Plasmonic Nanocavity Induced Coupling and Boost of Dark Excitons in Monolayer WSe2 at Room Temperature

Exciton Emission Enhancement in Two-Dimensional Monolayer Tungsten Disulfide on a Silicon Substrate via a Fabry-Pérot Microcavity

Exciton emitters in two-dimensional monolayer transition-metal dichalcogenides (TMDs) provide a boulevard for the emerging optoelectronic field, ranging from miniaturized light-emitting diodes to quantum emitters and optical communications. However, the low quantum efficiency from limited light-matter interactions and harmful substrate effects seriously hinders their applications. In this work, we achieve a ∼438-fold exciton photoluminescence enhancement by constructing a Fabry-Pérot cavity consisting of monolayer WS2 and a micron-scale hole on the SiO2/Si substrate. The overall enhancement results from the increased exciton-photon interaction due to the effective exciton-cavity mode coupling and decreased trion formation from the weakened substrate effect confirmed by transient spectroscopy. Moreover, the effective coupling improves the directivity of excitons' spontaneous radiation (fwhm ∼ 5°). This research reveals a practical platform for simultaneously enhancing exciton emission and attenuating the substrate effect, and it provides a blueprint for the development of two-dimensional monolayer TMDs-based emitters in integrated optoelectronic devices.

Plexciton Photoluminescence in Strongly Coupled 2D Semiconductor-Plasmonic Nanocavity Hybrid

Strong plasmon-exciton interaction between two-dimensional transition metal dichalcogenides and a plasmonic nanocavity under ambient conditions has been reported extensively. But the suspicion on whether it has reached a true "strong coupling" is always there because the commonly used dark-field scattering spectroscopy shows a larger spectral splitting and the splitting in the photoluminescence spectra is absent. Here, by using a nanobipyramid-over-mirror to enhance the in-plane vacuum field, we achieve spectral Rabi splitting in both scattering and differential reflection spectra and observe a clear photoluminescence emission of the lower plexciton branch. The established nanocavity offers two polarization-dependent gap plasmon resonances to provide excitation and quantum yield enhancement simultaneously, yielding a total photoluminescence enhancement of 2.1 × 104 times. This allows the acquisition of emission spectra from an individual coupled system regardless of the presence of an uncoupled emitting background in the collection area. The sharp tips of the nanobipyramid lead to a large single-exciton coupling strength up to a few meV. Correlated scattering, differential reflection, and photoluminescence spectra reveal the similarity between the scattering and normalized photoluminescence spectra. These correlative measurements on a single coupled system clear up the suspicions of strong plasmon-exciton interactions and will promote the development of light-emitting plexcitonic devices at room temperature.

Room-Temperature, Strong Emission of Momentum-Forbidden Interlayer Excitons in Nanocavity-Coupled Twisted van der Waals Heterostructures

The emission efficiency of interlayer excitons (IEs) in twisted 2D heterostructures has long suffered from momentum mismatch, limiting their applications in ultracompact excitonic devices. Here, we report strong room-temperature emission of the momentum-forbidden IEs in a 30°-twisted MoS2/WS2 heterobilayer. Utilizing the Purcell effect of a compact plasmonic nanocavity boosts the IE emission intensity in the cavity by over 2 orders of magnitude. We further study the interplay of this Purcell enhancement and phonon assistance in 30°- and 0°-twisted heterostructures. Temperature-dependent and time-resolved spectroscopic measurements reveal that the IE enhancement in the 30°-twisted case involves competition between IE and intralayer-exciton emissions, which is remarkably distinct from the 0°-twisted case. We propose an exciton decay model capturing the features of phonon-assisted momentum compensation and Purcell enhancement in the IE emission, showing consistency with the experimental measurements. Our results enrich the understanding of the nanocavity-assisted light-matter interaction for momentum-indirect excitonic transitions.

