Observation of Tunable Charged Exciton Polaritons in Hybrid Monolayer WS2-Plasmonic Nanoantenna System

Infrared routing and switching with tunable spectral bandwidth using arrays of metallic nanoantennas

We study infrared routing and switching with tunable spectral bandwidth using in-plane scattering of light by flat Au nanoantenna arrays. The base dimensions of these nanoantennas are approximately 250 by 850 nm, while their heights vary from 20 to 150 nm. Our results show that, with the increase in height, the arrays become more efficient scatterers while their spectra broaden within the 1-1.6µm range. Our findings demonstrate that such processes strongly depend on the incident light polarization. For a given polarization, the incident light is efficiently scattered in only two opposite directions along the plane of the arrays, with insignificant transmission. Switching such a polarization by 90∘, however, suppresses this process, allowing the light to mostly pass through the arrays with minimal scattering. These unique characteristics suggest a tunable beam splitter application in the 1-1.6µm range and even longer wavelengths.

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.

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.

Identifying high-order plasmon modes in silver nanoparticle-over-mirror configuration

Metallic nanoparticle-over-mirror (NPOM) represents as a versatile plasmonic configuration for surface enhanced spectroscopy, sensing and light-emitting metasurfaces. However, experimentally identifying the high-order localized surface plasmon modes in NPOM, especially for the best plasmonic material silver, is often hindered by the small scattering cross-section of high-order plasmon modes and the poor reproducibility of the spectra across different NPOMs, resulted from the polyhedral morphology of the colloidal nanoparticles or the rough surface of deposited polycrystalline metals. In this study, we identify the high-order localized surface plasmon modes in silver NPOM by using differential reflection spectroscopy. We achieved reproducible single-particle absorption spectra by constructing uniform NPOM consisting of silver nanospheres, single-crystallized silver microplates, and a self-assembled monolayer of 1,10-decanedithiol. For comparison, silver NPOM created from typical polycrystalline films exhibits significant spectral fluctuations, even when employing template stripping methods to minimize the film roughness. Identifying high-order plasmon modes in the NPOM configuration offers a pathway to construct high-quality plasmonic substrates for applications such as colloidal metasurface, surface-enhanced Raman spectroscopy, fluorescence, or infrared absorption.

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.

Photoluminescence of MoS2 on Plasmonic Gold Nanoparticles Depending on the Aggregate Size

Transition metal dichalcogenides (TMDs) are promising candidates for ultrathin functional semiconductor devices. In particular, incorporating plasmonic nanoparticles into TMD-based devices enhances the light-matter interaction for increased absorption efficiency and enables control of device performance such as electronic, electrical, and optical properties. In this heterohybrid structure, manipulating the number of TMD layers and the aggregate size of plasmonic nanoparticles is a straightforward approach to tailoring device performance. In this study, we use photoluminescence (PL) spectroscopy, which is a commonly employed technique for monitoring device performance, to analyze the changes in electronic and optical properties depending on the number of MoS2 layers and the size of the gold nanoparticle (AuNP) aggregate under nonresonant and resonant excitation conditions. The PL intensity in monolayer MoS2/AuNPs increases as the size of aggregates increases irrespective of the excitation conditions. The strain induced by AuNPs causes a red shift, but as the aggregates grow larger, the effect of p-doping increases and the blue shift becomes prominent. In multilayer MoS2/AuNPs, quenched PL intensity is observed under nonresonant excitation, while enhancement is noted under resonant excitation, which is mainly contributed by p-doping and LSPR, respectively. Remarkably, the alteration in the spectral shape due to resonant excitation is evident solely in small aggregates of AuNPs across all layers.

Strong Coupling of Two-Dimensional Excitons and Plasmonic Photonic Crystals: Microscopic Theory Reveals Triplet Spectra

Monolayers of transition metal dichalcogenides (TMDCs) are direct-gap semiconductors with strong light-matter interactions featuring tightly bound excitons, while plasmonic crystals (PCs), consisting of metal nanoparticles that act as meta-atoms, exhibit collective plasmon modes and allow one to tailor electric fields on the nanoscale. Recent experiments show that TMDC-PC hybrids can reach the strong-coupling limit between excitons and plasmons, forming new quasiparticles, so-called plexcitons. To describe this coupling theoretically, we develop a self-consistent Maxwell-Bloch theory for TMDC-PC hybrid structures, which allows us to compute the scattered light in the near- and far-fields explicitly and provide guidance for experimental studies. One of the key findings of the developed theory is the necessity to differentiate between bright and originally momentum-dark excitons. Our calculations reveal a spectral splitting signature of strong coupling of more than 100 meV in gold-MoSe2 structures with 30 nm nanoparticles, manifesting in a hybridization of the plasmon mode with momentum-dark excitons into two effective plexcitonic bands. The semianalytical theory allows us to directly infer the characteristic asymmetric line shape of the hybrid spectra in the strong coupling regime from the energy distribution of the momentum-dark excitons. In addition to the hybridized states, we find a remaining excitonic mode with significantly smaller coupling to the plasmonic near-field, emitting directly into the far-field. Thus, hybrid spectra in the strong coupling regime can contain three emission peaks.

