Plasmon-trion and plasmon-exciton resonance energy transfer from a single plasmonic nanoparticle to monolayer MoS2

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

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

Evaluating the plasmon-exciton interaction in ZnO tetrapods coupled with gold nanostructures by nanoscale cathodoluminescence

Plasmon-exciton coupling is gaining increasing interest for enhancing the performance of optoelectronic, photonic and photo-catalytic devices. Herein we evaluate the interaction of excitons in zinc oxide tetrapods with surface plasmons of gold nanostructures with different morphologies. The gold nanostructures are grown in situ on ZnO tetrapods by means of a photochemical process, resulting in clean interfaces. The modification of the synthesis parameters results in different morphologies, as isolated nanoparticles, nano-domes or nanoparticles aggregates. Plasmon-exciton interaction is evaluated by means of cathodoluminescence spectroscopy and mapping at the nanoscale. The ZnO excitonic emission is strongly blue-shifted and broadened in close proximity of the gold nanostructures. This effect is explained by the formation of a Schottky barrier that is strongly mediated by the morphology of metal nanostructures.

Suppressing material loss in the visible and near-infrared range for functional nanophotonics using bandgap engineering

All-dielectric nanostructures have recently opened exciting opportunities for functional nanophotonics, owing to their strong optical resonances along with low material loss in the near-infrared range. Pushing these concepts to the visible range is hindered by their larger absorption coefficient, thus encouraging the search for alternative dielectrics for nanophotonics. Here, we employ bandgap engineering to synthesize hydrogenated amorphous Si nanoparticles (a-Si:H NPs) offering ideal features for functional nanophotonics. We observe significant material loss suppression in a-Si:H NPs in the visible range caused by hydrogenation-induced bandgap renormalization, producing strong higher-order resonant modes in single NPs with Q factors up to ~100 in the visible and near-IR range. We also realize highly tunable all-dielectric meta-atoms by coupling a-Si:H NPs to photochromic spiropyran molecules. ~70% reversible all-optical tuning of light scattering at the higher-order resonant mode under a low incident light intensity is demonstrated. Our results promote the development of high-efficiency visible nanophotonic devices.

Large absorption of high-index semiconductors has hindered the application of all dielectric nanostructures in the visible range. Here, the authors present bandgap-engineered hydrogenated amorphous Si nanoparticles with Q-factors up to 100 and their integration with photochromic molecules as tunable meta-atoms.

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

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

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

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

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

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

Robust Synthesis of Gold Nanotriangles and their Self-Assembly into Vertical Arrays

We report an efficient, seed-mediated method for the synthesis of gold nanotriangles (NTs) which can be used for controlled self-assembly. The main advantage of the proposed synthetic protocol is that it relies on using stable (over the course of several days) intermediate seeds. This stability translates into increasing time efficiency of the synthesis and makes the protocol experimentally less demanding ('fast addition' not required, tap water can be used in the final steps) as compared to previously reported procedures, without compromising the size and shape monodispersity of the product. We demonstrate high reproducibility of the protocol in the hands of different researchers and in different laboratories. Additionally, this modified seed-mediated method can be used to produce NTs with edge lengths between ca. 45 and 150 nm. Finally, the high 'quality' of NTs allows the preparation of long-range ordered assemblies with vertically oriented building blocks, which makes them promising candidates for future optoelectronic technologies.

Photoluminescence enhancement of monolayer MoS2 using plasmonic gallium nanoparticles

2D monolayer molybdenum disulphide (MoS₂) has been the focus of intense research due to its direct bandgap compared with the indirect bandgap of its bulk counterpart; however its photoluminescence (PL) intensity is limited due to its low absorption efficiency. Herein, we use gallium hemispherical nanoparticles (Ga NPs) deposited by thermal evaporation on top of chemical vapour deposited MoS₂ monolayers in order to enhance its luminescence. The influence of the NP radius and the laser wavelength is reported in PL and Raman experiments. In addition, the physics behind the PL enhancement factor is investigated. The results indicate that the prominent enhancement is caused by the localized surface plasmon resonance of the Ga NPs induced by a charge transfer phenomenon. This work sheds light on the use of alternative metals, besides silver and gold, for the improvement of MoS₂ luminescence.

