Strong Light-Matter Coupling between Plasmons in Individual Gold Bi-pyramids and Excitons in Mono- and Multilayer WSe2

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.

Unconventional Thermophotonic Charge Density Wave

Charge-order states of broken symmetry, such as charge density wave (CDW), are able to induce exceptional physical properties, however, the precise understanding of the underlying physics is still elusive. Here, we combine fluctuational electrodynamics and density functional theory to reveal an unconventional thermophotonic effect in CDW-bearing TiSe_{2}, referred to as thermophotonic-CDW (tp-CDW). The interplay of plasmon polariton and CDW electron excitations give rise to an anomalous negative temperature dependency in thermal photons transport, offering an intuitive fingerprint for a transformation of the electron order. Additionally, the demonstrated nontrivial features of tp-CDW transition hold promise for a controllable manipulation of heat flow, which could be extensively utilized in various fields such as thermal science and electron dynamics, as well as in next-generation energy devices.

Influence of molecular structure on the coupling strength to a plasmonic nanoparticle and hot carrier generation

Strong coupling between metal nanoparticles and molecules mixes their excitations, creating new eigenstates with modified properties such as altered chemical reactivity, different relaxation pathways or modified phase transitions. Here, we explore excited state plasmon-molecule coupling and discuss how strong coupling together with a changed orientation and number of an asymmetric molecule affects the generation of hot carriers in the system. We used a promising plasmonic material, magnesium, for the nanoparticle and coupled it with CPDT molecules, which are used in organic optoelectronic materials for organic electronic applications due to their facile modification, electron-rich structure, low band gap, high electrical conductivity and good charge transport properties. By employing computational quantum electronic tools we demonstrate the existence of a strong coupling mediated charge transfer plasmon whose direction, magnitude, and spectral position can be tuned. We find that the orientation of CPDT changes the nanoparticle-molecule gap for which maximum charge separation occurs, while larger gaps result in trapping hot carriers within the moieties due to weaker interactions. This research highlights the potential for tuning hot carrier generation in strongly coupled plasmon-molecule systems for enhanced energy generation or excited state chemistry.

Strong coupling between plasmonic nanocavity gold nanorods and quantum dots emitter

The discovery of hybrid states in strong coupling interaction has gained growing attention in cavity-quantum electrodynamics research owing to its fundamental directives and potential in advanced optical applications. The ultra-confined mode volume of plasmonic cavity gold nanorods (AuNRs), particularly at the nanorod tip "hotspot" provides a large coupling strength, a prerequisite for a coherent energy exchange in the strong coupling regime. Here, we reported a remarkable Rabi splitting of ∼231 meV between gold nanorods longitudinal localized surface plasmon resonance (LLSPR) mode and quantum dots (QDs) at ambient conditions, monitored by dielectric medium tuning. Numerical simulations confirmed the result, displaying absorption spectral splitting.

Deterministic positioning and alignment of a single-molecule exciton in plasmonic nanodimer for strong coupling

Experimental realization of strong coupling between a single exciton and plasmons remains challenging as it requires deterministic positioning of the single exciton and alignment of its dipole moment with the plasmonic fields. This study aims to combine the host-guest chemistry approach with the cucurbit[7]uril-mediated active self-assembly to precisely integrate a single methylene blue molecule in an Au nanodimer at the deterministic position (gap center of the nanodimer) with the maximum electric field (EFmax) and perfectly align its transition dipole moment with the EFmax, yielding a large spectral Rabi splitting of 116 meV for a single-molecule exciton-matching the analytical model and numerical simulations. Statistical analysis of vibrational spectroscopy and dark-field scattering spectra confirm the realization of the single exciton strong coupling at room temperature. Our work may suggest an approach for achieving the strong coupling between a deterministic single exciton and plasmons, contributing to the development of room-temperature single-qubit quantum devices.

Validated enhancement and temperature modulated absorbance of a WS2 monolayer based on a planar structure

Transition-metal dichalcogenides (TMDCs), as emerging optoelectronic materials, necessitate the establishment of an experimentally viable system to study their interaction with light. In this study, we propose and analyze a WS2/PMMA/Ag planar Fabry-Perot (F-P) cavity, enabling the direct experimental measurement of WS2 absorbance. By optimizing the structure, the absorbance of A exciton of WS2 up to 0.546 can be experimentally achieved, which matches well with the theoretical calculations. Through temperature and thermal expansion strain induced by temperature, the absorbance of the A exciton can be tuned in situ. Furthermore, temperature-dependent photocurrent measurements confirmed the consistent absorbance of the A exciton under varying temperatures. This WS2/PMMA/Ag planar structure provides a straightforward and practical platform for investigating light interaction in TMDCs, laying a solid foundation for future developments of TMDC-based optoelectronic devices.

Ultrastrong exciton-plasmon couplings in WS2 multilayers synthesized with a random multi-singular metasurface at room temperature

Van der Waals semiconductors exemplified by two-dimensional transition-metal dichalcogenides have promised next-generation atomically thin optoelectronics. Boosting their interaction with light is vital for practical applications, especially in the quantum regime where ultrastrong coupling is highly demanded but not yet realized. Here we report ultrastrong exciton-plasmon coupling at room temperature in tungsten disulfide (WS2) layers loaded with a random multi-singular plasmonic metasurface deposited on a flexible polymer substrate. Different from seeking perfect metals or high-quality resonators, we create a unique type of metasurface with a dense array of singularities that can support nanometre-sized plasmonic hotspots to which several WS2 excitons coherently interact. The associated normalized coupling strength is 0.12 for monolayer WS2 and can be up to 0.164 for quadrilayers, showcasing the ultrastrong exciton-plasmon coupling that is important for practical optoelectronic devices based on low-dimensional semiconductors.

