All-optical polariton transistor

Ultrafast dynamics of exciton-polariton fluids at non-zero momenta

In this study, we have explored the ultrafast formation and decay dynamics of exciton-polariton fluids at non-zero momenta, non-resonantly excited by a small-spot femtosecond pump pulse in a ZnO microcavity. Using the femtosecond angle-resolved spectroscopic imaging technique, multidimensional dynamics in both the energy and momentum degrees of freedom have been obtained. Two distinct regions with different decay rate in the energy dimension and various decay-channels in the momentum dimension can be well-resolved. Theoretical simulations based on the generalized Gross-Pitaevskii equation can reach a qualitative agreement with the experimental observations, demonstrating the significance of the initial potential barrier induced by the pump pulse during the decay process. The finding of our study can provide additional insights into the fundamental understanding of exciton-polariton condensates, enabling further advancements for controlling the fluids and practical applications.

Room temperature, cascadable, all-optical polariton universal gates

Today, almost all information processing is performed using electronic logic circuits operating at several gigahertz frequency. All-optical logic holds the promise to allow for up to three orders of magnitude higher speed. Whereas essential all-optical transistor functionalities were demonstrated across a range of platforms, utilising them to implement a complete Boolean logic gate set and in particular negation, i.e. switching off an optical signal with another, weaker, optical signal, poses a major challenge. Here, we realize a cascadable NOT gate by introducing the concept of non-ground-state polariton amplification in organic semiconductor microcavities under non-resonant optical excitation. We unravel the importance of vibron-mediated stimulated scattering in room temperature operation of the inverter. Moreover, we extend the concept to a multi-input universal NOR logic gate, where in the presence of any of the input signals non-ground-state amplification supersedes spontaneous ground-state condensation, resulting in a NOR gate with ~1 ps switching time. The realisation of an ultrafast universal logic gate constitutes an essential step for more complex optical circuitry that could boost information processing applications.

Switchable Anisotropic/Isotropic Photon Transport in a Double-Dipole Metal-Organic Framework via Radical-Controlled Energy Transfer

Directional control of photon transport at micro/nanoscale holds great potential in developing multifunctional optoelectronic devices. Here, the switchable anisotropic/isotropic photon transport is reported in a double-dipole metal-organic framework (MOF) based on radical-controlled energy transfer. Double-dipole MOF microcrystals with transition dipole moments perpendicular to each other have been achieved by the pillared-layer coordination strategy. The energy transfer between the double dipolar chromophores can be modulated by the photogenerated radicals, which permits the in situ switchable output on both polarization (isotropy/anisotropy state) and wavelength information (blue/red-color emission). On this basis, the original MOF microcrystal with isotropic polarization state displays the isotropic photon transport and similar reabsorption losses at various directions, while the radical-affected MOF microcrystal with anisotropic polarization state shows the anisotropic photon transport with distinct reabsorption losses at different directions, finally leading to the in situ switchable anisotropic/isotropic photon transport. These results offer a novel strategy for the development of MOF-based photonic devices with tunable anisotropic performance.

Time-of-flight detection of terahertz phonon-polariton

A polariton is a fundamental quasiparticle that arises from strong light-matter interaction and as such has attracted wide scientific and practical interest. When light is strongly coupled to the crystal lattice, it gives rise to phonon-polaritons (PPs), which have been proven useful in the dynamical manipulation of quantum materials and the advancement of terahertz technologies. Yet, current detection and characterization methods of polaritons are still limited. Traditional techniques such as Raman or transient grating either rely on fine-tuning of external parameters or complex phase extraction techniques. To overcome these inherent limitations, we propose and demonstrate a technique based on a time-of-flight measurement of PPs. We resonantly launch broadband PPs with intense terahertz fields and measure the time-of-flight of each spectral component with time-resolved second harmonic generation. The time-of-flight information, combined with the PP attenuation, enables us to resolve the real and imaginary parts of the PP dispersion relation. We demonstrate this technique in the van der Waals magnets NiI2 and MnPS3 and reveal a hidden magnon-phonon interaction. We believe that this approach will unlock new opportunities for studying polaritons across diverse material systems and enhance our understanding of strong light-matter interaction.

Mechanical Control of Polaritonic States in Lead Halide Perovskite Phonons Strongly Coupled in THz Microcavity

We demonstrate the generation and control of polaritonic states in perovskite phonon polaritons, which are strongly coupled in the middle of a flexible Fabry-Perot cavity. We fabricated flexible perovskite films on a microporous substrate coated with graphene oxide, which led to a virtually free-standing film incorporated into the microcavity. Rabi splitting was observed when the cavity resonance was in tune with that of the phonons. The Rabi splitting energy increased as the film thickness increased, reaching 1.9 meV, which is 2.4-fold higher than the criterion for the strong coupling regime. We obtained dispersion curves for various perovskite film thicknesses exhibiting two polariton branches; clear beats between the two polaritonic branches were observed in the time domain. Flexible cavity devices with perovskite phonons enable macroscopic control over the polaritonic energy states through bending processes, which add an additional degree of freedom in the manipulation of polaritonic 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.