Emerging 2D Materials and Van der Waals Heterostructures for Advanced NIR, SWIR, and MWIR Emitters

Infrared (IR) emitters have drawn considerable attention for applications in deep-tissue imaging, optical communication, and thermal sensing. While III-V and II-VI semiconductors are traditionally used in these emitters, their reliance on complex epitaxial growth to overcome lattice mismatch and thermal expansion challenges leads to intricate device structures and limits their integrability. In contrast, 2D materials provide a more flexible solution, offering diverse optical bandgaps and the ability to be vertically restacked in arbitrary crystal orientations to form complex van der Waals (vdW) heterostructures, which can be further integrated onto diverse device platforms. This review highlights recent advancements in 2D-based IR emitters, focusing on the NIR, SWIR, and MWIR regions. It discusses the photoluminescence properties of 2D materials and innovative vdW engineering techniques used to develop IR light-emitting diodes (LEDs). The review also explores how external stimuli, such as electric fields and strain, can enable tunable emission wavelengths and examines the integration of 2D-based emitters with photonic structures, like cavities and waveguides, to create hybrid photonic devices. Finally, the review addresses the challenges and prospects of 2D-based IR technologies, highlighting their potential to transform IR light sources across various applications.

Boosting Light-Matter Interactions in Plasmonic Nanogaps

Plasmonic nanogaps in strongly coupled metal nanostructures can confine light to nanoscale regions, leading to huge electric field enhancement. This unique capability makes plasmonic nanogaps powerful platforms for boosting light-matter interactions, thereby enabling the rapid development of novel phenomena and applications. This review traces the progress of nanogap systems characterized by well-defined morphologies, controllable optical responses, and a focus on achieving extreme performance. The properties of plasmonic gap modes in far-field resonance and near-field enhancement are explored and a detailed comparative analysis of nanogap fabrication techniques down to sub-nanometer scales is provided, including bottom-up, top-down, and their combined approaches. Additionally, recent advancements and applications across various frontier research areas are highlighted, including surface-enhanced spectroscopy, plasmon-exciton strong coupling, nonlinear optics, optoelectronic devices, and other applications beyond photonics. Finally, the challenges and promising emerging directions in the field are discussed, such as light-driven atomic effects, molecular optomechanics, and alternative new materials.

Sub-100 fs Formation of Dark Excitons in Monolayer WS2

Two-dimensional semiconducting transition metal dichalcogenides are promising materials for optoelectronic applications due to their strongly bound excitons. While bright excitons have been thoroughly scrutinized, dark excitons have been much less investigated, as they are not directly observable with far-field spectroscopy. However, with their nonzero momenta, dark excitons are significant for applications requiring long-range transport or coupling to external fields. We access such dark excitons in WS2 monolayer using transient photoemission electron microscopy with subdiffraction limited spatial resolution (75 nm) and exceptionally high temporal resolution (13 fs). Image time series of the monolayer are recorded at several different fluences. We directly observe the ultrafast formation of dark K-Λ excitons occurring within 14-50 fs and follow their subsequent picosecond decay. We distinguish exciton dynamics between the monolayer's interior and edges and conclude that the picosecond-scale evolution of dark excitations is defect-mediated while intervalley scattering is not affected by the defects.

Achieving Ideal Magnetic Light Emission with Electric-Type Emitters

Optical magnetic dipole (MD) emission predominantly relies on emitters with significant MD transitions, which, however, rarely exist in nature. Here, we propose a strategy to transform electric dipole (ED) emission to a magnetic one by elegantly coupling an ED emitter to a silicon nanoparticle exhibiting a strong MD resonance. This emission mode transformation enables an artificially ideal magnetic dipole source with an MD purity factor of up to 99%. The far-field emission patterns of such artificial MD sources were experimentally measured, which unambiguously resolved their magnetic-type emission origin. This study opens the path to achieving ideal magnetic dipole emission with nonmagnetic emitters, largely extending the availability of magnetic light emitters conventionally limited by nature. Beyond the fundamental significance in science, we anticipate that this study will also facilitate the development of magnetic optical nanosource and enable potential photonic applications relying on magnetic light emission.