Realization of exciton-polariton optical chirality based on strong coupling between intrinsic chiral quasibound states in the continuum and monolayer WS2

Hybrid quasiparticles produced by the strong interaction between nanostructures and excitons will exhibit optical chirality when one of the coupled components is chiral. Due to the tunability of hybrid states, the coupled system has potential applications in chiral devices and chiral sensing. However, reported chiral materials including chiral molecules and three-dimensional chiral structures in the coupled system limit the application due to the weak chiroptical responses and difficult fabrication, respectively. In this paper, we design chiral quasibound states in the continuum (q-BIC) metasurface by introducing planar symmetry-breaking and z-axis perturbation into an array structure whose unit cell is a C4 rotational symmetric disk. By tuning the polarization state of the eigenmode, a significant chiroptical response is obtained in our q-BIC metasurface. Furthermore, mode splitting is observed not only in the reflection spectrum but also in the circular dichroism (CD) spectrum in the chiral q-BIC and monolayer WS2 strong coupling system, which indicates the realization of the exciton-polariton optical chirality. More importantly, one order of magnitude difference in the reflection to left and right circularly polarized light is achieved resulting in significant CD signals. Our work provides a new strategy to realize the exciton polaritons with significant chiroptical responses, which exhibits promising applications in on-chip chiral devices.

Biexcitons-plasmon coupling of Ag@Au hollow nanocube/MoS2 heterostructures based on scattering spectra

The strong interaction between light and matter is one of the current research hotspots in the field of nanophotonics, and provides a suitable platform for fundamental physics research such as on nanolasers, high-precision sensing in biology, quantum communication and quantum computing. In this study, double Rabi splitting was achieved in a composite structure monolayer MoS2 and a single Ag@Au hollow nanocube (HNC) in room temperature mainly due to the two excitons in monolayer MoS2. Moreover, the tuning of the plasmon resonance peak was realized in the scattering spectrum by adjusting the thickness of the shell to ensure it matches the energy of the two excitons. Two distinct anticrossings are observed at both excitons resonances, and large double Rabi splittings (90 meV and 120 meV) are obtained successfully. The finite-difference time domain (FDTD) method was also used to simulate the scattering spectra of the nanostructures, and the simulation results were in good agreement with the experimental results. Additionally, the local electromagnetic field ability of the Ag@Au hollow HNC was proved to be stronger by calculating and comparing the mode volume of different nanoparticles. Our findings provides a good platform for the realization of strong multi-mode coupling and open up a new way to construct nanoscale photonic devices.

Patterned growth of two-dimensional atomic layer semiconductors

Transition metal dichalcogenides (TMDCs), which are representative of two-dimensional (2D) semiconductors, have attracted tremendous attention over the last two decades. TMDCs are regarded as potential candidates in modern nano- and optoelectronic applications due to their unique crystal structures and outstanding electronic and optoelectronic properties. For practical use, 2D semiconductors need to be fabricated with diverse morphologies for integration into electronic devices and to perform different functionalities. Controlled patterning synthesis with programmable geometries is therefore highly desired. We review state-of-the-art strategies for the patterned growth of atomic layer TMDCs and their heterostructures, including additive manufacturing and subtractive manufacturing for patterning single TMDC materials and the introduction of other low-dimensional nanomaterials as growth templates or hetero-atoms for element conversion in patterning TMDC heterostructures. The optoelectronic and electronic applications of the as-grown monolayer TMDC patterns are introduced. Future challenges and the prospects for the patterned growth of 2D semiconductors are discussed based on present achievements.

Electrochemically Switchable Multimode Strong Coupling in Plasmonic Nanocavities

The strong-coupling interaction between quantum emitters and cavities provides the archetypical platform for fundamental quantum electrodynamics. Here we show that methylene blue (MB) molecules interact coherently with subwavelength plasmonic nanocavity modes at room temperature. Experimental results show that the strong coupling can be switched on and off reversibly when MB molecules undergo redox reactions which transform them to leuco-methylene blue molecules. In simulations we demonstrate the strong coupling between the second excited plasmonic cavity mode and resonant emitters. However, we also show that other detuned modes simultaneously couple efficiently to the molecular transitions, creating unusual cascades of mode spectral shifts and polariton formation. This is possible due to the relatively large plasmonic particle size resulting in reduced mode splittings. The results open significant potential for device applications utilizing active control of strong coupling.