Strongly enhanced light-matter coupling of monolayer WS2 from a bound state in the continuum

Exciton-polaritons derived from the strong light-matter interaction of an optical bound state in the continuum with an excitonic resonance can inherit an ultralong radiative lifetime and significant nonlinearities, but their realization in two-dimensional semiconductors remains challenging at room temperature. Here we show strong light-matter interaction enhancement and large exciton-polariton nonlinearities at room temperature by coupling monolayer tungsten disulfide excitons to a topologically protected bound state in the continuum moulded by a one-dimensional photonic crystal, and optimizing for the electric-field strength at the monolayer position through Bloch surface wave confinement. By a structured optimization approach, the coupling with the active material is maximized here in a fully open architecture, allowing to achieve a 100 meV photonic bandgap with the bound state in the continuum in a local energy minimum and a Rabi splitting of 70 meV, which results in very high cooperativity. Our architecture paves the way to a class of polariton devices based on topologically protected and highly interacting bound states in the continuum.

Photonic transistor based on a coupled-cavity system with polaritons

We investigate the transmission of probe fields in a coupled-cavity system with polaritons and propose a theoretical schema for realizing a polariton-based photonic transistor. When probe light passes through such a hybrid optomechanical device, its resonant point with Stokes or anti-Stokes scattered effects, intensity with amplification or attenuation effects, as well as group velocity with slow or fast light effects can be effectively controlled by another pump light. This controlling depends on the exciton-photon coupling and single-photon coupling. We also discover an asymmetric Fano resonance in transparency windows under the strong exciton-photon coupling, which is different from general symmetric optomechanically induced transparency. Our results open up exciting possibilities for designing photonic transistors, which may be useful for implementing polariton integrated circuits.

Flexible and Red-Emissive Organic Single-Crystal Microresonator for Efficient Active Waveguides

We fabricated a flexible and red-emissive microcrystal resonator for highly efficient optical waveguiding. The microfiber crystals of diketopyrrolopyrrole (DPP) used in this work possess a high photoluminescence (PL) quantum efficiency (Φ_PL = 0.45) and exhibit a micromechanical deformation shape in the curved state. The crystals show optical fringes in their PL spectra, suggesting the existence of a naturally formed Fabry-Pérot crystal resonator owing to its flat crystal surface. The group refractive index (n_g) and Rabi splitting energy (ℏΩ) indicating the coupling strength between excitons and resonator photons are large (n_g = 3.7-6.0, ℏΩ = 1.38 eV), suggesting the strong confinement of waveguiding photons. Spatially resolved PL measurements revealed that the PL in a crystal fiber is efficiently waveguided, even in a curved crystal with a very small curvature radius of 11 μm. Strong photon confinement inside a crystal resonator is a plausible origin of efficient optical waveguiding.

Sub-picosecond collapse of molecular polaritons to pure molecular transition in plasmonic photoswitch-nanoantennas

Molecular polaritons are hybrid light-matter states that emerge when a molecular transition strongly interacts with photons in a resonator. At optical frequencies, this interaction unlocks a way to explore and control new chemical phenomena at the nanoscale. Achieving such control at ultrafast timescales, however, is an outstanding challenge, as it requires a deep understanding of the dynamics of the collectively coupled molecular excitation and the light modes. Here, we investigate the dynamics of collective polariton states, realized by coupling molecular photoswitches to optically anisotropic plasmonic nanoantennas. Pump-probe experiments reveal an ultrafast collapse of polaritons to pure molecular transition triggered by femtosecond-pulse excitation at room temperature. Through a synergistic combination of experiments and quantum mechanical modelling, we show that the response of the system is governed by intramolecular dynamics, occurring one order of magnitude faster with respect to the uncoupled excited molecule relaxation to the ground state.

Molecular Chemistry in Cavity Strong Coupling

The coherent exchange of energy between materials and optical fields leads to strong light-matter interactions and so-called polaritonic states with intriguing properties, halfway between light and matter. Two decades ago, research on these strong light-matter interactions, using optical cavity (vacuum) fields, remained for the most part the province of the physicist, with a focus on inorganic materials requiring cryogenic temperatures and carefully fabricated, high-quality optical cavities for their study. This review explores the history and recent acceleration of interest in the application of polaritonic states to molecular properties and processes. The enormous collective oscillator strength of dense films of organic molecules, aggregates, and materials allows cavity vacuum field strong coupling to be achieved at room temperature, even in rapidly fabricated, highly lossy metallic optical cavities. This has put polaritonic states and their associated coherent phenomena at the fingertips of laboratory chemists, materials scientists, and even biochemists as a potentially new tool to control molecular chemistry. The exciting phenomena that have emerged suggest that polaritonic states are of genuine relevance within the molecular and material energy landscape.

