Exciton polariton interactions in Van der Waals superlattices at room temperature

Polariton-Mediated Ultrafast Nonlinear Energy Transfer in a van der Waals Superlattice

Exciton-polariton dynamics in 2D materials have garnered substantial attention across diverse scientific domains for fundamental research with potential applications in optoelectronics. However, practical implementation has been hindered by the challenge of maintaining stable and long-range polariton propagation. Here, we present an innovative material platform featuring extensive monolayer WS2/Al2O3 superlattices (a square with a length of >0.5 cm) coupled to a waveguide mode designed to host exciton-polaritons with operation at room temperature. Time-resolved transient absorption spectra show picosecond nonlinear energy transfer phenomena between upper and lower polariton states, clarifying the dynamic behavior within this quantum realm. In addition, we observed population inversion behavior between the two polariton states that facilitate potential avenues for creating polariton-based ultrafast modulators and switches. This research not only advances our fundamental understanding of polariton dynamics but also promotes the development of innovative technologies that harness these fascinating quantum phenomena.

A general model for designing the chirality of exciton-polaritons

Chirality of exciton-polaritons can be tuned by the chirality of photons, excitons, and their coupling strength. In this work, we propose a general analytical model based on coupled harmonic oscillators to describe the chirality of exciton-polaritons. Our model predicts the degree of circular polarization (DCP) of exciton-polaritons, which is determined by the DCPs and weight fractions of the constituent excitons and photons. At the anticrossing point, the DCP of exciton-polaritons is equally contributed from both constituents. Away from the anticrossing point, the DCP of exciton-polaritons relaxes toward the DCP of the dominant constituent, with the relaxation rate decreasing as the coupling strength increases. We validate our model through simulations of strongly coupled topological edge states and excitons, showing good agreement with model predictions. Our model provides a valuable tool for designing the chirality of strong coupling systems and offers a framework for the inverse design of exciton-polaritons with tailored chirality.

Room-temperature continuous-wave pumped exciton polariton condensation in a perovskite microcavity

Microcavity exciton polaritons (polaritons) as part-light part-matter quasiparticles garner considerable attention for Bose-Einstein condensation at elevated temperatures. Recently, halide perovskites have emerged as promising room-temperature polaritonic platforms because of their large exciton binding energies and superior optical properties. However, currently, inducing room-temperature nonequilibrium polariton condensation in perovskite microcavities requires optical pulsed excitations with high excitation densities. Here, we demonstrate continuous-wave optically pumped polariton condensation with an exceptionally low threshold of ~0.53 W cm-2 and a narrow linewidth of ~0.5 meV. Polariton condensation is unambiguously demonstrated by characterizing the nonlinear behavior and coherence properties. We also unveil the trapping potential landscape strategy to facilitate polariton relaxation and accumulation. Our findings lay the foundation for the next-generation energy-efficient polaritonic devices operating at room temperature.

Revealing Ultrafast Optical Nonlinearity of Trapped Exciton Polaritons in Atomically Thin Semiconductors

Nonlinearities are fundamental to modern optical technologies. Exciton polaritons in semiconductor microcavities provide a promising route to strong nonlinearities. Monolayer TMDs, with tightly bound excitons and strong oscillator strength, enable polaritonic phenomena under ambient conditions but face challenges from weak polariton interactions due to small exciton Bohr radius. Although spatial confinement can boost polariton nonlinearity, the dynamics of trapped polaritons remain underexplored. Here we study the transient nonlinearities of confined polaritons in monolayer WS2 mesa cavities. We observe increasingly pronounced blueshifts within the first few picoseconds as trapping sizes decrease or excitonic fractions increase. Furthermore, our findings reveal that exciton-photon detuning, not trapping size, predominantly influences the time to reach the peak of transient nonlinearity. This insight aligns with the experimentally observed and theoretically simulated relaxation dynamics of trapped polaritons. Our findings pave the way for developing ultrafast all-optical polaritonic devices in TMD systems.

