Phonon Sidebands in Monolayer Transition Metal Dichalcogenides

Abstract

Excitons dominate the optical properties of monolayer transition metal dichalcogenides (TMDs). Besides optically accessible bright exciton states, TMDs exhibit also a multitude of optically forbidden dark excitons. Here, we show that efficient exciton-phonon scattering couples bright and dark states and gives rise to an asymmetric excitonic line shape. The observed asymmetry can be traced back to phonon-induced sidebands that are accompanied by a polaron redshift. We present a joint theory-experiment study investigating the microscopic origin of these sidebands in different TMD materials taking into account intra- and intervalley scattering channels opened by optical and acoustic phonons. The gained insights contribute to a better understanding of the optical fingerprint of these technologically promising nanomaterials.

Ultra‐Long Lifetimes of Single Quantum Emitters in Monolayer WSe2/hBN Heterostructures

“…The PL is measured with the polarizer both parallel and perpendicular to the PL emission, showing a high degree of linear polarization with a polarization ratio of ≈9. By looking at higher resolution (Figure a) taken under the same conditions, we find that the emitter exhibits fine structure splitting into doublets, a spectral feature which has been reported previously in SQEs, as well as a prominent phonon sideband due to exciton–phonon coupling . The splitting is on the order of 0.45 meV, which is at the low end of the range of doublet splitting reported for WSe 2 SQEs.…”

“…However, the dark exciton states can decay faster by means of phonon‐assisted recombination, for example, electronic state decays thermally to or decays thermally to . The key role played by the neutral dark excitons on the optical properties of monolayer TMDs has been illustrated in various PL experiments and efficient exciton–phonon scattering, well known in TMDs, has recently been shown to couple bright and dark excitons . In ref.…”

Theory of Exciton–Exciton Interactions in Monolayer Transition Metal Dichalcogenides

“…For the derivation of excitonic TMDC Bloch equations, typical two band models, such as used for GaAs‐like semiconductors must be adopted to describe the characteristic valley structure of monolayer TMDCs. This includes new physical effects, such as Coulomb intervalley exchange, intrinsic and Dexter‐like intervalley scattering, as well as exciton–phonon intervalley coupling . This paper addresses the theoretical foundations and the coherent limit of exciton–exciton interactions on the level of the Hartree–Fock approximation during ultrafast optical excitation.…”

Theory of the absorption line shape in monolayers of transition metal dichalcogenides

“…Monolayers of transition metal dichalcogenides (TMDCs) are intensely studied direct semiconductors with tightly bound excitons [1][2][3][4][5] and exceptionally strong exciton-phonon interaction [6][7][8][9]. Remarkable signatures of these traits are strongly inhomogeneous lineshapes in absorption spectra and pronounced phonon-assisted transitions in photoluminescence [6][7][8]10]. This gives rise to the question how to theoretically describe the combined exciton and phonon physics.…”