Directional Emission from Electrically Injected Exciton-Polaritons in Perovskite Metasurfaces

We present a new approach to achieving strong coupling between electrically injected excitons and photonic bound states in the continuum of a dielectric metasurface. Here a high-finesse metasurface cavity is monolithically patterned in the channel of a perovskite light-emitting transistor to induce a large Rabi splitting of ∼200 meV and more than 50-fold enhancement of the polaritonic emission compared to the intrinsic excitonic emission of the perovskite film. Moreover, the directionality of polaritonic electroluminescence can be dynamically tuned by varying the source-drain bias, which induces an asymmetric distribution of exciton population within the transistor channel. We argue that this approach provides a new platform to study strong light-matter interactions in dispersion engineered photonic cavities under electrical injection and paves the way to solution-processed electrically pumped polariton lasers.

Uncovering temperature-dependent exciton-polariton relaxation mechanisms in hybrid organic-inorganic perovskites

Hybrid perovskites have emerged as a promising material candidate for exciton-polariton (polariton) optoelectronics. Thermodynamically, low-threshold Bose-Einstein condensation requires efficient scattering to the polariton energy dispersion minimum, and many applications demand precise control of polariton interactions. Thus far, the primary mechanisms by which polaritons relax in perovskites remains unclear. In this work, we perform temperature-dependent measurements of polaritons in low-dimensional perovskite wedged microcavities achieving a Rabi splitting of [Formula: see text] = 260 ± 5 meV. We change the Hopfield coefficients by moving the optical excitation along the cavity wedge and thus tune the strength of the primary polariton relaxation mechanisms in this material. We observe the polariton bottleneck regime and show that it can be overcome by harnessing the interplay between the different excitonic species whose corresponding dynamics are modified by strong coupling. This work provides an understanding of polariton relaxation in perovskites benefiting from efficient, material-specific relaxation pathways and intracavity pumping schemes from thermally brightened excitonic species.

Single-photon switches, beam splitters, and circulators based on the photonic Aharonov-Bohm effect

Single-photon devices such as switches, beam splitters, and circulators are fundamental components to construct photonic integrated quantum networks. In this paper, two V-type three-level atoms coupled to a waveguide are proposed to simultaneously realize these functions as a multifunctional and reconfigurable single-photon device. When both the two atoms are driven by the external coherent fields, the difference in the phases of the coherent driving induces the photonic Aharonov-Bohm effect. Based on the photonic Aharonov-Bohm effect and setting the two-atom distance to match the constructive or destructive interference conditions among photons travelling along different paths, a single-photon switch is achieved since the incident single photon can be controlled from complete transmission to complete reflection by adjusting the amplitudes and phases of the driving fields. When properly changing the amplitudes and phases of the driving fields, the incident photons are split equally into multiple components as a beam splitter operated with different frequencies. Meanwhile, the single-photon circulator with reconfigurable circulation directions can also be obtained.

All-Optical Switching Based on Sub-Bandgap Photoactivation of Charge Trapping in Metal Halide Perovskites

Controllable optical properties are crucial for the application of light-emitting materials in optical devices. In this work, controllable photoluminescence in metal halide perovskite crystals is realized via photoactivation of their defects. It is found that under continuous excitation, the photoluminescence intensity of a CH₃ NH₃ PbBr₃ crystal can be fully controlled by sub-bandgap energy photon illumination. Such optically controllable emission behavior is rather general as it is observed also in CsPbBr₃ and other perovskite materials. The switching mechanism is assigned to reversible light-induced activation/deactivation of nonradiative recombination centers, the presence of which relates to an excess of Pb during perovskite synthesis. Given the success of perovskites in photovoltaics and optoelectronics, it is believed that the discovery of green luminescence controlled by red illumination will extend the application scope of perovskites toward optical devices and intelligent control.

Triple Threshold Transitions and Strong Polariton Interaction in 2D Layered Metal-Organic Framework Microplates

Room-temperature interaction between light-matter hybrid particles such as exciton-polaritons under extremely low-pump plays a crucial role in future coherent quantum light sources. However, the practical and scalable realization of coherent quantum light sources operating under low-pump remains a challenge because of the insufficient polariton interaction strength. Here, at room temperature, a very large polariton interaction strength is demonstrated, g ≈ 128 ± 21 µeV µm² realized in a 2D nanolayered metal-organic framework (MOF). As a result, a polariton lasing at an extremely low pump fluence of P₁  ≈ 0.01 ± 0.0015 µJ cm^-2 (first threshold) is observed. Interestingly, as pump fluence increases to P₂  ≈ 0.031 ± 0.003 µJ cm^-2 (second threshold), a spontaneous transition to a polariton breakdown region occurs, which has not been reported before. Finally, an ordinary photon lasing occurs at P₃  ≈ 0.11 ± 0.077 µJ cm^-2 (third threshold), or above. These experiments and the theoretical model reveal new insights into the transition mechanisms characterized by three distinct optical regions. This work introduces MOF as a new type of quantum material, with naturally formed polariton cavities, that is a cost-effective and scalable solution to build microscale coherent quantum light sources and polaritonic devices.

