Multiscale Supercrystal Meta-atoms.

Meta-atoms are the building blocks of metamaterials, which are employed to control both generation and propagation of light as well as provide novel functionalities of localization and directivity of electromagnetic radiation. In many cases, simple dielectric or metallic resonators are employed as meta-atoms to create different types of electromagnetic metamaterials. Here, we fabricate and study supercrystal meta-atoms composed of coupled perovskite quantum dots. We reveal that these multiscale structures exhibit specific emission properties, such as spectrum splitting and polaritonic effects. We believe that such multiscale supercrystal meta-atoms will provide novel functionalities in the design of many novel types of active metamaterials and metasurfaces.

Long-Range Ballistic Propagation of 80% Excitonic Fraction Polaritons in a Perovskite Metasurface at Room Temperature.

Exciton-polaritons, hybrid light-matter excitations arising from the strong coupling between excitons in semiconductors and photons in photonic nanostructures, are crucial for exploring the physics of quantum fluids of light and developing all-optical devices. Achieving room temperature propagation of polaritons with a large excitonic fraction is challenging but vital, e.g., for nonlinear light transport. We report on room temperature propagation of exciton-polaritons in a metasurface made from a subwavelength lattice of perovskite pillars. The large Rabi splitting, much greater than the optical phonon energy, decouples the lower polariton band from the phonon bath of the perovskite. These cooled polaritons, in combination with the high group velocity achieved through the metasurface design, enable long-range propagation, exceeding hundreds of micrometers even with an 80% excitonic component. Furthermore, the design of the metasurface introduces an original mechanism for unidirectional propagation through polarization control, suggesting a new avenue for the development of advanced polaritonic devices.

Polarization-Controlled Exciton-Polaritons in WS2 Strongly Coupled with Low-Symmetry Photonic Crystal Nanostructures.

Atomically thin transition metal dichalcogenides (TMDs) with ambient stable exciton resonances have emerged as an ideal material platform for exciton-polaritons. In particular, the strong coupling between excitons in TMDs and optical resonances in anisotropic photonic nanostructures can form exciton-polaritons with polarization selectivity, which offers a new degree of freedom for the manipulation of the light-matter interaction. In this work, we present the experimental demonstration of polarization-controlled exciton-polaritons in tungsten disulfide (WS2) strongly coupled with polarization singularities in the momentum space of low-symmetry photonic crystal (PhC) nanostructures. The utilization of polarization singularities can not only effectively modulate the polarization states of exciton-polaritons in the momentum space but also facilitate or suppress their far field coupling capabilities by tuning the in-plane momentum. Our results provide new strategies for creating polarization-selective exciton-polaritons.

Microscopic Theory of Multimode Polariton Dispersion in Multilayered Materials.

We develop a microscopic theory for the multimode polariton dispersion in materials coupled to cavity radiation modes. Starting from a microscopic light-matter Hamiltonian, we devise a general strategy for obtaining simple matrix models of polariton dispersion curves based on the structure and spatial location of multilayered 2D materials inside the optical cavity. Our theory exposes the connections between seemingly distinct models that have been employed in the literature and resolves an ambiguity that has arisen concerning the experimental description of the polaritonic band structure. We demonstrate the applicability of our theoretical formalism by fabricating various geometries of multilayered perovskite materials coupled to cavities and demonstrating that our theoretical predictions agree with the experimental results presented here.

Polaron-Enhanced Polariton Nonlinearity in Lead Halide Perovskites.

Exciton-polaritons offer a versatile platform for realization of all-optical integrated logic gates due to the strong effective optical nonlinearity resulting from the exciton-exciton interactions. In most of the current excitonic materials there exists a direct connection between the exciton robustness to thermal fluctuations and the strength of the exciton-exciton interaction, making materials with the highest levels of exciton nonlinearity applicable at cryogenic temperatures only. Here, we show that strong polaronic effects, characteristic for perovskite materials, allow overcoming this limitation. Namely, we demonstrate a record-high value of the nonlinear optical response in the nanostructured organic-inorganic halide perovskite MAPbI3, experimentally detected as a 19.7 meV blueshift of the polariton branch under femtosecond laser irradiation. This is substantially higher than characteristic values for the samples based on conventional semiconductors and monolayers of transition-metal dichalcogenides. The observed strong polaron-enhanced nonlinearity exists for both tetragonal and orthorhombic phases of MAPbI3 and remains stable at elevated temperatures.

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.

Imaging Anisotropic Waveguide Exciton Polaritons in Tin Sulfide.