Quantifying Shape Transition in Anisotropic Plasmonic Nanoparticles through Geometric Inversion. Application to Gold Bipyramids

Unraveling the nuanced interplay between the morphology and the optical properties of plasmonic nanoparticles is crucial for targeted applications. Managing the relationship becomes significantly complex when dealing with anisotropic nanoparticles that defy a simple description using parameters like length, width, or aspect ratio. This complexity requires computationally intensive numerical modeling and advanced imaging techniques. To address these challenges, we propose a detailed structural parameter determination of gold nanoparticles using their two-dimensional projections (e.g., micrographs). Employing gold bipyramids (AuBPs) as a model morphology, we can determine their three-dimensional geometry and extract optical features computationally for comparison with the experimental data. To validate our inversion model's effectiveness, we apply it to derive the structural parameters of AuBPs undergoing shape modification through oxidative etching. In summary, our findings allow for the precise characterization of structural parameters for plasmonic nanoparticles during shape transitions, potentially enhancing the comprehension of nanocrystal growth and optimizing plasmonic material design for various applications.

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

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

Room-Temperature Strong Coupling of Few-Exciton in a Monolayer WS2 with Plasmon and Dispersion Deviation

Engineering room-temperature strong coupling of few-exciton in transition-metal dichalcogenides (TMDCs) with plasmons promises to construct compact and high-performance quantum optical devices. But it remains unimplemented due to their in-plane excitons. Here, we demonstrate the strong coupling of few-exciton within 10 in monolayer WS2 with the plasmonic mode with a large tangential component of the electric field tightly trapped around the sharp corners of an Au@Ag nanocuboid, the fewest number of excitons observed in the TMDC family so far. Furthermore, we for the first time report a significant deviation with a relative difference of up to 100.6% between the spectrum and eigenlevel splitting dispersions, which increases with decreasing coupling strength. It is also shown that the coupling strength obtained by the conventional concept of both being equal to the measured spectrum splitting is markedly overestimated. Our work enriches the understanding of strong light-matter interactions at room temperature.

Plasmonic Bound States in the Continuum to Tailor Exciton Emission of MoTe2

Plasmon resonances can greatly enhance light-matter interactions of two-dimensional van der Waals materials. However, the quality factor of plasmonic resonances is limited. Here, we demonstrate a plasmonic quasi-bound state in the continuum (quasi-BIC), which is composed of gold nanorod pairs. Through controlling the rotation angle of the nanorods, the quality factor of the plasmonic BIC mode can be tuned. Simulation results show that the plasmonic BIC combines the advantages of high-quality factor from the BIC effect and small mode volume from plasmonic resonance. Experiment results show that the designed plasmonic BIC mode exhibits a quality factor higher than 15 at the wavelength of around 1250 nm. Through integrating the plasmonic bound state structure with monolayer molybdenum ditelluride (MoTe₂), the exciton emission of MoTe₂ in the PL spectrum split into two exciton-polariton modes, which is attributed to the high Q factor and strong interaction between the BIC mode and excitons of MoTe₂.

Self-hybridized exciton-polaritons in thin films of transition metal dichalcogenides for narrowband perfect absorption

Monolayer direct-band gap transition metal dichalcogenides (TMDCs) have been extensively investigated in the context of light-matter interactions. To reach strong coupling, these studies make use of external optical cavities supporting well-defined resonant modes. However, use of an external cavity might limit the scope of possible applications of such systems. Here, we demonstrate that thin films of TMDCs can themselves serve as high-quality-factor cavities due to the guided optical modes they sustain in the visible and near-infrared ranges. Making use of the prism coupling, we achieve the strong coupling between excitons and guided-mode resonances lying below the light line, and show that the thickness of TMDC membranes can be used to tune and promote photon-exciton interactions within the strong-coupling regime. Additionally, we demonstrate narrowband perfect absorption in thin TMDC films through critical coupling with guided-mode resonances. Our work not only provides a simple and intuitive picture to tame interaction of light and matter in thin TMDC films, but also suggests that these simple systems are a promising platform for realizing polaritonic and optoelectronic devices.

Orientation-Dependent Interaction between the Magnetic Plasmons in Gold Nanocups and the Excitons in WS2 Monolayer and Multilayer

The integration of two-dimensional transition metal dichalcogenides with plasmonic nanostructures is extremely attractive for the investigation of the resonance coupling between plasmons and excitons, which offers a framework for the study of cavity quantum electrodynamics and is of great potential for exploring diverse quantum technologies. Herein we report on the coupling between the magnetic plasmons supported by individual asymmetric Au nanocups and the excitons in WS₂ monolayer and multilayer. Resonance coupling with the strength varying from weak to strong regimes is realized by adjusting the orientation of the individual Au nanocups on WS₂ monolayer. Different energy detunings between the magnetic plasmons and the excitons are achieved by varying the size of the Au nanocup. The Rabi splitting energies extracted at zero detuning are up to 106 meV. The anticrossing feature is observed in the measured scattering spectra and simulated absorption spectra, which indicates that the resonance coupling between the magnetic plasmons in the Au nanocup and the excitons in WS₂ monolayer enters the strongly coupled regime. A dependence of the coupling strength on the layer number is further observed when the Au nanocups are coupled with WS₂ multilayer. Our study suggests a promising approach toward the realization of different coupling regimes in a simple hybrid system made of individual Au nanocups and two-dimensional materials.