In-Plane and Out-of-Plane Investigation of Resonant Tunneling Polaritons in Metal-Dielectric-Metal Cavities

Polaritons can be generated by tuning the optical transitions of a light emitter to the resonances of a photonic cavity. We show that a dye-doped cavity generates resonant tunneling polaritons with Epsilon-Near-Zero (ENZ) effective permittivity. We studied the polariton spectral dispersion in dye-doped metal-dielectric-metal (MDM) cavities as a function of the in-plane (k_||) and out-of-plane (k_⊥) components of the incident wavevector. The dependence on k_|| was investigated through ellipsometry, revealing the ENZ modes. The k_⊥ dependence was measured by varying the cavity thickness under normal incidence using a Surface Force Apparatus (SFA). Both methods revealed a large Rabi splitting well exceeding 100 meV. The SFA-based investigation highlighted the collective nature of strong coupling by producing a splitting proportional to the square root of the involved photons. This study demonstrates the possibility of generating ENZ polaritons and introduces the SFA as a powerful tool for the characterization of strong light-matter interactions.

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.

Polaritons in a Polycrystalline Layer of Non-fullerene Acceptor

Non-fullerene acceptor molecules developed for organic solar cells feature a very intense absorption band in the near-infrared. In the solid phase, the strong interaction between light and the transition dipole moment for molecular excitation should induce formation of polaritons. The reflection spectra for polycrystalline films of a non-fullerene acceptor with a thienothienopyrrolo-thienothienoindole core of the so-called Y6 type indeed show a signature of polaritons. A local minimum in the middle of the reflection band is associated with the allowed molecular transition. The minimum in reflection allows efficient entry of light into the solid, resulting in a local maximum in external quantum efficiency of a photovoltaic cell made of the pure acceptor.

Organic molecules with inverted singlet-triplet gaps

According to Hund’s multiplicity rule, the energy of the lowest excited triplet state (T₁) is always lower than that of the lowest excited singlet state (S₁) in organic molecules, resulting in a positive singlet-triplet energy gap (ΔE_ST). Therefore, the up-converted reverse intersystem crossing (RISC) from T₁ to S₁ is an endothermic process, which may lead to the quenching of long-lived triplet excitons in electroluminescence, and subsequently the reduction of device efficiency. Interestingly, organic molecules with inverted singlet-triplet (INVEST) gaps in violation of Hund’s multiplicity rule have recently come into the limelight. The unique feature has attracted extensive attention in the fields of organic optoelectronics and photocatalysis over the past few years. For an INVEST molecule possessing a higher T₁ with respect to S₁, namely a negative ΔE_ST, the down-converted RISC from T₁ to S₁ does not require thermal activation, which is possibly conducive to solving the problems of fast efficiency roll-off and short lifetime of organic light-emitting devices. By virtue of this property, INVEST molecules are recently regarded as a new generation of organic light-emitting materials. In this review, we briefly summarized the significant progress of INVEST molecules in both theoretical calculations and experimental studies, and put forward suggestions and expectations for future research.

Electric Field Modulation of 2D Perovskite Excitonics

Multiquantum-well (MQW) perovskite is one of the forerunners in high-efficiency perovskite LED (PeLEDs) research. Despite the rapid inroads, PeLEDs suffer from the pertinent issue of efficiency decrease with increasing brightness, commonly known as "efficiency roll-off". The underlying mechanisms are presently an open question. Herein, we explicate the E-field effects on the exciton states in the archetypal MQW perovskite (C₆H₅C₂H₄NH₃)₂PbI₄, or PEPI, in a device-like architecture using field-assisted transient spectroscopy and theoretical modeling. The applied E-field results in a complex interplay of spectral blueshifts and enhancement/quenching of the different exciton modes. The former originates from the DC Stark shift, while the latter is attributed to the E-field modulation of the transfer rates between bright/dark exciton modes. Importantly, our findings uncover crucial insights into the photophysical processes under E-field modulation contributing to efficiency roll-off in MQW PeLEDs. Electrical modulation of exciton properties presents exciting possibilities for signal processing devices.

Optically Controlled Femtosecond Polariton Switch at Room Temperature

Exciton polaritons have shown great potential for applications such as low-threshold lasing, quantum simulation, and dissipation-free circuits. In this paper, we realize a room temperature ultrafast polaritonic switch where the Bose-Einstein condensate population can be depleted at the hundred femtosecond timescale with high extinction ratios. This is achieved by applying an ultrashort optical control pulse, inducing parametric scattering within the photon part of the polariton condensate via a four-wave mixing process. Using a femtosecond angle-resolved spectroscopic imaging technique, the erasure and revival of the polariton condensates can be visualized. The condensate depletion and revival are well modeled by an open-dissipative Gross-Pitaevskii equation including parametric scattering process. This pushes the speed frontier of all-optical controlled polaritonic switches at room temperature towards the THz regime.