In recent years, novel materials supporting in-plane anisotropic polaritons have attracted a great deal of research interest due to their capability of shaping nanoscale field distributions and controlling nanophotonic energy flows. Here we report a nano-optical imaging study of waveguide exciton polaritons (EPs) in tin sulfide (SnS) in the near-infrared (near-IR) region using scattering-type scanning near-field optical microscopy (s-SNOM). With s-SNOM, we mapped in real space the propagative EPs in SnS, which show sensitive dependence on the excitation energy and sample thickness. Moreover, we found that both the polariton wavelength and propagation length are anisotropic in the sample plane. In particular, in a narrow spectral range from 1.32 to 1.44 eV, the EPs demonstrate quasi-one-dimensional propagation, which is rarely seen in natural polaritonic materials. A further analysis indicates that the observed polariton anisotropy originates from the different optical band gaps and exciton binding energies along the two principal crystal axes of SnS.

Neuromorphic Binarized Polariton Networks.

The rapid development of artificial neural networks and applied artificial intelligence has led to many applications. However, current software implementation of neural networks is severely limited in terms of performance and energy efficiency. It is believed that further progress requires the development of neuromorphic systems, in which hardware directly mimics the neuronal network structure of a human brain. Here, we propose theoretically and realize experimentally an optical network of nodes performing binary operations. The nonlinearity required for efficient computation is provided by semiconductor microcavities in the strong quantum light-matter coupling regime, which exhibit exciton-polariton interactions. We demonstrate the system performance against a pattern recognition task, obtaining accuracy on a par with state-of-the-art hardware implementations. Our work opens the way to ultrafast and energy-efficient neuromorphic systems taking advantage of ultrastrong optical nonlinearity of polaritons.

Second Harmonic Generation from a Single Plasmonic Nanorod Strongly Coupled to a WSe2 Monolayer.

Monolayer transition metal dichalcogenides, coupled to metal plasmonic nanocavities, have recently emerged as new platforms for strong light-matter interactions. These systems are expected to have nonlinear-optical properties that will enable them to be used as entangled photon sources, compact wave-mixing devices, and other elements for classical and quantum photonic technologies. Here, we report the first experimental investigation of the nonlinear properties of these strongly coupled systems, by observing second harmonic generation from a WSe2 monolayer strongly coupled to a single gold nanorod. The pump-frequency dependence of the second-harmonic signal displays a pronounced splitting that can be explained by a coupled-oscillator model with second-order nonlinearities. Rigorous numerical simulations utilizing a nonperturbative nonlinear hydrodynamic model of conduction electrons support this interpretation and reproduce experimental results. Our study thus lays the groundwork for understanding the nonlinear properties of strongly coupled nanoscale systems.

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

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

Polaritonic Neuromorphic Computing Outperforms Linear Classifiers.

Machine learning software applications are ubiquitous in many fields of science and society for their outstanding capability to solve computationally vast problems like the recognition of patterns and regularities in big data sets. In spite of these impressive achievements, such processors are still based on the so-called von Neumann architecture, which is a bottleneck for faster and power-efficient neuromorphic computation. Therefore, one of the main goals of research is to conceive physical realizations of artificial neural networks capable of performing fully parallel and ultrafast operations. Here we show that lattices of exciton-polariton condensates accomplish neuromorphic computing with outstanding accuracy thanks to their high optical nonlinearity. We demonstrate that our neural network significantly increases the recognition efficiency compared with the linear classification algorithms on one of the most widely used benchmarks, the MNIST problem, showing a concrete advantage from the integration of optical systems in neural network architectures.

Tailoring Dispersion of Room-Temperature Exciton-Polaritons with Perovskite-Based Subwavelength Metasurfaces.

Exciton-polaritons represent a promising platform for studying quantum fluids of light and realizing prospective all-optical devices. Here we report on the experimental demonstration of exciton-polaritons at room temperature in resonant metasurfaces made from a sub-wavelength two-dimensional lattice of perovskite pillars. The strong coupling regime is revealed by both angular-resolved reflectivity and photoluminescence measurements, showing anticrossing between photonic modes and the exciton resonance with a Rabi splitting in the 200 meV range. Moreover, by tailoring the photonic Bloch mode to which perovskite excitons are coupled, polaritonic dispersions are engineered exhibiting linear, parabolic, and multivalley dispersions. All of our results are perfectly reproduced by both numerical simulations based on a rigorous coupled wave analysis and an elementary model based on a quantum theory of radiation-matter interaction. Our results suggest a new approach to study exciton-polaritons and pave the way toward large-scale and low-cost integrated polaritonic devices operating at room temperature.