Substrate engineering of plasmonic nanocavity antenna modes

Plasmonic nanocavities have emerged as a promising platform for next-generation spectroscopy, sensing and photonic quantum information processing technologies, benefiting from a unique confluence of nanoscale compactness and integrability, ultrafast functionality and room-temperature viability. Harnessing their unprecedented optical field confinement and enhancement properties for such diverse application domains, however, demands continued innovation in cavity design and robust strategies for engineering their plasmonic mode characteristics, with the aim of optimizing spatial and spectral matching conditions for strong light-matter interaction involving embedded quantum emitters. Adopting the canonical gold bowtie nanoantenna, we show that the complex refractive index, n + ik, of the substrate material provides additional design flexibility in tailoring the properties of plasmonic nanocavity modes, including their resonance wavelengths, hotspot locations, intracavity field polarization and radiative decay rates. In particular, we predict that highly refractive (n ≥ 4) or highly absorptive (k ≥ 4) substrates provide two complementary approaches to engineering nanocavity modes that are especially desirable for coupling two-dimensional quantum materials, featuring namely an elevated hotspot with a dominantly in-plane polarized near-field, as well as a strongly radiative character. Our study elucidates the benefits and intricacies of a largely unexplored facet of nanocavity mode manipulation, beyond the widely practiced synthetic control over the cavity topology or physical dimensions, and paves the way for plasmonic cavity quantum electrodynamics with two-dimensional excitonic matter.

Ultra-confined Propagating Exciton-Plasmon Polaritons Enabled by Cavity-Free Strong Coupling: Beating Plasmonic Trade-Offs

Hybrid coupling systems consisting of transition metal dichalcogenides (TMD) and plasmonic nanostructures have emerged as a promising platform to explore exciton-plasmon polaritons. However, the requisite cavity/resonator for strong coupling introduces extra complexities and challenges for waveguiding applications. Alternatively, plasmonic nano-waveguides can also be utilized to provide a non-resonant approach for strong coupling, while their utility is limited by the plasmonic confinement-loss and confinement-momentum trade-offs. Here, based on a cavity-free approach, we overcome these constraints by theoretically strong coupling of a monolayer TMD to a single metal nanowire, generating ultra-confined propagating exciton-plasmon polaritons (PEPPs) that beat the plasmonic trade-offs. By leveraging strong-coupling-induced reformations in energy distribution and combining favorable properties of surface plasmon polaritons (SPPs) and excitons, the generated PEPPs feature ultra-deep subwavelength confinement (down to 1-nm level with mode areas ~ 10^-4 of λ²), long propagation length (up to ~ 60 µm), tunable dispersion with versatile mode characters (SPP- and exciton-like mode characters), and small momentum mismatch to free-space photons. With the capability to overcome the trade-offs of SPPs and the compatibility for waveguiding applications, our theoretical results suggest an attractive guided-wave platform to manipulate exciton-plasmon interactions at the ultra-deep subwavelength scale, opening new horizons for waveguiding nano-polaritonic components and devices.

Theoretical Quantum Model of Two-Dimensional Propagating Plexcitons

When plasmonic excitations of metallic interfaces and nanostructures interact with electronic excitations in semiconductors, new states emerge that hybridize the characteristics of the uncoupled states. The engendered properties make these hybrid states appealing for a broad range of applications, ranging from photovoltaic devices to integrated circuitry for quantum devices. Here, through quantum modeling, the coupling of surface plasmon polaritons and mobile two-dimensional excitons such as those in atomically thin semiconductors is examined with emphasis on the case of strong coupling. Our model shows that at around the energy crossing of the dispersion relationships of the uncoupled species, they strongly interact and polariton states --propagating plexcitons -- emerge. The temporal evolution of the system where surface plasmon polaritons are continuously injected into the system is simulated to gain initial insight on potential experimental realizations of these states. The results show a steady state that is dominated by the lower-energy polariton. The study theoretically further establishes the possible existence of propagating plexcitons in atomically thin semiconductors and provides important guidance for the experimental detection and characterization of such states for a wide range of optoelectronic technologies.

Efficient Quadrature Squeezing from Biexcitonic Parametric Gain in Atomically Thin Semiconductors

Modification of electromagnetic quantum fluctuations in the form of quadrature squeezing is a central quantum resource, which can be generated from nonlinear optical processes. Such a process is facilitated by coherent two-photon excitation of the strongly bound biexciton in atomically thin semiconductors. We show theoretically that interfacing an atomically thin semiconductor with an optical cavity makes it possible to harness this two-photon resonance and use the biexcitonic parametric gain to generate squeezed light with input power an order of magnitude below current state-of-the-art devices with conventional third-order nonlinear materials that rely on far off-resonant nonlinearities. Furthermore, the squeezing bandwidth is found to be in the range of several meV. These results identify atomically thin semiconductors as a promising candidate for on-chip squeezed-light sources.

Femtosecond Pulsed Fiber Laser by an Optical Device Based on NaOH-LPE Prepared WSe2 Saturable Absorber

We report on all-optical devices prepared from WSe₂ combined with drawn tapered fibers as saturable absorbers to achieve ultrashort pulse output. The saturable absorber with a high damage threshold and high saturable absorption characteristics is prepared for application in erbium-doped fiber lasers by the liquid phase exfoliation method for WSe₂, and the all-optical device exhibited strong saturable absorption characteristics with a modulation depth of 15% and a saturation intensity of 100.58 W. The net dispersion of the erbium-doped fiber laser cavity is ~-0.1 ps², and a femtosecond pulse output with a bandwidth of 11.4 nm, a pulse width of 390 fs, and a single-pulse capability of 42 pJ is obtained. Results indicate that the proposed WSe₂ saturable absorbers are efficient, photonic devices to realize stable fiber lasers. The results demonstrate that the WSe₂ saturable absorber is an effective photonic device for realizing stable fiber lasers, which have a certain significance for the development of potential photonic devices.