Mechanically controllable nonreciprocal transmission and perfect absorption of photons

Photon absorption and nonreciprocal photon transmission are studied in a rotating optical resonator coupled with an atomic ensemble. It is demonstrated that the perfect photon absorption is accompanied by optical bistability when the resonator is static. If the spinning detune is adjusted to some particular values, we find that the amplified unidirectional photon transmission can be realized. We have explicitly given the perfect photon absorption conditions and the maximal adjustable amplification rate. It is found that the coupling of the resonator and the atomic ensemble is necessary for perfect photon absorption, and the phase difference of the two input fields only affects the perfect absorption point. It gives new insight into the design of photon absorbers and optical switches.

Rydberg exciton-polaritons in a Cu2O microcavity

Giant Rydberg excitons with principal quantum numbers as high as n = 25 have been observed in cuprous oxide (Cu₂O), a semiconductor in which the exciton diameter can become as large as ∼1 μm. The giant dimension of these excitons results in excitonic interaction enhancements of orders of magnitude. Rydberg exciton-polaritons, formed by the strong coupling of Rydberg excitons to cavity photons, are a promising route to exploit these interactions and achieve a scalable, strongly correlated solid-state platform. However, the strong coupling of these excitons to cavity photons has remained elusive. Here, by embedding a thin Cu₂O crystal into a Fabry-Pérot microcavity, we achieve strong coupling of light to Cu₂O Rydberg excitons up to n = 6 and demonstrate the formation of Cu₂O Rydberg exciton-polaritons. These results pave the way towards realizing strongly interacting exciton-polaritons and exploring strongly correlated phases of matter using light on a chip.

Nanometer-Scale Spatial and Spectral Mapping of Exciton Polaritons in Structured Plasmonic Cavities

Exciton polaritons (EPs) are ubiquitous light-matter excitations under intense investigation as test beds of fundamental physics and as components for all-optical computing. Owing to their unique attributes and facile experimental tunability, EPs potentially enable strong nonlinearities, condensation, and superfluidity at room temperature. However, the diffraction limit of light and the momentum content of fast electron probes preclude the characterization of EPs in nanoscale structured cavities exhibiting energy-momentum dispersion. Here we present fully relativistic analytical theory and companion numerical simulations showing that these limitations can be overcome to measure EPs in periodic nanophotonic cavities on their natural energy, momentum, and length scales via lattice electron energy gain spectroscopy. With the combined high momentum resolution of light and nanoscale spatial resolution of focused electron beams, lattice electron energy gain spectroscopy can expose deeply subwavelength EP features using currently available monochromated, aberration-corrected scanning transmission electron microscopes.

Optical nonlinearity goes ultrafast in 2D semiconductor-based nanocavities

Hybrid systems of silver nanodisks strongly coupled to monolayer tungsten-disulfide (WS₂) show giant room-temperature nonlinearity due to their deeply sub-wavelength localized nature, resulting in ultrafast modifications of nonlinear absorption in a solid-state system.

Nonlinear polariton parametric emission in an atomically thin semiconductor based microcavity

Parametric nonlinear optical processes are at the heart of nonlinear optics underpinning the central role in the generation of entangled photons as well as the realization of coherent optical sources. Exciton-polaritons are capable to sustain parametric scattering at extremely low threshold, offering a readily accessible platform to study bosonic fluids. Recently, two-dimensional transition-metal dichalcogenides (TMDs) have attracted great attention in strong light-matter interactions due to robust excitonic transitions and unique spin-valley degrees of freedom. However, further progress is hindered by the lack of realizations of strong nonlinear effects in TMD polaritons. Here, we demonstrate a realization of nonlinear optical parametric polaritons in a WS₂ monolayer microcavity pumped at the inflection point and triggered in the ground state. We observed the formation of a phase-matched idler state and nonlinear amplification that preserves the valley population and survives up to room temperature. Our results open a new door towards the realization of the future for all-optical valley polariton nonlinear devices.

Thermalization of Fluorescent Protein Exciton-Polaritons at Room Temperature

Fluorescent proteins (FPs) have recently emerged as a serious contender for realizing ultralow threshold room temperature exciton-polariton condensation and lasing. This contribution investigates the thermalization of FP microcavity exciton-polaritons upon optical pumping under ambient conditions. Polariton cooling is realized using a new FP molecule, called mScarlet, coupled strongly to the optical modes in a Fabry-Pérot cavity. Interestingly, at the threshold excitation energy (fluence) of ≈9 nJ per pulse (15.6 mJ cm^-2 ), an effective temperature is observed, T_eff  ≈ 350 ± 35 K close to the lattice temperature indicative of strongly thermalized exciton-polaritons at equilibrium. This efficient thermalization results from the interplay of radiative pumping facilitated by the energetics of the lower polariton branch and the cavity Q-factor. Direct evidence for dramatic switching from an equilibrium state into a metastable state is observed for the organic cavity polariton device at room temperature via deviation from the Maxwell-Boltzmann statistics at k_‖  = 0 above the threshold. Thermalized polariton gases in organic systems at equilibrium hold substantial promise for designing room temperature polaritonic circuits, switches, and lattices for analog simulation.