Efficient Bosonic Condensation of Exciton Polaritons in an H-Aggregate Organic Single-Crystal Microcavity.

Although organic polariton condensation has been recently demonstrated, they only utilize the photon part of polaritons and ignore the excitonic contribution because the polariton-polariton and polariton-reservoir interactions are weak in organic microcavities owing to the absence of Coulomb exchange-interactions between Frenkel excitons. We demonstrate highly efficient and strongly polarization-dependent polariton condensates in a microcavity consisting of an H-aggregate organic single-crystalline microbelt sandwiched between two silver reflectors. Benefitting from the advantages of vibronic coupling in H-aggregates and heavy exciton-like polaritons, both macroscopic coherent polariton ground-state population and high-energy quantized-modes are observed. The measurements are qualitatively reproduced based on simulations of the spatiotemporal polariton dynamics. The observation of low threshold polariton lasing, the ease of fabrication, and the potential for efficient electronic charge injection make microcrystals of organic semiconductors attractive candidates for continuous wave and electrically pumped functional photonic polariton circuits and organic polariton lasers, operating at room temperature.

Nonlinear Optics at Excited States of Exciton Polaritons in Two-Dimensional Atomic Crystals.

Exciton polaritons (EPs) are partial-light partial-matter quasiparticles in semiconductors demonstrating striking quantum phenomena such as Bose-Einstein condensation and single-photon nonlinearity. In these phenomena, the governing process is the EP relaxation into the ground states upon excitation, where various mechanisms are extensively investigated with thermodynamic limits. However, the relaxation process becomes drastically different and could significantly advance the understanding of EP dynamics for these quantum phenomena, when excited states of EPs are involved. Here, for the first time, we observe nonlinear optical responses at the EP excited states in a monolayer tungsten disulfide (WS2) microcavity, including dark excited states and dynamically metastable upper polariton bands. The nonlinear optics leads to unique emissions of ground states with prominent valley degree of freedom (DOF) via an anomalous relaxation process, which is applicable to a wide range of semiconductors from monolayer transition metal dichalcogenides (TMDs) to emerging halide perovskites. This work promises possible approaches to challenging experiments such as valley polariton condensation. Moreover, it also constructs a valley-dependent solid-state three-level system for terahertz photonics and stimulated Raman adiabatic passage.

Size-Dependent Control of Exciton-Polariton Interactions in WS2 Nanotubes.

Multiwall WS2 nanotubes (and fullerene-like nanoparticles thereof) are currently synthesized in large amounts, reproducibly. Other than showing interesting mechanical and tribological properties, which offer them a myriad of applications, they are recently shown to exhibit remarkable optical and electrical properties, including quasi-1D superconductivity, electroluminescence, and a strong bulk photovoltaic effect. Here, it is shown that, using a simple dispersion-fractionation technique, one can control the diameter of the nanotubes and move from pure excitonic to polaritonic features. While nanotubes of an average diameter >80 nm can support cavity modes and scatter light effectively via a strong coupling mechanism, the extinction of nanotubes with smaller diameter consists of pure absorption. The experimental work is complemented by finite-difference time-domain simulations, which shed new light on the cavity mode-exciton interaction in 2D materials. Furthermore, transient absorption experiments of the size-fractionated nanotubes fully confirm the steady-state observations. Moreover, it is shown that the tools developed here are useful for size control of the nanotubes, e.g., in manufacturing environment. The tunability of the light-matter interaction of such nanotubes offers them intriguing applications such as polaritonic devices, in photocatalysis, and for multispectral sensors.

Fano Resonance in Waveguide Coupled Surface Exciton Polaritons: Theory and Application in Biosensor.

Surface exciton polaritons (SEPs) are one of the three major elementary excitations: Phonons, plasmons and excitons. They propagate along the interface of the crystal and dielectric medium. Surface exciton polaritons hold a significant position in the aspect of novel sensor and optical devices. In this article, we have realized a sharp Fano resonance (FR) by coupling the planar waveguide mode (WGM) and SEP mode with Cytop (perfluoro (1-butenyl vinyl ether)) and J-aggregate cyanine dye. After analyzing the coupling mechanism and the localized field enhancement, we then applied our structure to the imaging biosensor. It was shown that the maximum imaging sensitivity of this sensor could be as high as 5858 RIU-1, which is more than three times as much as classical FR based on metal. A biosensor with ultra-high sensitivity, simple manufacturing technique and lower cost with J-aggregate cyanine dye provides us with the most appropriate substitute for the surface plasmon resonance sensors with the noble metals and paves the way for applications in new sensing technology and biological studies.