Plasmon-induced coherence, exciton-induced transparency, and Fano interference for hybrid plasmonic systems in strong coupling regime

We present an analytical model describing the transition to a strong coupling regime for an ensemble of emitters resonantly coupled to a localized surface plasmon in a metal-dielectric structure. The response of a hybrid system to an external field is determined by two distinct mechanisms involving collective states of emitters interacting with the plasmon mode. The first mechanism is the near-field coupling between the bright collective state and the plasmon mode, which underpins the energy exchange between the system components and gives rise to exciton-induced transparency minimum in scattering spectra in the weak coupling regime and to emergence of polaritonic bands as the system transitions to the strong coupling regime. The second mechanism is the Fano interference between the plasmon dipole moment and the plasmon-induced dipole moment of the bright collective state as the hybrid system interacts with the radiation field. The latter mechanism is greatly facilitated by plasmon-induced coherence in a system with the characteristic size below the diffraction limit as the individual emitters comprising the collective state are driven by the same alternating plasmon near field and, therefore, all oscillate in phase. This cooperative effect leads to scaling of the Fano asymmetry parameter and of the Fano function amplitude with the ensemble size, and therefore, it strongly affects the shape of scattering spectra for large ensembles. Specifically, with increasing emitter numbers, the Fano interference leads to a spectral weight shift toward the lower energy polaritonic band.

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.

Single-particle spectroscopic investigation on the scattering spectrum of Au@MoS2 core-shell nanosphere heterostructure

Owing to the uniform shape of the nanospheres, the Au@MoS₂ core-shell nanosphere heterostructure enables us to design nano-optoelectronic devices and nanosensors with highly tunable and reproducible optical properties. However, until now, at the single-particle level, there is still uncertainty as to how much the scattering characteristics depend on the particle size and the local environment. In this letter, we performed an in situ single-particle study of the scattering spectrum of the Au@MoS₂ core-shell nanosphere heterostructure before and after coating with the MoS₂ layer. Single-particle characterization confirms that the classic quasi-static approximation (QSA) theory can be used to predict the scattering spectra of Au@MoS₂ core-shell nanoparticles. Moreover, we have found that the A and B-exciton absorption peaks do not rely on the local refractive index change, while the position of the particle plasmon resonances does. Such features can be used as an internal reference for sensing applications against measurement errors, such as defocusing the imaging. Our results show that Au@MoS₂ core-shell nanoparticles have the potential to become one of the promising nanosensors in the field of single-particle sensing.

Enhanced light-matter interaction in two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS₂, WS₂, MoSe₂, and WSe₂, have received extensive attention in the past decade due to their extraordinary electronic, optical and thermal properties. They evolve from indirect bandgap semiconductors to direct bandgap semiconductors while their layer number is reduced from a few layers to a monolayer limit. Consequently, there is strong photoluminescence in a monolayer (1L) TMDC due to the large quantum yield. Moreover, such monolayer semiconductors have two other exciting properties: large binding energy of excitons and valley polarization. These properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak light-matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research.

Greatly Enhanced Plasmon-Exciton Coupling in Si/WS2/Au Nanocavities

A strong light-matter interaction is highly desirable from the viewpoint of both fundamental research and practical application. Here, we propose a dielectric-metal hybrid nanocavity composed of a silicon (Si) nanoparticle and a thin gold (Au) film and investigate numerically and experimentally the coupling between the plasmons supported by the nanocavity and the excitons in an embedded tungsten disulfide (WS₂) monolayer. When a Si/WS₂/Au nanocavity is excited by the surface plasmon polariton generated on the surface of the Au film, greatly enhanced plasmon-exciton coupling originating from the hybridization of the surface plasmon polariton, the mirror-image-induced magnetic dipole, and the exciton modes is clearly revealed in the angle- or size-resolved scattering spectra. A Rabi splitting as large as ∼240 meV is extracted by fitting the experimental data with a coupled harmonic oscillator model containing three oscillators. Our findings open new horizons for constructing nanoscale photonic devices by exploiting dielectric-metal hybrid nanocavities.

Steering Room-Temperature Plexcitonic Strong Coupling: A Diexcitonic Perspective

Plexcitonic strong coupling between a plasmon-polariton and a quantum emitter empowers ultrafast quantum manipulations in the nanoscale under ambient conditions. The main body of previous studies deals with homogeneous quantum emitters. To enable multiqubit states for future quantum computing and network, the strong coupling involving two excitons of the same material but different resonant energies has been investigated and observed primarily at very low temperature. Here, we report a room-temperature diexcitonic strong coupling (DiSC) nanosystem in which the excitons of a transition metal dichalcogenide monolayer and dye molecules are both strongly coupled to a single Au nanocube. Coherent information exchange in this DiSC nanosystem could be observed even when exciton energy detuning is about five times larger than the respective line widths. The strong coupling behaviors in such a DiSC nanosystem can be manipulated by tuning the plasmon resonant energies and the coupling strengths, opening up a paradigm of controlling plasmon-assisted coherent energy transfer.

