Limits to Strong Coupling of Excitons in Multilayer WS₂ with Collective Plasmonic Resonances

Abstract

We demonstrate the strong coupling of direct transition excitons in tungsten disulfide (WS2) with collective plasmonic resonances at room temperature. We use open plasmonic cavities formed by periodic arrays of metallic nanoparticles. We show clear anticrossings with monolayer, bilayer and thicker multilayer WS2 on top of the nanoparticle array.The Rabi energy of such hybrid system varies from 50 to 110 meV from monolayer to sixteen layers, while it does not scale with the square root of the number of layers as expected for collective strong coupling. We prove that out-of-plane coupling components can be disregarded since the normal field is screened due to the high refractive index contrast of the dielectric layers. Even though the in-plane dipole moments of the excitons decrease beyond monolayers, the strong in-plane field distributed in the flake can still enhance the coupling strength with multilayers. However, the screened out-of-plane field leads to the saturation of the Rabi energy.The achieved coherent coupling of TMD multilayers with open cavities could be exploited for

Preserving the Emission Lifetime and Efficiency of a Monolayer Semiconductor upon Transfer

“…Absorption Spectra of TMD Monolayers : In order to calculate the dielectric permittivity, the measured transmission spectra of WS 2 were fitted using the transfer matrix method (TMM) and a multi‐Lorentizan modelwhere E ,ε B , f j , E j , and γ j are, respectively, the photon energy, the nonresonant background dielectric permittivity, oscillator strength, resonance center energy, and the linewidth of the absorption band j . For monolayers, ε B = 1 and five oscillators were used.…”

“…To elucidate the effect of different dielectric substrates, we investigate additional WS 2 monolayers transferred to hydrophobic polystyrene, hydrophilic fused silica, and alumina/silica. We chose these substrates due to their extended use in nanophotonic applications . We plot the normalized time‐resolved PL decay measurements of monolayer WS 2 , obtained via FLIM, before and after soft‐transfer encapsulation in these dielectric substrates in Figure S3 (Supporting Information).…”

“…Further enhancement and tuning of light–matter interaction is still possible through the integration of monolayers into nanophotonic architectures. In structures such as microcavities or metal nanoantennas, dielectric layers are frequently placed on top of the TMD monolayer to tune optical resonances, to protect the samples from degradation, or as spacers to avoid charge transfer. In this way, TMD monolayers may be surrounded by a dielectric environment that could modify their absorption and emission, which if controllable, is desirable for engineering properties such as the optical density of states or dielectric screening .…”

“…Due to the significant variations in PL for similarly exfoliated samples, we suggest that comparative studies that correlate the effect of any substrate parameter must evaluate any observed changes by comparing each individual monolayer before and after transfer. These results underline that particular attention must be paid to the method used to encapsulate monolayers when fabricating TMD‐based devices, and that the combination of the soft‐transfer method with PL lifetime measurements provides a reliable method to produce controlled quality samples.…”

Strong coupling with directional absorption features of Ag@Au hollow nanoshell/J-aggregate heterostructures

“…The role of various optical parameters dictating the plasmon-exciton interactions is less understood. To achieve strong plasmon-exciton coupling, the emitter must have a high oscillator strength and a high exciton binding energy, such as the electronic excitations (excitons) in quantum dots [45][46][47] and two-dimensional monolayer transition mental dichalcogenides [37,39,40,[48][49][50], for the realization of SC. The molecular excitation of J-aggregates represents an ideal platform for the formation of exciton polaritons because of their exceptionally high oscillator strength and narrow resonances even at room temperature and in the liquid phase [26,27].…”