Photoluminescence modal splitting via strong coupling in hybrid Au/WS2/GaP nanoparticle-on-mirror cavities

By integrating dielectric and metallic components, hybrid nanophotonic devices present promising opportunities for manipulating nanoscale light-matter interactions. Here, we investigate hybrid nanoparticle-on-mirror optical cavities, where semiconductor WS2 monolayers are positioned between gallium phosphide (GaP) nanoantennas and a gold mirror, thereby establishing extreme confinement of optical fields. Prior to integration of the mirror, we observe an intermediate coupling regime from GaP nanoantennas covered with WS2 monolayers. Upon introduction of the mirror, enhanced interactions lead to modal splitting in the exciton photoluminescence spectra, spatially localized within the dielectric-metallic gap. Using a coupled harmonic oscillator model, we extract an average Rabi splitting energy of 22.6 meV at room temperature, at the onset of the strong coupling regime. Moreover, the characteristics of polaritonic emission are revealed by the increasing Lorentzian linewidth and energy blueshift with increasing excitation power. Our findings highlight hybrid nanophotonic structures as novel platforms for controlling light-matter coupling with atomically thin materials.

Deterministic Areal Enhancement of Interlayer Exciton Emission by a Plasmonic Lattice on Mirror

The emergence of interlayer excitons (IX) in atomically thin heterostructures of transition metal dichalcogenides (TMDCs) has drawn great attention due to their unique and exotic optical and optoelectronic properties. Because of the spatially indirect nature of IX, its oscillator strength is 2 orders of magnitude smaller than that of the intralayer excitons, resulting in a relatively low photoluminescence (PL) efficiency. Here, we achieve the PL enhancement of IX by more than 2 orders of magnitude across the entire heterostructure area with a plasmonic lattice on mirror (PLoM) structure. The significant PL enhancement mainly arises from resonant coupling between the amplified electric field strength within the PLoM gap and the out-of-plane dipole moment of IX excitons, increasing the emission efficiency by a factor of around 47.5 through the Purcell effect. This mechanism is further verified by detuning the PLoM resonance frequency with respect to the IX emission energy, which is consistent with our theoretical model. Moreover, our simulation results reveal that the PLoM structure greatly alters the far-field radiation of the IX excitons preferentially to the surface normal direction, which increases the collection efficiency by a factor of around 10. Our work provides a reliable and universal method to enhance and manipulate the emission properties of the out-of-plane excitons in a deterministic way and holds great promise for boosting the development of photoelectronic devices based on the IX excitons.

Strong Linearly Polarized Light Emission by Coupling Out-of-Plane Exciton to Anisotropic Gap Plasmon Nanocavity

With exceptional quantum confinement, 2D monolayer semiconductors support a strong excitonic effect, making them an ideal platform for exploring light-matter interactions and as building blocks for novel optoelectronic devices. Different from the well-known in-plane excitons in transition metal dichalcogenides (TMD), the out-of-plane excitons in indium selenide (InSe) usually show weak emission, which limits their applications as light sources. Here, by embedding InSe in an anisotropic gap plasmon nanocavity, we have realized plasmon-enhanced linearly polarized photoluminescence with an anisotropic ratio up to ∼140, corresponding to degree of polarization (DoP) of ∼98.6%. Such polarization selectivity, originating from the polarization-dependent plasmonic enhancement supported by the "nanowire-on-mirror" nanocavity, can be well tuned by the InSe thickness. Moreover, we have also realized an InSe-based light-emitting diode with polarized electroluminescence. Our research highlights the role of excitonic dipole orientation in designing nanophotonic devices and paves the way for developing InSe-based optoelectronic devices with polarization control.

Construction of nanoparticle-on-mirror nanocavities and their applications in plasmon-enhanced spectroscopy

Plasmonic nanocavities exhibit exceptional capabilities in visualizing the internal structure of a single molecule at sub-nanometer resolution. Among these, an easily manufacturable nanoparticle-on-mirror (NPoM) nanocavity is a successful and powerful platform for demonstrating various optical phenomena. Exciting advances in surface-enhanced spectroscopy using NPoM nanocavities have been developed and explored, including enhanced Raman, fluorescence, phosphorescence, upconversion, etc. This perspective emphasizes the construction of NPoM nanocavities and their applications in achieving higher enhancement capabilities or spatial resolution in dark-field scattering spectroscopy and plasmon-enhanced spectroscopy. We describe a systematic framework that elucidates how to meet the requirements for studying light-matter interactions through the creation of well-designed NPoM nanocavities. Additionally, it provides an outlook on the challenges, future development directions, and practical applications in the field of plasmon-enhanced spectroscopy.