Controllable Plexcitonic Coupling in a WS2-Ag Nanocavity with Solvents

Strong coupling between emitters and cavities underlies many of the current strategies aiming at generating and controlling quantum states at room temperature. Recent experiments reveal strong coupling between two-dimensional transition metal dichalcogenides (TMDCs) and individual plasmonic structures; however, the coupling strength is quite limited (<200 meV), and the active control of the coupling strength is challenging. Here, we demonstrate the active tuning of plexcitonic coupling in monolayer WS₂ coupled to a plasmonic nanocavity by immersing into a mixed solution of dichloromethane (DCM) and ethanol. By adjusting the mixture ratio, continuous tuning of the Rabi splitting energy ranged from 183 meV (in ethanol) to 273 meV (in DCM) is achieved. The results are mainly attributed to the remarkable increase of the neutral exciton density in monolayer WS₂ as the concentration of DCM is increased. It offers an important stepping stone toward a further study on plexcitonic coupling in layered materials, along with potential applications in quantum information processing and nonlinear optical materials.

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.

Tuning Hybrid exciton-Photon Fano Resonances in Two-Dimensional Organic-Inorganic Perovskite Thin Films

As easy-to-grow quantum wells with narrow excitonic features at room temperature, two-dimensional (2D) Ruddleson-Popper perovskites are promising for realizing novel nanophotonic devices based on exciton-photon interactions. Here, we demonstrate a distinct hybrid exciton-photon Fano resonance in (C₄H₉NH₃)₂PbI₄ thin films prepared via spin coating. Using a classical coupled-oscillator model and finite-difference time-domain simulations, we link the Fano interference to the coupling of the exciton with the Rayleigh-like scattering of the film microstructure. Combining colloidal plasmonic cavities with the 2D perovskite films, we demonstrate tuning of the Fano resonance. In combination with silver nanoparticles, the exciton-photon Fano interference couples to the in-plane plasmonic modes with indications of Rabi splitting. By creating a nanoparticle on mirror geometry, we address the out-of-plane excitonic component, reaching an intermediate coupling regime. These structures suggest possible photonic targets for biomolecular self-assembly applications.

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.

Strong plasmon-exciton coupling in transition metal dichalcogenides and plasmonic nanostructures

Achieving strong coupling between emitters and cavity photons holds an important position in the light-matter interaction due to its applications such as polariton lasing, all-optical switches, and quantum information processing. However, room-temperature polaritonic devices with subwavelength dimensions based on strong light-matter coupling are difficult to realize using traditional emitter-cavity coupled systems. In recent years, coupled systems constructed from plasmonic nanostructures and transition metal dichalcogenides (TMDs) have shown their potential in achieving room-temperature strong coupling and robustness in the nanofabrication processes. This minireview presents the recent progress in strong plasmon-exciton coupling in such plasmonic-TMD hybrid structures. Differing from a broader scope of strong coupling, we focus on the plasmon-exciton coupling between excitons in TMDs and plasmons in single nanoparticles, nanoparticle-over-mirrors, and plasmonic arrays. In addition, we discuss the future perspectives on the strong plasmon-exciton coupling at few-excitons level and the nonlinear response of these hybrid structures in the strong coupling regime.

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.

Plasmons in the van der Waals charge-density-wave material 2H-TaSe2

Plasmons in two-dimensional (2D) materials beyond graphene have recently gained much attention. However, the experimental investigation is limited due to the lack of suitable materials. Here, we experimentally demonstrate localized plasmons in a correlated 2D charge-density-wave (CDW) material: 2H-TaSe2. The plasmon resonance can cover a broad spectral range from the terahertz (40 μm) to the telecom (1.55 μm) region, which is further tunable by changing thickness and dielectric environments. The plasmon dispersion flattens at large wave vectors, resulted from the universal screening effect of interband transitions. More interestingly, anomalous temperature dependence of plasmon resonances associated with CDW excitations is observed. In the CDW phase, the plasmon peak close to the CDW excitation frequency becomes wider and asymmetric, mimicking two coupled oscillators. Our study not only reveals the universal role of the intrinsic screening on 2D plasmons, but also opens an avenue for tunable plasmons in 2D correlated materials.

Resonance Coupling in an Individual Gold Nanorod-Monolayer WS2 Heterostructure: Photoluminescence Enhancement with Spectral Broadening

Recently, resonance coupling between plasmonic nanocavity and two-dimensional semiconductors has attracted considerable attention. Most of the previous studies have focused on demonstrating this effect with light scattering or reflection spectroscopy, while the photoluminescence (PL) spectrum can help ascertain the underlying physics. Here, we report on the light-emitting characteristics of a monolayer WS₂ flake coupled with a plasmonic gold nanorod. We construct a heterostructure by integrating an individual gold nanorod on top of a small piece of monolayer WS₂, where the WS₂ area is determined by the projected area of the nanorod. In such a heterostructure, the background PL from the uncoupled WS₂ can be suppressed, which allows us to characterize the resonance coupling effect using correlated single-particle dark-field (DF) scattering and PL spectroscopies. Distinct mode splitting and anticrossing dispersion are observed in the scattering spectra, which originate from the resonance coupling between the excitons in the WS₂ and plasmon resonance in the gold nanorod. In addition, a 1187-fold enhancement is obtained for the light emitted from the heterostructure relative to that of the pristine monolayer WS₂. The emission spectra are broadened with mode-splitting features at room temperature, which can be further decomposed into two resonance modes using a coupled mode analysis. Moreover, two PL modes are polarized along the longitudinal axis of the gold nanorod. These findings show the potential of the designed individual gold nanorod-monolayer WS₂ heterostructure as a platform for studying the resonance coupling effect between plasmon resonance and two-dimensional excitons.

