Optical Study of Intervalley Excitons and Phonon Replicas in Bilayer WSe2
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Optical Study of Intervalley Excitons and Phonon Replicas in Bilayer WSe2


Atomically thin transition metal dichalcogenides (TMDs, e.g., MoS2, MoSe2, WS2, WSe2) exhibit intriguing tunable properties. For instance, their electronic band structure and optical spectra can be significantly modulated under the application of an electric field. Such electrical tunability allows for interesting scientific research and versatile applications of the materials. In this dissertation, we investigate 2H-stacked bilayer WSe2 dual-gated devices encapsulated by boron nitride. Bilayer WSe2 hosts two competing low-lying excitons, namely the QK and QΓ intervalley excitons. Although its electronic band structure suggests the QK exciton as the lowest-lying exciton, we observe the QΓ exciton at ~18 meV below the QK exciton. Our observation reveals that the QΓ exciton has larger binding energy than the QK exciton. The QK and QΓ excitons possess different interlayer electric dipole moments, which give rise to different Stark shifts under the application of an out-of-plane electric field. By controlling the electric field, we can switch the energy ordering of the QΓ and QK excitons, and control which exciton dominates the luminescence of bilayer WSe2. Remarkably, both QΓ and QK excitons exhibit unusually strong two-phonon replicas, which are comparable to or even stronger than the one-phonon replicas. By detailed theoretical simulation, we infer the existence of numerous two-phonon scattering paths involving (nearly) resonant exciton-phonon scattering in bilayer WSe2. Such electric-field-switchable intervalley excitons with strong two-phonon replicas make bilayer WSe2 a distinctive valleytronic material.We have also observed coherent states between multiple excitons with different center-of-mass momenta in bilayer WSe2. By studying the reflection spectra, we resolve two weak features arising from KQ and KQ’ intervalley excitons. Under the application of the out-of-plane electric field, both excitons exhibit strong Stark splitting, coupled with each other and with the nearby KK intravalley exciton. We can quantitatively simulate the results with a model that considers defect-mediated coherent coupling between the KQ, KQ’ and KK excitons. The good experiment-theory agreement strongly supports the emergence of coherent states between momentum-mismatched excitons and provides important insight into the role of defects in coherent excitonic coupling in semiconductors.

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