Room-Temperature Exciton Polaritons in a Monolayer Molecular Crystal

Strong coupling between excitons and photons in optical microcavities leads to the formation of exciton polaritons, which maintain both the coherence of light and the interaction of matter. Recently, atomically thin monolayer semiconductors with a large exciton oscillator strength and high exciton binding energy have been widely used for realizing room-temperature exciton polaritons. Here, we demonstrated room-temperature exciton polaritons with a monolayer molecular crystal. The molecular monolayers behave as J-aggregates with comparable oscillator strength and narrow line width as inorganic monolayers, enabling exciton-photon strong coupling at the monolayer limit. Moreover, the coupling strength can be tuned systematically via engineering the in-plane polarization or by using a vertical stack of multiple molecular monolayers. Our research provides a new material platform for realizing strong light-matter interactions inside optical microcavities at room temperature and may motivate the development of molecular-crystal-based exciton-polaritonic devices with novel functions and new possibilities.

Element-Specific Ultrafast Lattice Dynamics in Monolayer WSe2

We study monolayer WSe2 using ultrafast electron diffraction. We introduce an approach to quantitatively extract atomic-site-specific information, providing an element-specific view of incoherent atomic vibrations following femtosecond excitation. Via differences between W and Se vibrations, we identify stages in the nonthermal evolution of the phonon population. Combined with a calculated phonon dispersion, this element specificity enables us to identify a long-lasting overpopulation of specific optical phonons and to interpret the stages as energy transfer processes between specific phonon groups. These results demonstrate the appeal of resolving element-specific vibrational information in the ultrafast time domain.

Room temperature polariton spin switches based on Van der Waals superlattices

Transition-metal dichalcogenide monolayers possess large exciton binding energy and a robust valley degree of freedom, making them a viable platform for the development of spintronic devices capable of operating at room temperature. The development of such monolayer TMD-based spintronic devices requires strong spin-dependent interactions and effective spin transport. This can be achieved by employing exciton-polaritons. These hybrid light-matter states arising from the strong coupling of excitons and photons allow high-speed in-plane propagation and strong nonlinear interactions. Here, we demonstrate the operation of all-optical polariton spin switches by incorporating a WS2 superlattice into a planar microcavity. We demonstrate spin-anisotropic polariton nonlinear interactions in a WS2 superlattice at room temperature. As a proof-of-concept, we utilize these spin-dependent interactions to implement different spin switch geometries at ambient conditions, which show intrinsic sub-picosecond switching time and small footprint. Our findings offer new perspectives on manipulations of the polarization state in polaritonic systems and highlight the potential of atomically thin semiconductors for the development of next generation information processing devices.

Strong light-matter coupling in van der Waals materials

In recent years, two-dimensional (2D) van der Waals materials have emerged as a focal point in materials research, drawing increasing attention due to their potential for isolating and synergistically combining diverse atomic layers. Atomically thin transition metal dichalcogenides (TMDs) are one of the most alluring van der Waals materials owing to their exceptional electronic and optical properties. The tightly bound excitons with giant oscillator strength render TMDs an ideal platform to investigate strong light-matter coupling when they are integrated with optical cavities, providing a wide range of possibilities for exploring novel polaritonic physics and devices. In this review, we focused on recent advances in TMD-based strong light-matter coupling. In the foremost position, we discuss the various optical structures strongly coupled to TMD materials, such as Fabry-Perot cavities, photonic crystals, and plasmonic nanocavities. We then present several intriguing properties and relevant device applications of TMD polaritons. In the end, we delineate promising future directions for the study of strong light-matter coupling in van der Waals materials.

Monolayer Semiconductor Superlattices with High Optical Absorption

Optical absorption plays a central role in optoelectronic and photonic technologies. Strongly absorbing materials are thus needed for efficient and miniaturized devices. A uniform film much thinner than the wavelength can only absorb up to 50% of the incident light when embedded in a symmetric and homogeneous environment. Although deviating from these conditions allows higher absorption, finding the thinnest possible material with the highest intrinsic absorption is still desirable. Here, we demonstrate strong absorption by artificially stacking WS2 monolayers into superlattices. We compare three simple approaches based on different spacer materials to surpass the peak absorptance of a single WS2 monolayer, which stands at 16% on ideal substrates. Through direct monolayer stacking without an intentional spacer, we reach an absorptance of 27% for an artificial bilayer, although with limited control over interlayer distance. Using a molecular spacer via spin coating, we demonstrate controllable spacer thickness in a bilayer with 25% absorptance while increasing photoluminescence thanks to doping. Finally, we exploit the atomic layer deposition of alumina spacers to boost the absorptance to 31% for a 4-monolayer superlattice. Our results demonstrate that monolayer superlattices are a powerful platform directly applicable to improve strong light-matter coupling and enhance the performance of nanophotonic devices such as modulators and photodetectors.

