How to Obtain the Correct Rabi Splitting in a Subwavelength Interacting System

Quantum Dynamics of Plasmonic Coupling in Silver Nanoparticle Dimers: Enhanced Energy and Population Transfer via Emitter Interaction

Plasmonic nanoparticles (NPs), characterized by significant localized surface plasmon excitations, can generate exceptionally large electromagnetic fields. In the plasmonic cavity, the enhancement of population and energy transfer across closely spaced metallic NPs significantly influence the optical response of the emitter. The theoretical investigation of transport properties in plasmonic nanocavities in atomic-scale level of calculation is important to characterize the optical response of the system. We model the coupling of plasmonic excitations of silver NPs in a bowtie configuration and generate new bright and dark states according to symmetry. By varying the separation distance, the rate of population and energy transfer between two NPs are analyzed within the framework of quantum dynamics multiconfiguration time-dependent Hartree (MCTDH) algorithm. The coupling of the emitter with bright and dark states of the plasmonic cavity is investigated based on the dipole-dipole approximation. The Hermitian Hamiltonian parametrized with first-principles calculations is applied to model the whole system. These results can reveal a connection between atomistic properties and optical response in the subnanometric-scale.

Exciton hybridization in a WS2/MoS2 heterobilayer mediated by a surface wave via strong photon-exciton coupling

The hybridization of multiple excitons in a heterobilayer composed of two transition metal dichalcogenides (TMDCs) based on strong light-matter interaction is interesting from the viewpoint of both fundamental research and practical application. Here, we investigate numerically and experimentally the hybridization of three excitons in a heterobilayer mediated by the surface plasmon polaritons (SPPs) excited on a thin Au film and the transverse-electric (TE) polarized waves excited on a Si3N4/Ag heterostructure via photon-exciton coupling. Relying on numerical simulation, we observe anticrossing behaviors in the angle-resolved reflection spectra calculated for MoS2/WS2/Au and WS2/MoS2/Si3N4/Ag heterostructures, which reveal the coupling between the surface wave (SPPs or TE waves) and the multiple excitons in the heterobilayer. In experiments, we employ the oligomers of polystyrene (PS) nanospheres as scatters to transfer the surface waves into far-field radiations. Similarly, we observe anticrossing behaviors in the angle-resolved scattering spectra measured for the oligomers of PS nanospheres. Relying on the coupled oscillator model, we observe Rabi splitting energies of ΩSPP ∼206.79 meV for the SPPs and ΩTE ∼237.60 meV for the TE waves. Based on the calculated current density distributions and Hopfield coefficients, we demonstrate the hybridization of the three excitons in the WS2/MoS2 heterobilayer mediated by the TE waves. Our findings open new horizons for manipulating light-matter interaction in TMDC heterobilayers and suggest the potential applications of exciton hybridization in energy transfer.

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.

Probing plexciton emission from 2D materials on gold nanotrenches

Probing strongly coupled quasiparticle excitations at their intrinsic length scales offers unique insights into their properties and facilitates the design of devices with novel functionalities. In this work, we investigate the formation and emission characteristics of plexcitons, arising from the interaction between surface plasmons in narrow gold nanotrenches and excitons in monolayer WSe2. We study this strong plasmon-exciton coupling in both the far-field and the near-field. Specifically, we observe a Rabi splitting in the far-field reflection spectra of about 80 meV under ambient conditions, consistent with our theoretical modeling. Using a custom-designed near-field probe, we find that plexciton emission originates predominantly from the lower-frequency branch, which we can directly probe and map its local field distribution. We precisely determine the plexciton's spatial extension, similar to the trench width, with nanometric precision by collecting spectra at controlled probe locations. Our work opens exciting prospects for nanoscale mapping and engineering of plexcitons in complex nanostructures with potential applications in nanophotonic devices, optoelectronics, and quantum electrodynamics in nanoscale cavities.

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.

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

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

Tuning the Plexcitonic Optical Chirality Using Discrete Structurally Chiral Plasmonic Nanoparticles

Constructing chiral plexcitonic systems with tunable plasmon-exciton coupling may advance the scientific exploitation of strong light-matter interactions. Because of their intriguing chiroptical properties, chiral plasmonic materials have shown promising applications in photonics, sensing, and biomedicine. However, the strong coupling of chiral plasmonic nanoparticles with excitons remains largely unexplored. Here we demonstrate the construction of a chiral plasmon-exciton system using chiral AuAg nanorods and J aggregates for tuning the plexcitonic optical chirality. Circular dichroism spectroscopy was employed to characterize chiral plasmon-exciton coupling, in which Rabi splitting and anticrossing behaviors were observed, whereas the extinction spectra exhibited less prominent phenomena. By controlling the number of molecular excitons and the energy detuning between plasmons and excitons, we have been able to fine-tune the plexcitonic optical chirality. The ability to fine-tune the plexcitonic optical chirality opens up unique opportunities for exploring chiral light-matter interactions and boosting the development of emerging chiroptical devices.

Polaritonic linewidth asymmetry in the strong and ultrastrong coupling regime

The intriguing properties of polaritons resulting from strong and ultrastrong light-matter coupling have been extensively investigated. However, most research has focused on spectroscopic characteristics of polaritons, such as their eigenfrequencies and Rabi splitting. Here, we study the decay rates of a plasmon-microcavity system in the strong and ultrastrong coupling regimes experimentally and numerically. We use a classical scattering matrix approach, approximating our plasmonic system with an effective Lorentz model, to obtain the decay rates through the imaginary part of the complex quasinormal mode eigenfrequencies. Our classical model automatically includes all the interaction terms necessary to account for ultrastrong coupling without dealing with the rotating-wave approximation and the diamagnetic term. We find an asymmetry in polaritonic decay rates, which deviate from the expected average of the uncoupled system's decay rates at zero detuning. Although this phenomenon has been previously observed in exciton-polaritons and attributed to their disorder, we observe it even in our homogeneous system. As the coupling strength of the plasmon-microcavity system increases, the asymmetry also increases and can become so significant that the lower (upper) polariton decay rate reduction (increase) goes beyond the uncoupled decay rates, γ - < γ 0,c < γ +. Furthermore, our findings demonstrate that polaritonic linewidth asymmetry is a generic phenomenon that persists even in the case of bulk polaritons.

Observation of ultra-large Rabi splitting in the plasmon-exciton polaritons at room temperature

Modifying the light-matter interactions in the plasmonic structures and the two-dimensional (2D) materials not only advances the deeper understanding of the fundamental studies of many-body physics but also provides the opportunities for exploration of novel 2D plasmonic polaritonic devices. Here, we report the plasmon-exciton coupling in the hybrid system with a plasmonic metasurface which can confine the electric field in an extremely compact mode volume. Because of the 2D feature of the designed and fabricated Al plasmonic metasurface, the confined electronic field is distributed in the plane with the same orientation as that of the exciton dipole moment in the transition metal dichalcogenides monolayers. By finely tuning the geometric size of the plasmonic nanostructures, we can significantly modify the dispersion relation of the coupled plasmon and the exciton. Our system shows a strong coupling behavior with an achieved Rabi splitting up to ∼200 meV at room temperature, in ambient conditions. The effective tailoring of the plasmon-exciton coupling with the plasmonic metasurfaces provides the testing platform for studying the quantum electromagnetics at the subwavelength scale as well as exploring plasmonic polariton Bose-Einstein condensation at room temperature.