Exciton Polaritons in Emergent Two-Dimensional Semiconductors

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

Photoluminescence Enhancement of Monolayer WS2 by n-Doping with an Optically Excited Gold Disk

In nanophotonics and quantum optics, we aim to control and manipulate light with tailored nanoscale structures. Hybrid systems of nanostructures and atomically thin materials are of interest here, as they offer rich physics and versatility due to the interaction between photons, plasmons, phonons, and excitons. In this study, we explore the optical and electronic properties of a hybrid system, a naturally n-doped monolayer WS2 covering a gold disk. We demonstrate that the nonresonant excitation of the gold disk in the high absorption regime efficiently generates hot carriers via localized surface plasmon excitation, which n-dope the monolayer WS2 and enhance the photoluminescence emission by regulating the multiexciton population and stabilizing the neutral exciton emission. The results are relevant to the further development of nanotransistors in photonic circuits and optoelectronic applications.

Room-temperature high-speed electrical modulation of excitonic distribution in a monolayer semiconductor

Excitons in monolayer semiconductors, benefitting from their large binding energies, hold great potential towards excitonic circuits bridging nano-electronics and photonics. However, achieving room-temperature ultrafast on-chip electrical modulation of excitonic distribution and flow in monolayer semiconductors is nontrivial. Here, utilizing lateral bias, we report high-speed electrical modulation of the excitonic distribution in a monolayer semiconductor junction at room temperature. The alternating charge trapping/detrapping at the two monolayer/electrode interfaces induces a non-uniform carrier distribution, leading to controlled in-plane spatial variations of excitonic populations, and mimicking a bias-driven excitonic flow. This modulation increases with the bias amplitude and eventually saturates, relating to the energetic distribution of trap density of states. The switching time of the modulation is down to 5 ns, enabling high-speed excitonic devices. Our findings reveal the trap-assisted exciton engineering in monolayer semiconductors and offer great opportunities for future two-dimensional excitonic devices and circuits.

Charged biexciton polaritons sustaining strong nonlinearity in 2D semiconductor-based nanocavities

Controlling the interaction between light and matter at micro- and nano-scale can provide new opportunities for modern optics and optoelectronics. An archetypical example is polariton, a half-light-half-matter quasi particle inheriting simultaneously the robust coherence of light and the strong interaction of matter, which plays an important role in many exotic phenomena. Here, we open up a new kind of cooperative coupling between plasmon and different excitonic complexes in WS2-silver nanocavities, namely plasmon-exciton-trion-charged biexciton four coupling states. Thanks to the large Bohr radius of up to 5 nm, the charged biexciton polariton exhibits strong saturation nonlinearity, ~30 times higher than the neutral exciton polariton. Transient absorption dynamics further reveal the ultrafast many-body interaction nature, with a timescale of <100 fs. The demonstration of biexciton polariton here combines high nonlinearity, simple processing and strong scalability, permitting access for future energy-efficient optical switching and information processing.

Strong coupling in plasmonic metal nanoparticles

The study of strong coupling between light and matter has gained significant attention in recent years due to its potential applications in diverse fields, including artificial light harvesting, ultraefficient polariton lasing, and quantum information processing. Plasmonic cavities are a compelling alternative of conventional photonic resonators, enabling ultracompact polaritonic systems to operate at room temperature. This review focuses on colloidal metal nanoparticles, highlighting their advantages as plasmonic cavities in terms of their facile synthesis, tunable plasmonic properties, and easy integration with excitonic materials. We explore recent examples of strong coupling in single nanoparticles, dimers, nanoparticle-on-a-mirror configurations, and other types of nanoparticle-based resonators. These systems are coupled with an array of excitonic materials, including atomic emitters, semiconductor quantum dots, two-dimensional materials, and perovskites. In the concluding section, we offer perspectives on the future of strong coupling research in nanoparticle systems, emphasizing the challenges and potentials that lie ahead. By offering a thorough understanding of the current state of research in this field, we aim to inspire further investigations and advances in the study of strongly coupled nanoparticle systems, ultimately unlocking new avenues in nanophotonic applications.

Multi-mode strong coupling in Fabry-Pérot cavity-WS2 photonic crystal hybrid structures

Optical microcavities embedded with transition metal dichalcogenide (TMDC) membranes have been demonstrated as excellent platforms to explore strong light-matter interactions. Most of the previous studies focus on strong coupling between excitons of unpatterned TMDC membranes and optical resonances of various microcavities. It is recently found that TMDC membranes patterned into photonic crystal (PhC) slabs can sustain guided-mode resonances that can be excited and probed by far-fields. Here, we present a comprehensive theoretical and numerical study on optical responses of Fabry-Pérot (F-P) cavity-WS₂ PhC hybrid structures to investigate the multi-mode coupling effects between excitons, guided-mode resonances and F-P modes. We show that both the exciton resonance and the guide-mode resonance of the WS₂ PhC can strongly interact with F-P modes of the cavity to reach strong coupling regime. Moreover, a Rabi splitting as large as 63 meV is observed for the strong coupling between the guided-mode resonance and the F-P mode, which is much larger than their average dissipation rate. We further demonstrate that it is even possible to realize a triple mode strong coupling by tuning the guide-mode resonances spectrally overlapped with the exciton resonance and the F-P modes. The hybrid polariton states generated from the triple mode coupling exhibit a Rabi splitting of 120 meV that greatly exceeds the criterion of a triple mode strong coupling (∼29.3 meV). Our results provide that optical microcavities embedded with TMDC PhCs can serve as promising candidates for polariton devices based on multi-mode strong coupling.