Strong Light-Matter Interactions between Gap Plasmons and Two-Dimensional Excitons under Ambient Conditions in a Deterministic Way

Strong exciton-plasmon interactions between layered two-dimensional (2D) semiconductors and gap plasmons show a great potential to implement cavity quantum electrodynamics under ambient conditions. However, achieving a robust plasmon-exciton coupling with nanocavities is still very challenging, because the layer area is usually small in the conventional approaches. Here, we report on a robust strong exciton-plasmon coupling between the gap mode of a bowtie and the excitons in MoS₂ layers with gold-assisted mechanical exfoliation and nondestructive wet transfer techniques for a large-area layer. Due to the ultrasmall mode volume and strong in-plane field, the estimated effective exciton number contributing to the coupling is largely reduced. With a corrected exciton transition dipole moment, the exciton numbers are extracted as being 40 for the case of a single layer and 48 for eight layers. Our work paves the way to realize strong coupling with 2D materials with a small number of excitons at room temperature.

Femtosecond Dynamics of a Polariton Bosonic Cascade at Room Temperature

Whispering gallery modes in a microwire are characterized by a nearly equidistant energy spectrum. In the strong exciton-photon coupling regime, this system represents a bosonic cascade: a ladder of discrete energy levels that sustains stimulated transitions between neighboring steps. Here, by using a femtosecond angle-resolved spectroscopic imaging technique, the ultrafast dynamics of polaritons in a bosonic cascade based on a one-dimensional ZnO whispering gallery microcavity are explicitly visualized. Clear ladder-form build-up processes from higher to lower energy branches of the polariton condensates are observed, which are well reproduced by modeling using rate equations. Remarkably, a pronounced superbunching feature, which could serve as solid evidence for bosonic cascades, is demonstrated by the measured second-order time correlation factor. In addition, the nonlinear polariton parametric scattering dynamics on a time scale of hundreds of femtoseconds are revealed. Our understandings pave the way toward ultrafast coherent control of polaritons at room temperature.

Dissipative Phase Transition with Driving-Controlled Spatial Dimension and Diffusive Boundary Conditions

We investigate theoretically and experimentally a first-order dissipative phase transition, with diffusive boundary conditions and the ability to tune the spatial dimension of the system. The considered physical system is a planar semiconductor microcavity in the strong light-matter coupling regime, where polariton excitations are injected by a quasiresonant optical driving field. The spatial dimension of the system from 1D to 2D is tuned by designing the intensity profile of the driving field. We investigate the emergence of criticality by increasing the spatial size of the driven region. The system is nonlinear due to polariton-polariton interactions and the boundary conditions are diffusive because the polaritons can freely diffuse out of the driven region. We show that no phase transition occurs using a 1D driving geometry, while for a 2D geometry we do observe both in theory and experiments the emergence of a first-order phase transition. The demonstrated technique allows all-optical and in situ control of the system geometry, providing a versatile platform for exploring the many-body physics of photons.

Quantum Squeezing Induced Optical Nonreciprocity

We propose an all-optical approach to achieve optical nonreciprocity on a chip by quantum squeezing one of two coupled resonator modes. By parametric pumping a χ^{(2)}-nonlinear resonator unidirectionally with a classical coherent field, we squeeze the resonator mode in a selective direction due to the phase-matching condition, and induce a chiral photon interaction between two resonators. Based on this chiral interresonator coupling, we achieve an all-optical diode and a three-port quasicirculator. By applying a second squeezed-vacuum field to the squeezed resonator mode, our nonreciprocal device also works for single-photon pulses. We obtain an isolation ratio of >40  dB for the diode and fidelity of >98% for the quasicirculator, and insertion loss of <1  dB for both. We also show that nonreciprocal transmission of strong light can be switched on and off by a relative weak pump light. This achievement implies a nonreciprocal optical transistor. Our protocol opens up a new route to achieve integrable all-optical nonreciprocal devices permitting chip-compatible optical isolation and nonreciporcal quantum information processing.

Engineering Photon Statistics in a Spinor Polariton Condensate

We implement full polarization tomography on photon correlations in a spinor exciton-polariton condensate. Our measurements reveal condensate pseudospin mean-field dynamics spanning from stochastic switching between linear polarization components, limit cycles, and stable fixed points, and their intrinsic relation to the condensate photon statistics. We optically harness the cavity birefringence, polariton interactions, and the optical orientation of an incoherent exciton reservoir to engineer photon statistics with precise control. Our results demonstrate a smooth transition from a highly coherent to a super-thermal state of the condensate polarization components.