Influence of Gold Nano-Bipyramid Dimensions on Strong Coupling with Excitons of Monolayer MoS2

Rabi splitting between the longitudinal plasmon of a gold nano-bipyramid and the A exciton of monolayer MoS₂ is observed at room temperature. The dependence of the Rabi splitting on the physical dimensions of the nano-bipyramid is reported. The impact of bipyramid length, aspect ratio, and tip radius on the coupling strength is investigated. The mode volume of the nanoresonator is significantly reduced because of the sharp tips of the bipyramid, and the Rabi splitting increases with tip sharpness. The results also reveal that greater Rabi splitting is observed for larger bipyramids, contrasting with results previously reported for different nanoresonator shapes. This shows, for the first time, how the magnitude of the splitting has a different response for particular nanoresonators when tuning the size, without increasing the number of excitons coupled into the system. The Rabi splitting, at zero energy detuning between plasmon and A exciton, increases from ∼55 meV with a 70 nm-long bipyramid to ∼80 meV with a 100 nm-long bipyramid. The increase in coupling strength with size arises because of increasing confinement of the field enhancement at the bipyramid tip.

On the applicability of quantum-optical concepts in strong-coupling nanophotonics

Rooted in quantum optics and benefiting from its well-established foundations, strong coupling in nanophotonics has experienced increasing popularity in recent years. With nanophotonics being an experiment-driven field, the absence of appropriate theoretical methods to describe ground-breaking advances has often emerged as an important issue. To address this problem, the temptation to directly transfer and extend concepts already available from quantum optics is strong, even if a rigorous justification is not always available. In this review we discuss situations where, in our view, this strategy has indeed overstepped its bounds. We focus on exciton-plasmon interactions, and particularly on the idea of calculating the number of excitons involved in the coupling. We analyse how, starting from an unfounded interpretation of the term N/V that appears in theoretical descriptions at different levels of complexity, one might be tempted to make independent assumptions for what the number N and the volume V are, and attempt to calculate them separately. Such an approach can lead to different, often contradictory results, depending on the initial assumptions (e.g. through different treatments of V as the-ambiguous in plasmonics-mode volume). We argue that the source of such contradictions is the question itself-How many excitons are coupled?, which disregards the true nature of the coupled components of the system, has no meaning and often not even any practical importance. If one is interested in validating the quantum nature of the system-which appears to be the motivation driving the pursuit of strong coupling with small N-one could instead focus on quantities such as the photon emission rate or the second-order correlation function. While many of the issues discussed here may appear straightforward to specialists, our target audience is predominantly newcomers to the field, either students or scientists specialised in different disciplines. We have thus tried to minimise the occurrence of proofs and overly-technical details, and instead provide a qualitative discussion of analyses that should be avoided, hoping to facilitate further growth of this promising area.

Strong plasmon-exciton coupling between lithographically defined single metal nanoparticles and monolayer WSe2

Strong coupling between surface plasmons and excitons leads to the formation of plexcitons with new energy states, providing a versatile platform for a range of frontier research subjects. Single plasmonic nanoparticles have recently attracted much attention for realizing strong coupling due to their small mode volume. However, the usually used chemically synthesized metal nanoparticles are unfavorable for accurately tailoring the surface plasmon resonances and adverse to the implementation of on-chip integration. Here, we report for the first time the realization of strong coupling between monolayer WSe2 and lithographically defined single metal nanoparticles. Focusing on gold nanobowties, the large Rabi splitting of 187 meV is achieved. The excitons around the nanogaps in the nanobowties contribute dominantly to the coupling strength, and the coupling strength is larger for smaller nanobowties due to the smaller mode volume. Moreover, the hybrid systems of monolayer WSe2 and gold nanoparticle monomers of nanorods, nanotriangles, and nanodisks are found to closely satisfy the criterion of strong coupling. The strong plasmon-exciton coupling realized by single plasmonic nanostructures fabricated by advanced nanofabrication techniques and monolayer semiconductors can provide new opportunities for manipulating strong light-matter interactions at the nanoscale and facilitate the development of plexciton-based nanodevices with ultrasmall footprints.

Electrical Control of Hybrid Monolayer Tungsten Disulfide-Plasmonic Nanoantenna Light-Matter States at Cryogenic and Room Temperatures

Hybrid light-matter states-polaritons-have attracted considerable scientific interest recently, motivated by their potential for development of nonlinear and quantum optical schemes. To realize such states, monolayer transition metal dichalcogenides (TMDCs) have been widely employed as excitonic materials. In addition to neutral excitons, TMDCs host charged excitons, which enables active tuning of hybrid light-matter states by electrical means. Although several reports demonstrated charged exciton-polaritons in various systems, the full-range interaction control attainable at room temperature has not been realized. Here, we demonstrate electrically tunable charged exciton-plasmon polaritons in a hybrid tungsten disulfide (WS₂) monolayer-plasmonic nanoantenna system. We show that electrical gating of monolayer WS₂ allows tuning the oscillator strengths of neutral and charged excitons not only at cryogenic but also at room temperature, both at vacuum and atmospheric pressure. Such electrical control enables a full-range tunable switching from strong neutral exciton-plasmon coupling to strong charged exciton-plasmon coupling. Our experimental findings allow discussing beneficial and limiting factors of charged exciton-plasmon polaritons, as well as offer routes toward realization of charged polaritonic devices at ambient conditions.