Plexcitonics: plasmon-exciton coupling for enhancing spectroscopy, optical chirality, and nonlinearity

Plexcitonics is a rapidly developing interdisciplinary field that holds immense potential for the creation of innovative optical technologies and devices. This field focuses on investigating the interactions between plasmons and excitons in hybrid systems. In this review, we provide an overview of the fundamental principles of plasmonics and plexcitonics and discuss the latest advancements in plexcitonics. Specifically, we highlight the ability to manipulate plasmon-exciton interactions, the emerging field of tip-enhanced spectroscopy, and advancements in optical chirality and nonlinearity. These recent developments have spurred further research in the field of plexcitonics and offer inspiration for the design of advanced materials and devices with enhanced optical properties and functionalities.

Routing the Exciton Emissions of WS2 Monolayer with the High-Order Plasmon Modes of Ag Nanorods

Locally routing the exciton emissions in two-dimensional (2D) transition-metal dichalcogenides along different directions at the nanophotonic interface is of great interest in exploiting the promising 2D excitonic systems for functional nano-optical components. However, such control has remained elusive. Herein we report on a facile plasmonic approach for electrically controlled spatial modulation of the exciton emissions in a WS2 monolayer. The emission routing is enabled by the resonance coupling between the WS2 excitons and the multipole plasmon modes in individual silver nanorods placed on a WS2 monolayer. Different from prior demonstrations, the routing effect can be modulated by the doping level of the WS2 monolayer, enabling electrical control. Our work takes advantage of the high-quality plasmon modes supported by simple rod-shaped metal nanocrystals for the angularly resolved manipulation of 2D exciton emissions. Active control is achieved, which offers great opportunities for the development of nanoscale light sources and nanophotonic devices.

Enhanced Exciton-to-Trion Conversion by Proton Irradiation of Atomically Thin WS2

Defect engineering of van der Waals semiconductors has been demonstrated as an effective approach to manipulate the structural and functional characteristics toward dynamic device controls, yet correlations between physical properties with defect evolution remain underexplored. Using proton irradiation, we observe an enhanced exciton-to-trion conversion of the atomically thin WS₂. The altered excitonic states are closely correlated with nanopore induced atomic displacement, W nanoclusters, and zigzag edge terminations, verified by scanning transmission electron microscopy, photoluminescence, and Raman spectroscopy. Density functional theory calculation suggests that nanopores facilitate formation of in-gap states that act as sinks for free electrons to couple with excitons. The ion energy loss simulation predicts a dominating electron ionization effect upon proton irradiation, providing further evidence on band perturbations and nanopore formation without destroying the overall crystallinity. This study provides a route in tuning the excitonic properties of van der Waals semiconductors using an irradiation-based defect engineering approach.

Strong coupling of second harmonic generation scattering spectrum in a diexcitionic nanosystem

Diexcitonic strong coupling between quantum emitters and localized surface plasmon has attracted more attention recently because it can provide multiple qubit states for future quantum information technology at room temperature. In a strong coupling regime, nonlinear optical effects can offer new routes for developing quantum devices, but it is rarely reported. In this paper, we established the hybrid system consisting of J-aggregates-WS₂-cuboid Au@Ag nanorods, which can realize diexcitonic strong coupling and second harmonic generation (SHG). We find that multimode strong coupling has been achieved not only in the fundamental frequency scattering spectrum but also in the SHG scattering spectrum. SHG scattering spectrum shows three plexciton branches, similar to the splitting in the fundamental frequency scattering spectrum. Furthermore, the SHG scattering spectrum can be modulated by tuning the armchair direction of the crystal lattice, pump polarization direction, and plasmon resonance frequency, which makes our system very promising in the quantum device at room temperature. Moreover, we develop coupled nonlinear harmonic oscillator model theory to explain the nonlinear diexcitonic strong coupling mechanism. The calculated results by the finite element method accord well with our theory. The nonlinear optical properties of the diexcitonic strong coupling can provide potential applications such as quantum manipulation, entanglement, and integrated logic devices.

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.