All-optical switching based on interacting exciton polaritons in self-assembled perovskite microwires

Ultrafast all-optical switches and integrated circuits call for giant optical nonlinearity to minimize energy consumption and footprint. Exciton polaritons underpin intrinsic strong nonlinear interactions and high-speed propagation in solids, thus affording an intriguing platform for all-optical devices. However, semiconductors sustaining stable exciton polaritons at room temperature usually exhibit restricted nonlinearity and/or propagation properties. Delocalized and strongly interacting Wannier-Mott excitons in metal halide perovskites highlight their advantages in integrated nonlinear optical devices. Here, we report all-optical switching by using propagating and strongly interacting exciton-polariton fluids in self-assembled CsPbBr₃ microwires. Strong polariton-polariton interactions and extended polariton fluids with a propagation length of around 25 μm have been reached. All-optical switching on/off of polariton propagation can be realized in picosecond time scale by locally blue-shifting the dispersion with interacting polaritons. The all-optical switching, together with the scalable self-assembly method, highlights promising applications of solution-processed perovskites toward integrated photonics operating in strong coupling regime.

Quantum Plasmonics: Energy Transport Through Plasmonic Gap

At the interfaces of metal and dielectric materials, strong light-matter interactions excite surface plasmons; this allows electromagnetic field confinement and enhancement on the sub-wavelength scale. Such phenomena have attracted considerable interest in the field of exotic material-based nanophotonic research, with potential applications including nonlinear spectroscopies, information processing, single-molecule sensing, organic-molecule devices, and plasmon chemistry. These innovative plasmonics-based technologies can meet the ever-increasing demands for speed and capacity in nanoscale devices, offering ultrasensitive detection capabilities and low-power operations. Size scaling from the nanometer to sub-nanometer ranges is consistently researched; as a result, the quantum behavior of localized surface plasmons, as well as those of matter, nonlocality, and quantum electron tunneling is investigated using an innovative nanofabrication and chemical functionalization approach, thereby opening a new era of quantum plasmonics. This new field enables the ultimate miniaturization of photonic components and provides extreme limits on light-matter interactions, permitting energy transport across the extremely small plasmonic gap. In this review, a comprehensive overview of the recent developments of quantum plasmonic resonators with particular focus on novel materials is presented. By exploring the novel gap materials in quantum regime, the potential quantum technology applications are also searched for and mapped out.

Ultrafast dynamics of exciton-polariton in optically tailored potential landscapes at room temperature

In this work, by using femtosecond angle-resolved spectroscopic imaging technique, the ultrafast dynamics of confined exciton-polaritons in an optical induced potential well based on a ZnO whispering-gallery microcavity is explicitly visualized. The sub-picosecond transition between succeeding quantum harmonic oscillator states can be experimentally distinguished. The landscape of the potential well can be modified by the pump power, the spatial distance and the time delay of the two input laser pulses. Clarifying the underlying mechanism of the polariton harmonic oscillator is interesting for the applications of polariton-based optoelectronic devices and quantum information processing.

III-Nitride Light-Emitting Devices

III-nitride light-emitting devices have been subjects of intense research for the last several decades owing to the versatility of their applications for fundamental research, as well as their widespread commercial utilization. Nitride light-emitters in the form of light-emitting diodes (LEDs) and lasers have made remarkable progress in recent years, especially in the form of blue LEDs and lasers. However, to further extend the scope of these devices, both below and above the blue emission region of the electromagnetic spectrum, and also to expand their range of practical applications, a number of issues and challenges related to the growth of materials, device design, and fabrication need to be overcome. This review provides a detailed overview of nitride-based LEDs and lasers, starting from their early days of development to the present state-of-the-art light-emitting devices. Besides delineating the scientific and engineering milestones achieved in the path towards the development of the highly matured blue LEDs and lasers, this review provides a sketch of the prevailing challenges associated with the development of long-wavelength, as well as ultraviolet nitride LEDs and lasers. In addition to these, recent progress and future challenges related to the development of next-generation nitride emitters, which include exciton-polariton lasers, spin-LEDs and lasers, and nanostructured emitters based on nanowires and quantum dots, have also been elucidated in this review. The review concludes by touching on the more recent topic of hexagonal boron nitride-based light-emitting devices, which have already shown significant promise as deep ultraviolet and single-photon emitters.

Perovskite semiconductors for room-temperature exciton-polaritonics

Lead-halide perovskites are generally excellent light emitters and can have larger exciton binding energies than thermal energy at room temperature, exhibiting great promise for room-temperature exciton-polaritonics. Rapid progress has been made recently, although challenges and mysteries remain in lead-halide perovskite semiconductors to push polaritons to room-temperature operation. In this Perspective, we discuss fundamental aspects of perovskite semiconductors for exciton-polaritons and review the recent rapid experimental advances using lead-halide perovskites for room-temperature polaritonics, including the experimental realization of strong light-matter interaction using various types of microcavities as well as reaching the polariton condensation regime in planar microcavities and lattices.