Hexagonal Boron Nitride Slab Waveguides for Enhanced Spectroscopy of Encapsulated 2D Materials

The layered insulator hexagonal boron nitride (hBN) is a critical substrate that brings out the exceptional intrinsic properties of two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs). In this work, the authors demonstrate how hBN slabs tuned to the correct thickness act as optical waveguides, enabling direct optical coupling of light emission from encapsulated layers into waveguide modes. Molybdenum selenide (MoSe2 ) and tungsten selenide (WSe2 ) are integrated within hBN-based waveguides and demonstrate direct coupling of photoluminescence emitted by in-plane and out-of-plane transition dipoles (bright and dark excitons) to slab waveguide modes. Fourier plane imaging of waveguided photoluminescence from MoSe2 demonstrates that dry etched hBN edges are an effective out-coupler of waveguided light without the need for oil-immersion optics. Gated photoluminescence of WSe2 demonstrates the ability of hBN waveguides to collect light emitted by out-of-plane dark excitons.Numerical simulations explore the parameters of dipole placement and slab thickness, elucidating the critical design parameters and serving as a guide for novel devices implementing hBN slab waveguides. The results provide a direct route for waveguide-based interrogation of layered materials, as well as a way to integrate layered materials into future photonic devices at arbitrary positions whilst maintaining their intrinsic properties.

Mechanical, electronic, optical, piezoelectric and ferroic properties of strained graphene and other strained monolayers and multilayers: an update

This is an update of a previous review (Naumiset al2017Rep. Prog. Phys.80096501). Experimental and theoretical advances for straining graphene and other metallic, insulating, ferroelectric, ferroelastic, ferromagnetic and multiferroic 2D materials were considered. We surveyed (i) methods to induce valley and sublattice polarisation (P) in graphene, (ii) time-dependent strain and its impact on graphene's electronic properties, (iii) the role of local and global strain on superconductivity and other highly correlated and/or topological phases of graphene, (iv) inducing polarisationPon hexagonal boron nitride monolayers via strain, (v) modifying the optoelectronic properties of transition metal dichalcogenide monolayers through strain, (vi) ferroic 2D materials with intrinsic elastic (σ), electric (P) and magnetic (M) polarisation under strain, as well as incipient 2D multiferroics and (vii) moiré bilayers exhibiting flat electronic bands and exotic quantum phase diagrams, and other bilayer or few-layer systems exhibiting ferroic orders tunable by rotations and shear strain. The update features the experimental realisations of a tunable two-dimensional Quantum Spin Hall effect in germanene, of elemental 2D ferroelectric bismuth, and 2D multiferroic NiI2. The document was structured for a discussion of effects taking place in monolayers first, followed by discussions concerning bilayers and few-layers, and it represents an up-to-date overview of exciting and newest developments on the fast-paced field of 2D materials.

Photoluminescence upconversion in monolayer WSe2 activated by plasmonic cavities through resonant excitation of dark excitons

Anti-Stokes photoluminescence (PL) is light emission at a higher photon energy than the excitation, with applications in optical cooling, bioimaging, lasing, and quantum optics. Here, we show how plasmonic nano-cavities activate anti-Stokes PL in WSe2 monolayers through resonant excitation of a dark exciton at room temperature. The optical near-fields of the plasmonic cavities excite the out-of-plane transition dipole of the dark exciton, leading to light emission from the bright exciton at higher energy. Through statistical measurements on hundreds of plasmonic cavities, we show that coupling to the dark exciton leads to a near hundred-fold enhancement of the upconverted PL intensity. This is further corroborated by experiments in which the laser excitation wavelength is tuned across the dark exciton. We show that a precise nanoparticle geometry is key for a consistent enhancement, with decahedral nanoparticle shapes providing an efficient PL upconversion. Finally, we demonstrate a selective and reversible switching of the upconverted PL via electrochemical gating. Our work introduces the dark exciton as an excitation channel for anti-Stokes PL in WSe2 and paves the way for large-area substrates providing nanoscale optical cooling, anti-Stokes lasing, and radiative engineering of excitons.

