Electrically Switchable Intervalley Excitons with Strong Two-Phonon Scattering in Bilayer WSe₂

Abstract

We report the observation of QΓ intervalley exciton in bilayer WSe2 devices encapsulated by boron nitride. The QΓ exciton resides at ∼18 meV below the QK exciton. The QΓ and QK excitons exhibit different Stark shifts under an out-of-plane electric field due to their different interlayer dipole moments. By controlling the electric field, we can switch their energy ordering 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 reveal the existence of numerous (≥14) two-phonon scattering paths involving (nearly) resonant exciton–phonon scattering in bilayer WSe2. To our knowledge, such electric-field-switchable intervalley excitons with strong two-phonon replicas have not been found in any other two-dimensional semiconductors. These make bilayer WSe2 a distinctive valleytronic material with potential novel applications.

Tunable phononic coupling in excitonic quantum emitters

“…This turn-on behavior may be attributed to the IXs being bound to a defect that becomes charged at the activation voltage. After the QE is turned on, its emission can be red-shifted by more than 6.4 meV (~3.3 nm in wavelength) with the application of gate voltage up to -7 V. We calculate the tunability to be 313 meV•nm/V, which is among the largest in deterministically created 2D QEs, consistent with the large dipole moment of IX in WSe2 29,33,34 . Even higher tunability can be achieved with QEs in heterobilayers, such as moiré excitons 42 , which have an even larger dipole moment.…”

“…The bilayer WSe2 is an ideal system to explore the phonon-photon interaction as it affords many Raman-active phonon modes covering a broad frequency range 28,29 . Particularly, as shown in Fig.…”

“…The optically excited IXs are funneled into the traps where they are bound with natural defects and recombine to emit single photons. In comparison to excitons in monolayer TMDs that have no electric dipole moment, IXs in bilayer WSe2 have a large out-ofplane electric dipole moment 29,[33][34][35] . Therefore, they couple efficiently to both a perpendicular electric field that generates a Stark shift and the interlayer vibration.…”

“…1e shows the density functional theory calculated band structure of bilayer WSe2 in its pristine state and strained state with 1% tensile strain. In pristine bilayer WSe2, indirect bandgap transitions including Q-K and Q-Γ excitons can occur, assisted by various single-phonon or two-phonon processes 29 . When strained, the conduction band minimum is shifted from the Q point to the K point, such that direct K-K transition becomes favorable in energy, enabling exciton coupling to zero-momentum phonons 29,[38][39][40][41] .…”

Tuning exciton complexes in twisted bilayer WSe2 at intermediate misorientation

“…2(b), we also observe a broad low-energy indirect exciton X I peak ≈ 1.55 eV. Specific domes of high PL intensity of X I peaks of BL WSe 2 emerge as electrostatic doping is modified, which could possibly be related to resonant excitonphonon scattering [39]. The domes of high PL have not been studied in a second device but are hinted by previous work [23,40].…”