Valley-Selective Response of Nanostructures Coupled to 2D Transition-Metal Dichalcogenides

Monolayer (1L) transition-metal dichalcogenides (TMDCs) are attractive materials for several optoelectronic applications because of their strong excitonic resonances and valley-selective response. Valley excitons in 1L-TMDCs are formed at opposite points of the Brillouin zone boundary, giving rise to a valley degree of freedom that can be treated as a pseudospin, and may be used as a platform for information transport and processing. However, short valley depolarization times and relatively short exciton lifetimes at room temperature prevent using valley pseudospins in on-chip integrated valley devices. Recently, it was demonstrated how coupling these materials to optical nanoantennas and metasurfaces can overcome this obstacle. Here, we review the state-of-the-art advances in valley-selective directional emission and exciton sorting in 1L-TMDC mediated by nanostructures and nanoantennas. We briefly discuss the optical properties of 1L-TMDCs paying special attention to their photoluminescence/absorption spectra, dynamics of valley depolarization, and the valley Hall effect. Then, we review recent works on nanostructures for valley-selective directional emission from 1L-TMDCs.

Nanophotonics with 2D transition metal dichalcogenides [Invited]

Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) have recently become attractive materials for several optoelectronic applications, such as photodetection, light harvesting, phototransistors, light-emitting diodes, and lasers. Their bandgap lies in the visible and near-IR range, and they possess strong excitonic resonances, high oscillator strengths, and valley-selective response. Coupling these materials to optical nanocavities enhances the quantum yield of exciton emission, enabling advanced quantum optics and nanophotonics devices. Here, we review the state-of-the-art advances of hybrid exciton-polariton structures based on monolayer TMDCs coupled to plasmonic and dielectric nanocavities. We discuss the optical properties of 2D WS₂, WSe₂, MoS₂ and MoSe₂ materials, paying special attention to their energy bands, photoluminescence/absorption spectra, excitonic fine structure, and to the dynamics of exciton formation and valley depolarization. We also discuss light-matter interactions in such hybrid exciton-polariton structures. Finally, we focus on weak and strong coupling regimes in monolayer TMDCs-based exciton-polariton systems, envisioning research directions and future opportunities for this material platform.

Tunable Resonance Coupling in Single Si Nanoparticle-Monolayer WS2 Structures

Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) are extremely attractive materials for optoelectronic applications in the visible and near-infrared range. Coupling these materials to optical nanocavities enables advanced quantum optics and nanophotonic devices. Here, we address the issue of resonance coupling in hybrid exciton-polariton structures based on single Si nanoparticles (NPs) coupled to monolayer (1L)-WS₂. We predict a strong coupling regime with a Rabi splitting energy exceeding 110 meV for a Si NP covered by 1L-WS₂ at the magnetic optical Mie resonance because of the symmetry of the mode. Further, we achieve a large enhancement in the Rabi splitting energy up to 208 meV by changing the surrounding dielectric material from air to water. The prediction is based on the experimental estimation of TMDC dipole moment variation obtained from the measured photoluminescence spectra of 1L-WS₂ in different solvents. An ability of such a system to tune the resonance coupling is realized experimentally for optically resonant spherical Si NPs placed on 1L-WS₂. The Rabi splitting energy obtained for this scenario increases from 49.6 to 86.6 meV after replacing air by water. Our findings pave the way to develop high-efficiency optoelectronic, nanophotonic, and quantum optical devices.

Bright monolayer tungsten disulfide via exciton and trion chemical modulations

Atomically thin transition metal dichalcogenides (TMDCs) with exceptional electrical and optical properties have drawn tremendous attention for use in novel optoelectronic applications as photodetectors, transistors, light emitters, etc. However, electron bound trions formed through the combination of neutral excitons and electrons significantly decrease the photoluminescence (PL) efficiency of TMDCs. In this study, we report a simple yet efficient chemical doping strategy to modulate the optical properties of monolayer tungsten disulfide (WS2). As a demonstrative example, a chemically doped monolayer of WS2 exhibits remarkable PL enhancement of about one order of magnitude higher than that of pristine WS2. This outstanding PL enhancement is attributed to the fact that excess electrons, which promote the formation of electron-bound trions, are reduced in number through charge transfer from WS2 to the chemical dopant. Furthermore, an improved degree of circular polarization from ∼9.0% to ∼41.5% was also observed in the chemically doped WS2 monolayer. This work describes a feasible strategy to manipulate the optical properties of TMDCs via exciton modulation, making TMDCs promising candidates for versatile semiconductor-based photonic devices.

Greatly enhanced light emission of MoS2 using photonic crystal heterojunction

We present theoretical study on developing a one-dimensional (1D) photonic crystal heterojunction (h-PhC) that consists of a monolayer molybdenum disulfide (MoS2) structure. By employing the transfer matrix method, we obtained the analytical solution of the light absorption and emission of two-dimensional materials in 1D h-PhC. Simultaneously enhancing the light absorption and emission of the medium in multiple frequency ranges is easy as h-PhC has more modes of photon localization than the common photonic crystal. Our numerical results demonstrate that the proposed 1D h-PhC can simultaneously enhance the light absorption and emission of MoS2 and enhance the photoluminescence spectrum of MoS2 by 2-3 orders of magnitude.