Planar Optical Cavities Hybridized with Low-Dimensional Light-Emitting Materials

Low-dimensional light-emitting materials have been actively investigated due to their unprecedented optical and optoelectronic properties that are not observed in their bulk forms. However, the emission from low-dimensional light-emitting materials is generally weak and difficult to use in nanophotonic devices without being amplified and engineered by optical cavities. Along with studies on various planar optical cavities over the last decade, the physics of cavity-emitter interactions as well as various integration methods are investigated deeply. These integrations not only enhance the light-matter interaction of the emitters, but also provide opportunities for realizing nanophotonic devices based on the new physics allowed by low-dimensional emitters. In this review, the fundamentals, strengths and weaknesses of various planar optical resonators are first provided. Then, commonly used low-dimensional light-emitting materials such as 0D emitters (quantum dots and upconversion nanoparticles) and 2D emitters (transition-metal dichalcogenide and hexagonal boron nitride) are discussed. The integration of these emitters and cavities and the expect interplay between them are explained in the following chapters. Finally, a comprehensive discussion and outlook of nanoscale cavity-emitter integrated systems is provided.

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.

Optical Introduction and Manipulation of Plasmon-Exciton-Trion Coupling in a Si/WS2/Au Nanocavity

Strong plasmon-exciton coupling, which has potential applications in nanophotonics, plasmonics, and quantum electrodynamics, has been successfully demonstrated by using metallic nanocavities and two-dimensional materials. Dynamical control of plasmon-exciton coupling strength, especially by using optical methods, remains a big challenge although it is highly desirable. Here, we report the optical introduction and manipulation of plasmon-exciton-trion coupling realized in a dielectric-metal hybrid nanocavity, which is composed of a silicon (Si) nanoparticle and a thin gold (Au) film, with an embedded tungsten disulfide (WS₂) monolayer. We employ scattering and photoluminescence spectra to characterize the coupling strength between plasmons and excitons in Si/WS₂/Au nanocavities constructed by using Si nanoparticles with different diameters. We enhance the plasmon-exciton and plasmon-trion coupling strength by injecting excitons and trions into the WS₂ monolayer with a 488 nm laser beam. It is revealed that the emission intensities of excitons and trions with respect to the reference WS₂ monolayer can be modified through the change in the coupling strength induced by the laser light. Interestingly, the coupling strength between the plasmons and the excitons/trions can be manipulated from weak to strong coupling regime by simply increasing the laser power, which is clearly resolved in the scattering spectra of Si/WS₂/Au nanocavities. More importantly, the plasmon-exciton-trion coupling induced by the laser light is confirmed by the energy exchange between excitons and trions. Our findings indicate the possibility for optically manipulating plasmon-exciton interaction and suggest the practical applications of dielectric-metal hybrid nanocavities in nanoscale plasmonic devices.

Nanocavity-induced trion emission from atomically thin WSe2

Exciton is a bosonic quasiparticle consisting of a pair of electron and hole, with promising potentials for optoelectronic device applications, such as exciton transistors, photodetectors and light emitting devices. However, the charge-neutral nature of excitons renders them challenging to manipulate using electronics. Here we present the generation of trions, a form of charged excitons, together with enhanced exciton resonance in monolayer WSe₂. The excitation of the trion quasiparticles is achieved by the hot carrier transport from the integrated gold plasmonic nanocavity, formed by embedding monolayer WSe₂ between gold nanoparticles and a gold film. The nanocavity-induced negatively charged trions provide a promising route for the manipulation of excitons, essential for the construction of all-exciton information processing circuits.

Transition metal dichalcogenide metaphotonic and self-coupled polaritonic platform grown by chemical vapor deposition

Transition metal dichalcogenides (TMDCs) have recently attracted growing attention in the fields of dielectric nanophotonics because of their high refractive index and excitonic resonances. Despite the recent realizations of Mie resonances by patterning exfoliated TMDC flakes, it is still challenging to achieve large-scale TMDC-based photonic structures with a controllable thickness. Here, we report a bulk MoS₂ metaphotonic platform realized by a chemical vapor deposition (CVD) bottom-up method, supporting both pronounced dielectric optical modes and self-coupled polaritons. Magnetic surface lattice resonances (M-SLRs) and their energy-momentum dispersions are demonstrated in 1D MoS₂ gratings. Anticrossing behaviors with Rabi splitting up to 170 meV are observed when the M-SLRs are hybridized with the excitons in multilayer MoS₂. In addition, distinct Mie modes and anapole-exciton polaritons are also experimentally demonstrated in 2D MoS₂ disk arrays. We believe that the CVD bottom-up method would open up many possibilities to achieve large-scale TMDC-based photonic devices and enrich the toolbox of engineering exciton-photon interactions in TMDCs.

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.

Tunable Strong Coupling in Transition Metal Dichalcogenide Nanowires

Subwavelength optical resonators with spatiotemporal control of light are essential to the miniaturization of optical devices. In this work, chemically synthesized transition metal dichalcogenide (TMDC) nanowires are exploited as a new type of dielectric nanoresonators to simultaneously support pronounced excitonic and Mie resonances. Strong light-matter couplings and tunable exciton polaritons in individual nanowires are demonstrated. In addition, the excitonic responses can be reversibly modulated with excellent reproducibility, offering the potential for developing tunable optical nanodevices. Being in the mobile colloidal state with highly tunable optical properties, the TMDC nanoresonators will find promising applications in integrated active optical devices, including all-optical switches and sensors.