Motional narrowing, ballistic transport, and trapping of room-temperature exciton polaritons in an atomically-thin semiconductor

Monolayer transition metal dichalcogenide crystals (TMDCs) hold great promise for semiconductor optoelectronics because their bound electron-hole pairs (excitons) are stable at room temperature and interact strongly with light. When TMDCs are embedded in an optical microcavity, excitons can hybridise with cavity photons to form exciton polaritons, which inherit useful properties from their constituents. The ability to manipulate and trap polaritons on a microchip is critical for applications. Here, we create a non-trivial potential landscape for polaritons in monolayer WS₂, and demonstrate their trapping and ballistic propagation across tens of micrometers. We show that the effects of dielectric disorder, which restrict the diffusion of WS₂ excitons and broaden their spectral resonance, are dramatically reduced for polaritons, leading to motional narrowing and preserved partial coherence. Linewidth narrowing and coherence are further enhanced in the trap. Our results demonstrate the possibility of long-range dissipationless transport and efficient trapping of TMDC polaritons in ambient conditions.

Exciton-Photonics: From Fundamental Science to Applications

Semiconductors in all dimensionalities ranging from 0D quantum dots and molecules to 3D bulk crystals support bound electron-hole pair quasiparticles termed excitons. Over the past two decades, the emergence of a variety of low-dimensional semiconductors that support excitons combined with advances in nano-optics and photonics has burgeoned an advanced area of research that focuses on engineering, imaging, and modulating the coupling between excitons and photons, resulting in the formation of hybrid quasiparticles termed exciton-polaritons. This advanced area has the potential to bring about a paradigm shift in quantum optics, as well as classical optoelectronic devices. Here, we present a review on the coupling of light in excitonic semiconductors and previous investigations of the optical properties of these hybrid quasiparticles via both far-field and near-field imaging and spectroscopy techniques. Special emphasis is given to recent advances with critical evaluation of the bottlenecks that plague various materials toward practical device implementations including quantum light sources. Our review highlights a growing need for excitonic material development together with optical engineering and imaging techniques to harness the utility of excitons and their host materials for a variety of applications.

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.

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.

Nonlinear infrared polaritonic interaction between cavities mediated by molecular vibrations at ultrafast time scale

Realizing nonlinear interactions between spatially separated particles can advance molecular science and technology, including remote catalysis of chemical reactions, ultrafast processing of information in infrared (IR) photonic circuitry, and advanced platforms for quantum simulations with increased complexity. Here, we achieved nonlinear interactions at ultrafast time scale between polaritons contained in spatially adjacent cavities in the mid-IR regime, altering polaritons in one cavity by pumping polaritons in an adjacent one. This was done by strong coupling molecular vibrational modes with photon modes, a process that combines characteristics of both photon delocalization and molecular nonlinearity. The dual photon/molecule character of polaritons enables delocalized nonlinearity-a property that neither molecular nor cavity mode would have alone.

Enhanced nonlinear interaction of polaritons via excitonic Rydberg states in monolayer WSe2

Strong optical nonlinearities play a central role in realizing quantum photonic technologies. Exciton-polaritons, which result from the hybridization of material excitations and cavity photons, are an attractive candidate to realize such nonlinearities. While the interaction between ground state excitons generates a notable optical nonlinearity, the strength of such interactions is generally not sufficient to reach the regime of quantum nonlinear optics. Excited states, however, feature enhanced interactions and therefore hold promise for accessing the quantum domain of single-photon nonlinearities. Here we demonstrate the formation of exciton-polaritons using excited excitonic states in monolayer tungsten diselenide (WSe2) embedded in a microcavity. The realized excited-state polaritons exhibit an enhanced nonlinear response ∼[Formula: see text] which is ∼4.6 times that for the ground-state exciton. The demonstration of enhanced nonlinear response from excited exciton-polaritons presents the potential of generating strong exciton-polariton interactions, a necessary building block for solid-state quantum photonic technologies.

Enhancement of Parametric Effects in Polariton Waveguides Induced by Dipolar Interactions

Exciton-polaritons are hybrid light-matter excitations arising from the nonperturbative coupling of a photonic mode and an excitonic resonance. Behaving as interacting photons, they show optical third-order nonlinearities providing effects such as optical parametric oscillation or amplification. It has been suggested that polariton-polariton interactions can be greatly enhanced by inducing aligned electric dipoles in their excitonic part. However, direct evidence of a true particle-particle interaction, such as superfluidity or parametric scattering, is still missing. In this Letter, we demonstrate that dipolar interactions can be used to enhance parametric effects such as self-phase modulation in waveguide polaritons. By quantifying these optical nonlinearities, we provide a reliable experimental measurement of the direct dipolar enhancement of polariton-polariton interactions.

All-optical RF spectrum analyzer with a 5 THz bandwidth based on CMOS-compatible high-index doped silica waveguides

We report an all-optical radio-frequency (RF) spectrum analyzer with a bandwidth greater than 5 THz, based on a 50 cm long spiral waveguide in a CMOS-compatible high-index doped silica platform. By carefully mapping out the dispersion profile of the waveguides for different thicknesses, we identify the optimal design to achieve near-zero dispersion in the C-band. To demonstrate the capability of the RF spectrum analyzer, we measure the optical output of a femtosecond fiber laser with an ultrafast optical RF spectrum in the terahertz regime.