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Electron–phonon interactions strongly influence exciton dynamics, enabling control at the nanoscale over light‐matter interactions and coherent phenomena for emerging quantum sensing applications. In transition metal dichalcogenides, momentum‐forbidden dark excitons occupy the energy ground state and exhibit exceptional sensitivity to their local environment. Here, we optically characterize these dark excitonic states in WS2and WSe2monolayers by tuning their band alignment through strain. We unveil their pronounced localization, extended lifetimes, and high linear polarization, signatures of strong exciton–phonon coupling and environmental responsiveness. Multiple distinct emission peaks are consistent with phonon‐dressed excitonic replicas with an estimated Huang‐Rhys factor of about 5. Density functional theory calculations, combined with measured strain‐dependent emission energies, constrain the relevant energy scales and suggest possible phonon‐assisted pathways underlying these phonon‐dressed states. Because replica energies, relative spectral weights, polarization axis, and lifetime respond to environmental factors such as local strain, dielectric environment, and phonon populations, these phonon‐dressed dark excitons provide a materials platform for exciton‐based nanoscale sensing and coherent control in 2D semiconductors.
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Narrow-band high-lying excitons with negative-mass electrons in monolayer WSe2
Abstract Monolayer transition-metal dichalcogenides (TMDCs) show a wealth of exciton physics. Here, we report the existence of a new excitonic species, the high-lying exciton (HX), in single-layer WSe 2 with an energy of ~3.4 eV, almost twice the band-edge A-exciton energy, with a linewidth as narrow as 5.8 meV. The HX is populated through momentum-selective optical excitation in the K -valleys and is identified in upconverted photoluminescence (UPL) in the UV spectral region. Strong electron-phonon coupling results in a cascaded phonon progression with equidistant peaks in the luminescence spectrum, resolvable to ninth order. Ab initio GW -BSE calculations with full electron-hole correlations explain HX formation and unmask the admixture of upper conduction-band states to this complex many-body excitation. These calculations suggest that the HX is comprised of electrons of negative mass. The coincidence of such high-lying excitonic species at around twice the energy of band-edge excitons rationalizes the excitonic quantum-interference phenomenon recently discovered in optical second-harmonic generation (SHG) and explains the efficient Auger-like annihilation of band-edge excitons.
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- Award ID(s):
- 2114081
- PAR ID:
- 10322097
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 12
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1038/s41467-021-25499-2