High Efficiency of Exciton-Polariton Lasing in a 2D Multilayer Structure

We have placed a van der Waals homostructure, formed by stacking three two-dimensional layers of WS2 separated by insulating hBN, similar to a multiple-quantum well structure, inside a microcavity, which facilitates the formation of quasiparticles known as exciton-polaritons. The polaritons are a combination of light and matter, allowing laser emission to be enhanced by nonlinear scattering, as seen in prior polariton lasers. In the experiments reported here, we have observed laser emission with an ultralow threshold. The threshold was approximately 59 nW/μm2, with a lasing efficiency of 3.82%. These findings suggest a potential for efficient laser operations using such homostructures.

Origin of Exciton-Polariton Interactions and Decoupled Dark States Dynamics in 2D Hybrid Perovskite Quantum Wells

The realization of efficient optical devices depends on the ability to harness strong nonlinearities, which are challenging to achieve with standard photonic systems. Exciton-polaritons formed in hybrid organic-inorganic perovskites offer a promising alternative, exhibiting strong interactions at room temperature (RT). Despite recent demonstrations showcasing a robust nonlinear response, further progress is hindered by an incomplete understanding of the microscopic mechanisms governing polariton interactions in perovskite-based strongly coupled systems. Here, we investigate the nonlinear properties of quasi-2D dodecylammonium lead iodide perovskite (n3-C12) crystals embedded in a planar microcavity. Polarization-resolved pump-probe measurements reveal the contribution of indirect exchange interactions assisted by dark states formation. Additionally, we identify a strong dependence of the unique spin-dependent interaction of polaritons on sample detuning. The results are pivotal for the advancement of polaritonics, and the tunability of the robust spin-dependent anisotropic interaction in n3-C12 perovskites makes this material a powerful choice for the realization of polaritonic circuits.

Raman scattering owing to magneto-polaron states in monolayer transition metal dichalcogenides

Magneto-optical measurements are fundamental research tools that allow for studying the hitherto unexplored optical transitions and the related applications of topological two-dimensional (2D) transition metal dichalcogenides (TMDs). A theoretical model is developed for the first-order magneto-resonant Raman scattering in a monolayer of TMD. A significant number of avoided crossing points involving optical phonons in the magneto-polaron (MP) spectrum, a superposition of the electron and hole states in the excitation branches, and their manifestations in optical transitions at various light scattering configurations are unique features for these 2D structures. The Raman intensity reveals three resonant splittings of double avoided-crossing levels. The three excitation branches are present in the MP spectrum provoked by the coupling of the Landau levels in the conduction and valence bands via an out-of-plane A 1 -optical phonon mode. The energy gaps at the anticrossing points in the MP scattering spectrum are revealed as a function of the electron and hole optical deformation potential constants. The resonant MP Raman scattering efficiency profile allows for quantifying the relative contribution of the conduction and valence bands in the formation of MPs. The results obtained are a guideline for controlling MP effects on the magneto-optical properties of TMD semiconductors, which open pathways to novel optoelectronic devices based on 2D TMDs.

Energy transfer driven brightening of MoS2 by ultrafast polariton relaxation in microcavity MoS2/hBN/WS2 heterostructures

Energy transfer is a ubiquitous phenomenon that delivers energy from a blue-shifted emitter to a red-shifted absorber, facilitating wide photonic applications. Two-dimensional (2D) semiconductors provide unique opportunities for exploring novel energy transfer mechanisms in the atomic-scale limit. Herein, we have designed a planar optical microcavity-confined MoS2/hBN/WS2 heterojunction, which realizes the strong coupling among donor exciton, acceptor exciton, and cavity photon mode. This configuration demonstrates an unconventional energy transfer via polariton relaxation, brightening MoS2 with a record-high enhancement factor of ~440, i.e., two-order-of-magnitude higher than the data reported to date. The polariton relaxation features a short characteristic time of ~1.3 ps, resulting from the significantly enhanced intra- and inter-branch exciton-exciton scattering. The polariton relaxation dynamics is associated with Rabi energies in a phase diagram by combining experimental and theoretical results. This study opens a new direction of microcavity 2D semiconductor heterojunctions for high-brightness polaritonic light sources and ultrafast polariton carrier dynamics.