Near-Field Coupling with a Nanoimprinted Probe for Dark Exciton Nanoimaging in Monolayer WSe2

Tip-enhanced photoluminescence (TRPL) is a powerful technique for spatially and spectrally probing local optical properties of 2-dimensional (2D) materials that are modulated by the local heterogeneities, revealing inaccessible dark states due to bright state overlap in conventional far-field microscopy at room temperature. While scattering-type near-field probes have shown the potential to selectively enhance and reveal dark exciton emission, their technical complexity and sensitivity can pose challenges under certain experimental conditions. Here, we present a highly reproducible and easy-to-fabricate near-field probe based on nanoimprint lithography and fiber-optic excitation and collection. The novel near-field measurement configuration provides an ∼3 orders of magnitude out-of-plane Purcell enhancement, diffraction-limited excitation spot, and subdiffraction hyperspectral imaging resolution (below 50 nm) of dark exciton emission. The effectiveness of this high spatial X_D mapping technique was then demonstrated through reproducible hyperspectral mapping of oxidized sites and bubble areas.

Giant Out-of-Plane Exciton Emission Enhancement in Two-Dimensional Indium Selenide via a Plasmonic Nanocavity

Out-of-plane (OP) exciton-based emitters in two-dimensional semiconductor materials are attractive candidates for novel photonic applications, such as radially polarized sources, integrated photonic chips, and quantum communications. However, their low quantum efficiency resulting from forbidden transitions limits their practicality. In this work, we achieve a giant enhancement of up to 34000 for OP exciton emission in indium selenide (InSe) via a designed Ag nanocube-over-Au film plasmonic nanocavity. The large photoluminescence enhancement factor (PLEF) is attributed to the induced OP local electric field (E_z) within the nanocavity, which facilitates effective OP exciton-plasmon interaction and subsequent tremendous enhancement. Moreover, the nanoantenna effect resulting from the effective interaction improves the directivity of spontaneous radiation. Our results not only reveal an effective photoluminescence enhancement approach for OP excitons but also present an avenue for designing on-chip photonic devices with an OP dipole orientation.

Structure modulation of two-dimensional transition metal chalcogenides: recent advances in methodology, mechanism and applications

Together with the development of two-dimensional (2D) materials, transition metal dichalcogenides (TMDs) have become one of the most popular series of model materials for fundamental sciences and practical applications. Due to the ever-growing requirements of customization and multi-function, dozens of modulated structures have been introduced in TMDs. In this review, we present a systematic and comprehensive overview of the structure modulation of TMDs, including point, linear and out-of-plane structures, following and updating the conventional classification for silicon and related bulk semiconductors. In particular, we focus on the structural characteristics of modulated TMD structures and analyse the corresponding root causes. We also summarize the recent progress in modulating methods, mechanisms, properties and applications based on modulated TMD structures. Finally, we demonstrate challenges and prospects in the structure modulation of TMDs and forecast potential directions about what and how breakthroughs can be achieved.

Tip-Enhanced Dark Exciton Nanoimaging and Local Strain Control in Monolayer WSe2

Dark excitons in transition-metal dichalcogenides, with their long lifetimes and strong binding energies, provide potential platforms from photonic and optoelectronic applications to quantum information science even at room temperature. However, their spatial heterogeneity and sensitivity to strain is not yet understood. Here, we combine tip-enhanced photoluminescence spectroscopy with atomic force induced strain control to nanoimage dark excitons in WSe₂ and their response to local strain. Dark exciton emission is facilitated by out-of-plane picocavity Purcell enhancement giving rise to spatially highly localized emission, providing for higher spatial resolution compared to bright exciton nanoimaging. Further, tip-antenna-induced dark exciton emission is enhanced in areas of higher strain associated with bubbles. In addition, active force control shows dark exciton emission to be more sensitive to strain with both compressive and tensile lattice deformation facilitating emission. This interplay between localized strain and Purcell effects provides novel pathways for nanomechanical exciton emission control.