Generation and Detection of Strain-Localized Excitons in WS2 Monolayer by Plasmonic Metal Nanocrystals

Excitons in a transition-metal dichalcogenide (TMDC) monolayer can be modulated through strain with spatial and spectral control, which offers opportunities for constructing quantum emitters for applications in on-chip quantum communication and information processing. Strain-localized excitons in TMDC monolayers have so far mainly been observed under cryogenic conditions because of their subwavelength emission area, low quantum yield, and thermal-fluctuation-induced delocalization. Herein, we demonstrate both generation and detection of strain-localized excitons in WS₂ monolayer through a simple plasmonic structure design, where WS₂ monolayer covers individual Au nanodisks or nanorods. Enhanced emission from the strain-localized excitons of the deformed WS₂ monolayer near the plasmonic hotspots is observed at room temperature with a photoluminescence energy redshift up to 200 meV. The emission intensity and peak energy of the strain-localized excitons can be adjusted by the nanodisk size. Furthermore, the excitation and emission polarization of the strain-localized excitons are modulated by anisotropic Au nanorods. Our results provide a promising strategy for constructing nonclassical integrated light sources, high-sensitivity strain sensors, or tunable nanolasers for future dense nanophotonic integrated circuits.

Room-Temperature Strong Coupling Between a Single Quantum Dot and a Single Plasmonic Nanoparticle

A single quantum dot (QD) strongly coupled with a plasmonic nanoparticle yields a promising qubit for scalable solid-state quantum information processing at room temperature. However, realizing such a strong coupling remains challenging due to the difficulty of spatial overlap of the QD excitons with the plasmonic electric fields (EFs). Here, by using a transmission electron microscope we demonstrate for the first time that this overlap can be realized by integrating a deterministic single QD with a single Au nanorod. When a wedge nanogap cavity consisting of them and the substrate is constructed, the plasmonic EFs can be more effectively "dragged" and highly confined in the QD's nanoshell where the excitons mainly reside. With these advantages, we observed the largest spectral Rabi splitting (reported so far) of ∼234 meV for a single QD strong coupling with plasmons. Our work opens a pathway to the massive construction of room-temperature strong coupling solid qubits.

Giant excitonic upconverted emission from two-dimensional semiconductor in doubly resonant plasmonic nanocavity

Phonon-assisted upconverted emission is the heart of energy harvesting, bioimaging, optical cryptography, and optical refrigeration. It has been demonstrated that emerging two-dimensional (2D) semiconductors can provide an excellent platform for efficient phonon-assisted upconversion due to the enhanced optical transition strength and phonon-exciton interaction of 2D excitons. However, there is little research on the further enhancement of excitonic upconverted emission in 2D semiconductors. Here, we report the enhanced multiphoton upconverted emission of 2D excitons in doubly resonant plasmonic nanocavities. Owing to the enhanced light collection, enhanced excitation rate, and quantum efficiency enhancement arising from the Purcell effect, an upconverted emission amplification of >1000-fold and a decrease of 2~3 orders of magnitude in the saturated excitation power are achieved. These findings pave the way for the development of excitonic upconversion lasing, nanoscopic thermometry, and sensing, revealing the possibility of optical refrigeration in future 2D electronic or excitonic devices.

Theory of Excitons in Atomically Thin Semiconductors: Tight-Binding Approach

Atomically thin semiconductors from the transition metal dichalcogenide family are materials in which the optical response is dominated by strongly bound excitonic complexes. Here, we present a theory of excitons in two-dimensional semiconductors using a tight-binding model of the electronic structure. In the first part, we review extensive literature on 2D van der Waals materials, with particular focus on their optical response from both experimental and theoretical points of view. In the second part, we discuss our ab initio calculations of the electronic structure of MoS2, representative of a wide class of materials, and review our minimal tight-binding model, which reproduces low-energy physics around the Fermi level and, at the same time, allows for the understanding of their electronic structure. Next, we describe how electron-hole pair excitations from the mean-field-level ground state are constructed. The electron-electron interactions mix the electron-hole pair excitations, resulting in excitonic wave functions and energies obtained by solving the Bethe-Salpeter equation. This is enabled by the efficient computation of the Coulomb matrix elements optimized for two-dimensional crystals. Next, we discuss non-local screening in various geometries usually used in experiments. We conclude with a discussion of the fine structure and excited excitonic spectra. In particular, we discuss the effect of band nesting on the exciton fine structure; Coulomb interactions; and the topology of the wave functions, screening and dielectric environment. Finally, we follow by adding another layer and discuss excitons in heterostructures built from two-dimensional semiconductors.