Exciton Polaritons in Emergent Two-Dimensional Semiconductors

The "marriage" of light (i.e., photon) and matter (i.e., exciton) in semiconductors leads to the formation of hybrid quasiparticles called exciton polaritons with fascinating quantum phenomena such as Bose-Einstein condensation (BEC) and photon blockade. The research of exciton polaritons has been evolving into an era with emergent two-dimensional (2D) semiconductors and photonic structures for their tremendous potential to break the current limitations of quantum fundamental study and photonic applications. In this Perspective, the basic concepts of 2D excitons, optical resonators, and the strong coupling regime are introduced. The research progress of exciton polaritons is reviewed, and important discoveries (especially the recent ones of 2D exciton polaritons) are highlighted. Subsequently, the emergent 2D exciton polaritons are discussed in detail, ranging from the realization of the strong coupling regime in various photonic systems to the discoveries of attractive phenomena with interesting physics and extensive applications. Moreover, emerging 2D semiconductors, such as 2D perovskites (2DPK) and 2D antiferromagnetic (AFM) semiconductors, are surveyed for the manipulation of exciton polaritons with distinct control degrees of freedom (DOFs). Finally, the outlook on the 2D exciton polaritons and their nonlinear interactions is presented with our initial numerical simulations. This Perspective not only aims to provide an in-depth overview of the latest fundamental findings in 2D exciton polaritons but also attempts to serve as a valuable resource to prospect explorations of quantum optics and topological photonic applications.

Recent progress of exciton transport in two-dimensional semiconductors

Spatial manipulation of excitonic quasiparticles, such as neutral excitons, charged excitons, and interlayer excitons, in two-dimensional semiconductors offers unique capabilities for a broad range of optoelectronic applications, encompassing photovoltaics, exciton-integrated circuits, and quantum light-emitting systems. Nonetheless, their practical implementation is significantly restricted by the absence of electrical controllability for neutral excitons, short lifetime of charged excitons, and low exciton funneling efficiency at room temperature, which remain a challenge in exciton transport. In this comprehensive review, we present the latest advancements in controlling exciton currents by harnessing the advanced techniques and the unique properties of various excitonic quasiparticles. We primarily focus on four distinct control parameters inducing the exciton current: electric fields, strain gradients, surface plasmon polaritons, and photonic cavities. For each approach, the underlying principles are introduced in conjunction with its progression through recent studies, gradually expanding their accessibility, efficiency, and functionality. Finally, we outline the prevailing challenges to fully harness the potential of excitonic quasiparticles and implement practical exciton-based optoelectronic devices.

Bose Condensation of Upper-Branch Exciton-Polaritons in a Transferable Microcavity

Exciton-polaritons are composite quasiparticles that result from the coupling of excitonic transitions and optical modes. They have been extensively studied because of their quantum phenomena and potential applications in unconventional coherent light sources and all-optical control elements. In this work, we report the observation of Bose-Einstein condensation of the upper polariton branch in a transferable WS2 monolayer microcavity. Near the condensation threshold, we observe a nonlinear increase in upper polariton intensity accompanied by a decrease in line width and an increase in temporal coherence, all of which are hallmarks of Bose-Einstein condensation. Simulations show that this condensation occurs within a specific particle density range, depending on the excitonic properties and pumping conditions. The manifestation of upper polariton condensation unlocks new possibilities for studying the condensate competition while linking it to practical realizations in polaritonic lasers. Our findings contribute to the understanding of bosonic systems and offer potential for the development of polaritonic devices.

Unraveling the Ultrafast Coherent Dynamics of Exciton Polariton Propagation at Room Temperature

Exciton polaritons are widely considered as promising platforms for developing room-temperature polaritonic devices, owing to the high-speed propagation and nonlinear interactions. However, it remains challenging to explore the dynamics of exciton polaritons specifically at room temperature, where the lifetime could be as small as a few picoseconds and the prevailing time-averaged measurement cannot give access to the true nature of it. Herein, by using the time-resolved photoluminescence, we have successfully traced the ultrafast coherent dynamics of a moving exciton polariton condensate in a one-dimensional perovskite microcavity. The propagation speed is directly measured to be ∼12.2 ± 0.8 μm/ps. Moreover, we have developed a time-resolved Michelson interferometry to quantify the time-dependent phase coherence, which reveals that the actual coherence time of exciton polaritons could be much longer (nearly 100%) than what was believed before. Our work sheds new light on the ultrafast coherent propagation of exciton polaritons at room temperature.