Strong plasmon-exciton coupling in MIM waveguide-resonator systems with WS2 monolayer

The room-temperature strong plasmon-exciton coupling is first investigated in a metal-insulator-metal (MIM) waveguide-resonator system with WS₂ monolayer. Finite-difference time-domain (FDTD) simulated results exhibit that the Fabry-Pérot (F-P) cavity is realized by the MIM plasmonic waveguide with two separated metal bars. When the F-P resonance is tuned to overlap with the A-exciton absorption peak of WS₂ monolayer, the strong plasmon-exciton coupling is obtained at visible wavelengths. As a result, the spectral splitting response confirmed by the coupled-mode theory (CMT) appears in the transmission spectrum. Intriguingly, the switching response is handily witnessed by tuning the orientation of WS₂ monolayer along the cavity, and the coupling strength is dynamically tuned by changing the position of the WS₂ monolayer. Simultaneously, the anticrossing behavior with the Rabi splitting up to 109 meV is achieved by small changes in the length of the F-P cavity and the refractive index of dielectric in the cavity, respectively. The underlying physics is further revealed by the coupled oscillator model (COM). The proposed strong plasmon-exciton coupling can find utility in highly integrated plasmonic circuits for optical switching.

Polarization-Dependent Optical Properties and Optoelectronic Devices of 2D Materials

The development of optoelectronic devices requires breakthroughs in new material systems and novel device mechanisms, and the demand recently changes from the detection of signal intensity and responsivity to the exploration of sensitivity of polarized state information. Two-dimensional (2D) materials are a rich family exhibiting diverse physical and electronic properties for polarization device applications, including anisotropic materials, valleytronic materials, and other hybrid heterostructures. In this review, we first review the polarized-light-dependent physical mechanism in 2D materials, then present detailed descriptions in optical and optoelectronic properties, involving Raman shift, optical absorption, and light emission and functional optoelectronic devices. Finally, a comment is made on future developments and challenges. The plethora of 2D materials and their heterostructures offers the promise of polarization-dependent scientific discovery and optoelectronic device application.

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.

In situ scattering of single gold nanorod coupling with monolayer transition metal dichalcogenides

We investigated in situ the interaction between a single gold nanorod and monolayer transition metal dichalcogenides (TMDCs) by atomic force microscopy nanomanipulation and single-particle spectroscopy. We observed that the resonant scattering peak of the hybrid redshifted, the full width at half maximum of the scattering resonance narrowed and the scattering intensity increased compared with those of the same nanorod before coupling with monolayer TMDCs. These results were understood with the aid of finite-difference time-domain simulations, the Fano model, and the classical oscillator model. Also, the spectral features varied with the distance between the nanorod and TMDCs, and the interaction was mainly attributed to the resonant energy transfer effect. Our findings clarify the influence of TMDCs on the plasmonic resonance and contribute to a deeper understanding of the plasmon exciton interaction. These results are beneficial for the optimization of plasmonic nanostructure-TMDC hybrids and their corresponding applications.

Visualizing Spatial Variations of Plasmon-Exciton Polaritons at the Nanoscale Using Electron Microscopy

Polaritons are compositional light-matter quasiparticles that have enabled remarkable breakthroughs in quantum and nonlinear optics, as well as in material science. Recently, plasmon-exciton polaritons (plexcitons) have been realized in hybrid material systems composed of transition metal dichalcogenide (TMDC) materials and metal nanoparticles, expanding polaritonic concepts to room temperature and nanoscale systems that also benefit from the exotic properties of TMDC materials. Despite the enormous progress in understanding TMDC-based plexcitons using optical-based methods, experimental evidence of plexcitons formation has remained indirect and mapping their nanometer-scale characteristics has remained an open challenge. Here, we demonstrate that plexcitons generated by a hybrid system composed of an individual silver nanoparticle and a few-layer WS₂ flake can be spectroscopically mapped with nanometer spatial resolution using electron energy loss spectroscopy in a scanning transmission electron microscope. Experimental anticrossing measurements using the absorption-dominated extinction signal provide the ultimate evidence for plexciton hybridization in the strong coupling regime. Spatially resolved EELS maps reveal the existence of unexpected nanoscale variations in the deep-subwavelength nature of plexcitons generated by this system. These findings pioneer new possibilities for in-depth studies of the local atomic structure dependence of polariton-related phenomena in TMDC hybrid material systems with nanometer spatial resolution.

Chasing Plasmons in Flatland

Two-dimensional layered crystals, including graphene and transition metal dichalcogenides, represent an interesting avenue for studying light-matter interactions at the nanoscale in confined geometries. They offer several attractive properties, such as large exciton binding energies, strong excitonic resonances, and tunable bandgaps from the visible to the near-IR along with large spin-orbit coupling, direct band gap transitions, and valley-selective responses.

Strong coupling of emitters to single plasmonic nanoparticles: exciton-induced transparency and Rabi splitting

Strong coupling between plasmons in metal nanoparticles and single excitons in molecules or semiconductor nanomaterials has recently attracted considerable experimental effort for potential applications in quantum-mechanical and classical optical information processing and for fundamental studies of light-matter interaction. Here, we review the theory behind strong plasmon-exciton coupling and provide analytical expressions that can be used for fitting experimental data, particularly the commonly measured scattering spectra. We re-analyze published data using these expressions, providing a uniform method for evaluating and quantifying claims of strong coupling that avoids ambiguities in distinguishing between Rabi splitting and exciton-induced transparency (or Fano-like interference between plasmons and excitons).