Strong Coupling of Carbon Quantum Dots in Liquid Crystals

Carbon quantum dots (CDs) have recently received a tremendous amount of interest owing to their attractive optical properties. However, CDs have broad absorption and emission spectra limiting their application ranges. We herein, for the first time, show synthesis of water-soluble red emissive CDs with a very narrow line width (∼75 meV) spectral absorbance and hence demonstrate strong coupling of CDs and plasmon polaritons in liquid crystalline mesophases. The excited state dynamics of CDs has been studied by ultrafast transient absorption spectroscopy, and CDs display very stable and strong photoluminescence emission with a quantum yield of 35.4% and a lifetime of ∼2 ns. More importantly, we compare J-aggregate dyes with CDs in terms of their absorption line width, photostability, and ability to do strong coupling, and we conclude that highly fluorescent CDs have a bright future in the mixed light-matter states for emerging applications in future quantum technologies.

Unified Scattering and Photoluminescence Spectra for Strong Plasmon-Exciton Coupling

The strong coupling between excitons and single plasmonic nanocavities enables plexcitonic states in nanoscale systems at room temperature. Here we demonstrate the strong coupling of surface plasmon modes of metal nanowires and excitons in monolayer semiconductors, with Rabi splitting manifested in both scattering and photoluminescence (PL) spectra. By utilizing the propagation properties of surface plasmons on the nanowires, the PL emitted through the scattering of plasmon-exciton hybrid modes is extracted. The analytically calculated scattering and PL spectra well reproduce the experimental results. These findings unify the scattering and PL spectra in the plexcitonic system and eliminate the ambiguities of PL emission, shedding new light on understanding the rich spectral phenomena in the plasmon-exciton strong coupling regime.

Principle and Applications of Multimode Strong Coupling Based on Surface Plasmons

In the past decade, strong coupling between light and matter has transitioned from a theoretical idea to an experimental reality. This represents a new field of quantum light-matter interaction, which makes the coupling strength comparable to the transition frequencies in the system. In addition, the achievement of multimode strong coupling has led to such applications as quantum information processing, lasers, and quantum sensors. This paper introduces the theoretical principle of multimode strong coupling based on surface plasmons and reviews the research related to the multimode interactions between light and matter. Perspectives on the future development of plasmonic multimode coupling are also discussed.

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.

Strong Dipole-Quadrupole-Exciton Coupling Realized in a Gold Nanorod Dimer Placed on a Two-Dimensional Material

Simple systems in which strong coupling of different excitations can be easily realized are highly important, not only for fundamental research but also for practical applications. Here, we proposed a T-shaped gold nanorod (GNR) dimer composed of a long GNR and a short GNR perpendicular to each other and revealed that the dark quadrupole mode of the long GNR can be activated by utilizing the dipole mode excited in the short GNR. It was found that the strong coupling between the dipole and quadrupole modes can be achieved by exciting the T-shaped GNR dimer with a plane wave. Then, we demonstrated the realization of strong dipole-quadrupole-exciton coupling by placing a T-shaped GNR on a tungsten disulfide (WS2) monolayer, which leads to a Rabi splitting as large as ~299 meV. It was confirmed that the simulation results can be well fitted by using a Hamiltonian based on the coupled harmonic oscillator model and the coupling strengths for dipole-quadrupole, dipole-exciton and quadrupole-exciton can be extracted from the fitting results. Our findings open new horizons for realizing strong plasmon-exciton coupling in simple systems and pave the way for constructing novel plasmonic devices for practical applications.

Control of light-valley interactions in 2D transition metal dichalcogenides with nanophotonic structures

Electronic valley in two-dimensional transition-metal dichalcogenides (2D TMDCs) offers a new degree of freedom for information storage and processing. The valley pseudospin can be optically encoded by photons with specific helicity, enabling the construction of electronic information devices with both high performance and low power consumption. Robust detection, manipulation and transport of the valley pseudospins at room temperature are still challenging because of the short lifetime of valley-polarized carriers and excitons. Integrating 2D TMDCs with nanophotonic objects such as plasmonic nanostructures provides a competitive solution to address the challenge. The research in this field is of practical interest and can also present rich physics of light-matter interactions. In this minireview, recent progress on using nanophotonic strategies to enhance the valley polarization degree, especially at room temperature, is highlighted. Open questions, major challenges, and interesting future developments in manipulating the valley information in 2D semiconductors with the help of nanophotonic structures will also be discussed.

Exciton-Trion Polaritons in Doped Two-Dimensional Semiconductors

We present a many-body theory of exciton-trion polaritons (ETPs) in doped two-dimensional semiconductor materials. ETPs are robust coherent hybrid excitations involving excitons, trions, and photons. In ETPs, the 2-body exciton states are coupled to the material ground state via exciton-photon interaction, and the 4-body trion states are coupled to the exciton states via Coulomb interaction. The trion states are not directly optically coupled to the material ground state. The energy-momentum dispersion of ETPs exhibit three bands. We calculate the energy band dispersions and the compositions of ETPs at different doping densities using Green's functions. The energy splittings between the polariton bands, as well as the spectral weights of the polariton bands, depend on the strength of the Coulomb coupling between the excitons and the trions, which in turn depends sensitively on the doping density. The doping density dependence of the ETP bands and the charged nature of the trion states could enable novel electrical and optical control of ETPs.