Plasmonic Crystals for Strong Light-Matter Coupling in Carbon Nanotubes.

Their high oscillator strength and large exciton binding energies make single-walled carbon nanotubes (SWCNTs) highly promising materials for the investigation of strong light-matter interactions in the near infrared and at room temperature. To explore their full potential, high-quality cavities-possibly with nanoscale field localization-are required. Here, we demonstrate the room temperature formation of plasmon-exciton polaritons in monochiral (6,5) SWCNTs coupled to the subdiffraction nanocavities of a plasmonic crystal created by a periodic gold nanodisk array. The interaction strength is easily tuned by the number of SWCNTs that collectively couple to the plasmonic crystal. Angle- and polarization resolved reflectivity and photoluminescence measurements combined with the coupled-oscillator model confirm strong coupling (coupling strength ∼120 meV). The combination of plasmon-exciton polaritons with the exceptional charge transport properties of SWCNTs should enable practical polariton devices at room temperature and at telecommunication wavelengths.

Giant Optical Anisotropy in 2D Metal-Organic Chalcogenates.

Optical anisotropy is a fundamental attribute of some crystalline materials and is quantified via birefringence. A birefringent crystal gives rise to not only asymmetrical light propagation but also attenuation along two distinct polarizations, a phenomenon called linear dichroism (LD). Two-dimensional (2D) layered materials with high in-plane and out-of-plane anisotropy have garnered interest in this regard. Mithrene, a 2D metal-organic chalcogenate (MOCHA) compound, exhibits strong excitonic resonances due to its naturally occurring multiquantum well (MQW) structure and in-plane anisotropic response in the blue wavelength (∼400-500 nm) regime. The MQW structure and the large refractive indices of mithrene allow the hybridization of the excitons with photons to form self-hybridized exciton-polaritons in mithrene crystals with appropriate thicknesses. Here, we report the giant birefringence (∼1.01) and the tunable in-plane anisotropic response of mithrene, which stem from its low symmetry crystal structure and strong excitonic properties. We show that the LD in mithrene can be tuned by leveraging the anisotropic exciton-polariton formation via the cavity coupling effect, exhibiting giant in-plane LD (∼77.1%) at room temperature. Our results indicate that mithrene is a polaritonic birefringent material for polarization-sensitive nanophotonic applications in the short wavelength regime.

Room Temperature Polariton Condensation from Whispering Gallery Modes in CsPbBr3 Microplatelets.

Room temperature (RT) polariton condensate holds exceptional promise for revolutionizing various fields of science and technology, encompassing optoelectronics devices to quantum information processing. Using perovskite materials, like all-inorganic cesium lead bromide (CsPbBr3) single crystal, provides additional advantages, such as ease of synthesis, cost-effectiveness, and compatibility with existing semiconductor technologies. In this work, the formation of whispering gallery modes (WGM) in CsPbBr3 single crystals with controlled geometry is shown, synthesized using a low-cost and efficient capillary bridge method. Through the implementation of microplatelets geometry, enhanced optical properties and performance are achieved due to the presence of sharp edges and a uniform surface, effectively avoiding non-radiative scattering losses caused by defects. This allows not only to observe strong light matter coupling and formation of whispering gallery polaritons, but also to demonstrate the onset of polariton condensation at RT. This investigation not only contributes to the advancement of the knowledge concerning the exceptional optical properties of perovskite-based polariton systems, but also unveils prospects for the exploration of WGM polariton condensation within the framework of a 3D perovskite-based platform, working at RT. The unique characteristics of polariton condensate, including low excitation thresholds and ultrafast dynamics, open up unique opportunities for advancements in photonics and optoelectronics devices.

Tunable Exciton Polaritons in Band-Gap Engineered Hexagonal Boron Nitride.

We show that hexagonal boron nitride (hBN), a two-dimensional insulator, when subjected to an external superlattice potential forms a paradigm for electrostatically tunable excitons in the near- and mid-ultraviolet (UV). With a combination of analytical and numerical methods, we see that the imposed potential has three consequences: (i) It renormalizes the effective mass tensor, leading to anisotropic effective masses. (ii) It renormalizes the band gap, eventually reducing it. (iii) It reduces the exciton binding energies. All these consequences depend on a single dimensionless parameter, which includes the product of strength of the external potential with its period. In addition to the excitonic energy levels, we compute the optical conductivity along two orthogonal directions and from it the absorption spectrum. The results for the latter show that our system is able to mimic a grid polarizer. These characteristics make one-dimensional hBN superlattices a viable and meaningful platform for fine-tuned polaritonics in the UV to visible spectral range.