Perfect absorption and phase singularities induced by surface lattice resonances for plasmonic nanoparticle array on a metallic film

The formation of pairs of perfect absorption associated with phase singularities in the parameter space using the hybridized structure constructed with a metallic nanoparticle array and a metallic film is promising to enhance light-mater interactions. However, the localized plasmon resonances of the array possess strong radiative losses, which is an obstacle to improve the performances for many applications. On the contrary with the subwavelength array hybridized structure, this study shows that by enlarging the lattice spacing, the oscillator strength of the nanoparticles can be enhanced with the formation of surface lattice resonance, thereby leading to similar but much narrower pairs of perfect absorption due to the interactions with the Fabry-Pérot cavity modes. Furthermore, when the surface plasmon polariton mode shift to the same spectral range associated with the enlarged lattice spacing, the coupling and mode hybridization with the surface lattice resonance result in an anticrossing in the spectra. Although the resonance coupling does not enter the strong coupling regime, the quality factors (∼ 134) and near-field enhancements (∼ 44) are strongly enhanced for the hybridized resonance modes due to the effectively suppressed radiative losses compared with that of the localized plasmon resonances, which make the hybridized structure useful for the design of functional nanophotonic device such as biosensing, multi-model nanolasing, and high-quality imaging.

Nanoparticle-on-mirror pairs: building blocks for remote spectroscopies

Surface-enhanced spectroscopies, such as surface-enhanced Raman scattering (SERS), fluorescence (SEF), circular dichroism, etc., are powerful tools for investigating nano-entities with high sensitivities. Owing to the giant local electric field confined in a plasmonic nanogap, nanogap-enhanced spectroscopies could detect samples with ultralow concentrations, even down to the single-molecule level for SERS and SEF. This great ability to detect analytes with ultralow concentrations provides opportunities for early diagnosis and monitoring in modern biomedicine. However, local laser excitations would inevitably bring about unwanted disruptive background perturbations, local heating, and the consequent geometry reshaping and biological analyte damages. Remote spectroscopies avoiding direct laser exposure to the samples can be treated as remarkable solutions. Here, we combined the nanoparticle-on-mirror (NPoM) family with the philosophy of remote spectroscopy to construct so-called "NPoM pairs" structures. They consist of two identical NPoMs with matched resonances yet separate functions either as receiving or transmitting antennas. A figure of merit for evaluating the remote spectroscopies was put forward, which accounts for the efficiencies in three processes, i.e., receiving, transporting, and transmitting. In addition, we experimentally demonstrated the performances of these NPoM pairs by proof-of-principle applications on the remote SERS and SEF. The optical access of the spectral information in these NPoM pairs both locally and remotely manifests themselves as fundamental building blocks for remote spectroscopies.

Nanoscale Valley Modulation by Surface Plasmon Interference

Excitons in two-dimensional (2D) materials have attracted the attention of the community to develop improved photoelectronic devices. Previous reports are based on direct excitation where the out-of-plane illumination projects a uniform single-mode light spot. However, because of the optical diffraction limit, the minimal spot size is a few micrometers, inhibiting the precise manipulation and control of excitons at the nanoscale level. Herein, we introduced the in-plane coherent surface plasmonic interference (SPI) field to excite and modulate excitons remotely. Compared to the out-of-plane light, a uniform in-plane SPI suggests a more compact spatial volume and an abundance of mode selections for a single or an array of device modulation. Our results not only build up a fundamental platform for operating and encoding the exciton states at the nanoscale level but also provide a new avenue toward all-optical integrated valleytronic chips for future quantum computation and information applications.

Manipulating the light-matter interactions in plasmonic nanocavities at 1 nm spatial resolution

The light-matter interaction between plasmonic nanocavity and exciton at the sub-diffraction limit is a central research field in nanophotonics. Here, we demonstrated the vertical distribution of the light-matter interactions at ~1 nm spatial resolution by coupling A excitons of MoS₂ and gap-mode plasmonic nanocavities. Moreover, we observed the significant photoluminescence (PL) enhancement factor reaching up to 2800 times, which is attributed to the Purcell effect and large local density of states in gap-mode plasmonic nanocavities. Meanwhile, the theoretical calculations are well reproduced and support the experimental results.