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.

Plexcitonic Optical Chirality: Strong Exciton-Plasmon Coupling in Chiral J-Aggregate-Metal Nanoparticle Complexes

Understanding the unique characteristics of plexcitons, hybridized states resulting from the strong coupling between plasmons and excitons, is vital for both fundamental studies and practical applications in nano-optics. However, the research of plexcitons from the perspective of chiral optics has been rarely reported. Here, we experimentally investigate the optical chirality of plexcitonic systems consisting of composite metal nanoparticles and chiral J-aggregates in the strong coupling regime. Mode splitting and anticrossing behavior are observed in both the circular dichroism (CD) and extinction spectra of the hybrid nanosystems. A large mode splitting (at zero detuning) of up to 136 meV/214 meV in CD/extinction measurements confirms that the systems attain the strong coupling regime. This phenomenon indicates that the formation of plexcitons modifies not only the extinction but also the optical chirality of the hybrid systems. We develop a quasistatic theory to elucidate the chiral optical responses of hybrid systems. Furthermore, we propose and justify a criterion of strong plasmon-exciton interaction: the mode splitting in the CD spectra (at zero detuning) is larger than half of that in the extinction spectra. Our findings give a chiral perspective on the study of strong plasmon-exciton coupling and have potential applications in the chiral optical field.

Nonequilibrium Lattice Dynamics in Monolayer MoS2

The coupled nonequilibrium dynamics of electrons and phonons in monolayer MoS₂ is investigated by combining first-principles calculations of the electron-phonon and phonon-phonon interactions with the time-dependent Boltzmann equation. Strict phase-space constraints in the electron-phonon scattering are found to influence profoundly the decay path of excited electrons and holes, restricting the emission of phonons to crystal momenta close to a few high-symmetry points in the Brillouin zone. As a result of momentum selectivity in the phonon emission, the nonequilibrium lattice dynamics is characterized by the emergence of a highly anisotropic population of phonons in reciprocal space, which persists for up to 10 ps until thermal equilibrium is restored by phonon-phonon scattering. Achieving control of the nonequilibrium dynamics of the lattice may provide unexplored opportunities to selectively enhance the phonon population of two-dimensional crystals and, thereby, transiently tailor electron-phonon interactions over subpicosecond time scales.

Enhancing plasmonic hot-carrier generation by strong coupling of multiple resonant modes

Plasmon-induced hot carriers have recently attracted considerable interest, but the energy efficiency in visible light is often low due to the short lifetime of hot carriers and the limited optical absorption of plasmonic architectures. To increase the generation of hot carriers, we propose to exert multiple plasmonic resonant modes and their strong coupling using a metal-dielectric-metal (MDM) nanocavity that comprises an Au nanohole array (AuNHA), a TiO₂ thin film and an Au reflector. Unlike common MDM structures, in addition to the Fabry-Pérot mode in the dielectric layer, AuNHA as the top layer is special because it excites the localized surface plasmon resonance (LSPR) mode in the Au nanoholes and launches the gap surface plasmon polariton (GSPP) mode in the Au reflector surface. The spatial field overlapping of the three resonance modes enables strong mode coupling by optimizing the TiO₂ thickness, which leads to notably enhanced average IPCE (∼1.5%) and broadband photocurrent (170 μA·cm^-2). This MDM structure would be useful for photochemistry and photovoltaics using sunlight.

Fine-tuning of polariton energies in a tailored plasmon cavity and J-aggregates hybrid system

Strong coupling systems enable coherent energy exchange between a light field and material electrons in nanoscale space. Active manipulation of this phenomenon by external stimuli is crucial for the design of advanced optoelectronic devices. Two neglected points severely hinder the improvement of tuning accuracy: irreversible variation in cavity morphology and lack of control over the dielectric environment which may change during the coupling process. Here we present a chemical fine-tuning of the strong plasmon-exciton coupling process in tailored Au@Ag nanocavities. The silver shell thickness was carefully controlled to tune the plasmon resonance wavelength with an accuracy of ∼8 nm and facilitate hot spots at the edges to boost the plasmon-exciton coupling strength. Hybrid polariton states were further regulated across the zero-detuning point with a spectral accuracy of less than 1 nm via tuning the solvent refractive index, and a Rabi splitting as large as 194 meV was observed at room temperature. The fine-tuning of strong plasmon-exciton coupling by an adjacent dielectric environment provides a novel route to manipulate excitons in molecules and possesses great potential for chemical or biological sensing.