 alleviates this issue owing to their formation via hybridization of interlayer charge transfer exciton with intralayer B exciton. Here we demonstrate the formation of dipolar exciton polaritons in bilayer MoS2 resulting in unprecedented nonlinear interaction strengths. A ten-fold increase in nonlinearity is observed for the interlayer dipolar excitons compared to the conventional A excitons. These highly nonlinear dipolar polaritons will likely be a frontrunner in the quest for solid-state quantum nonlinear devices.

Ultrafast All-Optical Polarization Switch Controlled by Optically Excited Picosecond Acoustic Perturbation of Exciton Resonance in Planar Microcavities

All-optical switching of the polarization of exciton polaritons is studied using a picosecond acoustic perturbation of the exciton resonance in high-$Q$ planar $\mathrm{Ga}\mathrm{As}$/$\mathrm{Al}\mathrm{As}$ microcavities. An irreversible switching of the degree of circular polarization from $22\mathrm{%}$ up to $82\mathrm{%}$ is realized in a microcavity pumped by a laser with a photon energy slightly exceeding the lower polariton resonance. The polarization switch is performed by a short-term, about 30-ps-long, blueshift of the exciton energy of ($\mathrm{In},\mathrm{Ga}$)$\mathrm{As}$ quantum wells in the cavity active layer by a 10-ps strain pulse generated in the $\mathrm{Ga}\mathrm{As}$ substrate by a violet femtosecond laser pulse and injected into the microcavity. The proposed all-optical control of light polarization using acoustic modulation of a polariton resonance opens the way for fast and easily tunable optical polarization switches.

Influence of the Angular Symmetry of an Exciting Optical Beam on the Dynamics of Cavity Exciton Polaritons in Coupled Potential Traps

Influence of the Angular Symmetry of an Exciting Optical Beam on the Dynamics of Cavity Exciton Polaritons in Coupled Potential Traps