Understanding the Different Exciton–Plasmon Coupling Regimes in Two-Dimensional Semiconductors Coupled with Plasmonic Lattices: A Combined Experimental and Unified Equation of Motion Approach

Abstract

We study exciton-plasmon coupling in two-dimensional semiconductors coupled with Ag plasmonic lattices via angle-resolved reflectance spectroscopy and by solving the equations of motion (EOMs) in a coupled oscillator model accounting for all the resonances of the system. Five resonances are considered in the EOM model: semiconductor A and B excitons, localized surface plasmon resonances (LSPRs) of plasmonic nanostructures and the lattice diffraction modes of the plasmonic array. We investigated the exciton-plasmon coupling in different 2D semiconductors and plasmonic lattice geometries, including monolayer MoS2 and 2 WS2 coupled with Ag nanodisk and bowtie arrays, and examined the dispersion and lineshape evolution in the coupled systems via the EOM model with different exciton-plasmon coupling parameters. The EOM approach provides a unified description of the exciton-plasmon interaction in the weak, intermediate and strong coupling cases with correctly explaining the dispersion and lineshapes of the complex system. This study provides a much deeper understanding of lightmatter interactions in multilevel systems in general and will be useful to instruct the design of novel two-dimensional exciton-plasmonic devices for a variety of optoelectronic applications with precisely tailored responses. Keywords: 2D semiconductor, exciton plasmon, polariton, Purcell enhancement, Fano resonance, MoS2, WS2 Study of light-matter interactions is essential in understanding and manipulating the optical properties of materials to enable new and unprecedented functionalities. When light interacts with matter, in particularly, a direct bandgap semiconductor, absorption of a photon leads to the formation of a coupled electron-hole pair (excitons) bonded via coulombic interaction. The exciton can then recombine radiatively and emit a photon, allowing energy transfer back and forth between the photon and the exciton. Depending on the relative magnitudes between such energy transfer rate (coupling strength, ), and the dissipation rate, of each state, the system can be broadly classified into three light-matter coupling regimes 1 : (1) weak coupling regime, where the coupling strength ≪ − , with and representing the decay rate of the excitonic and 3 photonic states, respectively. In this regime, the eigenstates of the coupled system remain unchanged from their initial uncoupled states, and the system can be described by a perturbation theory where the Purcell effect 2 , i.e., the modification of the spontaneous emission rate via engineering the photon density of states, can be observed. Purcell effect has been extensively studied for enhancing and suppressing the spontaneous emission rate in various cavity geometries 3-4 , with applications in photonic and plasmonic lasers 5-6 , brighter single-photon sources 7-8 , hot luminescence 9-10 and quantum cryptography 11 . (2) intermediate coupling regime with − < < + , in which normal mode splitting occurs in the frequency domain, and an anti-crossing behavior generally observed in the far-fie...

Recent Progress on Exciton Polaritons in Layered Transition‐Metal Dichalcogenides

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Strong light–matter coupling and exciton-polariton condensation in lattices of plasmonic nanoparticles [Invited]

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