Angle-independent strong coupling between plasmonic magnetic resonances and excitons in monolayer WS2

The room-temperature strong coupling between plasmonic magnetic resonances (MRs) of metal gratings and excitons in monolayer WS₂ is intensively investigated. Both numerical simulations and theoretical calculations indicate that the strong coupling between MRs and excitons enables the remarkable spectral splitting. The typical anticrossing behavior with the Rabi splitting up to 86.5 meV is realized on the color-coded absorption spectra by changing the nanogroove depth, width, and the refractive index of the dielectric filled into nanogrooves, respectively. Interestingly, such strong coupling can also be achieved by using WS₂ ribbons instead of the monolayer and simultaneously is dynamically controlled by varying the interaction area. More importantly, the observed MR-exciton coupling is angle-independent. Our findings thus suggest a possible way toward enhancing light-matter interactions in monolayer transition-metal dichalcogenides and play significant roles in quantum and nonlinear nanophotonic devices at ambient conditions.

Dark-Exciton-Mediated Fano Resonance from a Single Gold Nanostructure on Monolayer WS2 at Room Temperature

Strong spatial confinement and highly reduced dielectric screening provide monolayer transition metal dichalcogenides with strong many-body effects, thereby possessing optically forbidden excitonic states (i.e., dark excitons) at room temperature. Herein, the interaction of surface plasmons with dark excitons in hybrid systems consisting of stacked gold nanotriangles and monolayer WS₂ is explored. A narrow Fano resonance is observed when the hybrid system is surrounded by water, and the narrowing of the spectral Fano linewidth is attributed to the plasmon-enhanced decay of dark K-K excitons. These results reveal that dark excitons in monolayer WS₂ can strongly modify Fano resonances in hybrid plasmon-exciton systems and can be harnessed for novel optical sensors and active nanophotonic devices.

Transition metal dichalcogenide nanodisks as high-index dielectric Mie nanoresonators

Monolayer transition metal dichalcogenides (TMDCs) have recently been proposed as an excitonic platform for advanced optical and electronic functionalities^1-3. However, in spite of intense research efforts, it has not been widely appreciated that TMDCs also possess a high refractive index^4,5. This characteristic opens up the possibility to utilize them to construct resonant nanoantennas based on subwavelength geometrical modes^6,7. Here, we show that nanodisks, fabricated from exfoliated multilayer WS₂, support distinct Mie resonances and anapole states⁸ that can be tuned in wavelength over the visible and near-infrared range by varying the nanodisk size and aspect ratio. As a proof of concept, we demonstrate a novel regime of light-matter interaction-anapole-exciton polaritons-which we realize within a single WS₂ nanodisk. We argue that the TMDC material anisotropy and the presence of excitons enrich traditional nanophotonics approaches based on conventional high-index materials and/or plasmonics.

Tunable strong exciton-plasmon-exciton coupling in WS2-J-aggregates-plasmonic nanocavity

A coupling system is proposed to active control of strong exciton-plasmon-exciton coupling, which consists of a silver nanoprism separated from a monolayer WS₂ by J-aggregates. The scattering spectrum of the hybrid system calculated by the finite-difference time-domain (FDTD) method is well reproduced by the coupled oscillator model theory. The calculation results show that strong couplings among WS₂ excitons, J-aggregate excitons, and localized surface plasmon resonances (LSPRs) are achieved in the hybrid nanostructure, and result in three plexciton branches. We further analyze the exciton-plasmon-exciton coupling behaviors and obtain the weighting efficiencies of the original modes in three plexciton branches. The strong couplings between two different excitons and LSPRs can be active manipulated by tuning the temperature or the concentration of J-aggregates. The proposed systems make up a simple platform for the dynamic control of exciton-plasmon-exciton couplings and have potential applications in optical modulators at the nanoscale.

Exciton-Plasmon Energy Exchange Drives the Transition to a Strong Coupling Regime

We present a model for exciton-plasmon coupling based on an energy exchange mechanism between quantum emitters (QE) and localized surface plasmons in metal-dielectric structures. Plasmonic correlations between QEs give rise to a collective state exchanging its energy cooperatively with a resonant plasmon mode. By defining carefully the plasmon mode volume for a QE ensemble, we obtain a relation between QE-plasmon coupling and a cooperative energy transfer rate that is expressed in terms of local fields. For a single QE near a sharp metal tip, we find analytically the enhancement factor for QE-plasmon coupling relative to QE coupling to a cavity mode. For QEs distributed in an extended region enclosing a plasmonic structure, we find that the ensemble QE-plasmon coupling saturates to a universal value independent of system size and shape, consistent with the experiment.

Resonance Coupling in Heterostructures Composed of Silicon Nanosphere and Monolayer WS2: A Magnetic-Dipole-Mediated Energy Transfer Process

Light-matter resonance coupling is a long-studied topic for both fundamental research and photonic and optoelectronic applications. Here we investigated the resonance coupling between the magnetic dipole mode of a dielectric nanosphere and 2D excitons in a monolayer semiconductor. By coating an individual silicon nanosphere with a monolayer of WS₂, we theoretically demonstrated that, because of the strong energy transfer between the magnetic dipole mode of the nanosphere and the A-exciton in WS₂, resonance coupling evidenced by anticrossing behavior in the scattering energy diagram was observed, with a mode splitting of 43 meV. In contrast to plexcitons, which involve plasmonic nanocavities, the resonance coupling in this all-dielectric heterostructure was insensitive to the spacing between the silicon nanosphere core and the WS₂ shell. Additionally, the two split modes exhibited distinct light-scattering directionality. We further experimentally demonstrated the resonance coupling effect by depositing silicon nanospheres with different diameters onto a WS₂ monolayer and collecting the scattering spectra of the resulting heterostructures under ambient conditions. We further demonstrated active control of the resonance coupling by temperature scanning. Our findings highlighted the potential of our all-dielectric heterostructure as a solid platform for studying strong light-matter interactions at the nanoscale.