Observation of ~100% valley-coherent excitons in monolayer MoS₂ through giant enhancement of valley coherence time

Valley excitons in monolayer TMDs are created selectively in the K and K' valleys and linear excitation can prepare a coherent superposition (valley-coherent exciton). However, valley coherence typically decays within sub-picosecond timescales due to fast momentum scattering and inter-valley exchange, yielding coherence times (98–520 fs) much shorter than the ~1 ps radiative lifetime, which hampers optical readout and coherent manipulation for quantum information applications. The research aims to substantially enhance the valley coherence time so that the generated valley coherence is retained throughout the exciton lifetime. The study demonstrates near-100% linear polarization in steady-state PL from A1s excitons in MoS2 encapsulated between few-layer graphene, indicating fully valley-coherent excitons limited only by their lifetime and establishing a pathway toward practical valleytronic qubits.

Prior works report rapid valley decoherence in monolayer TMDs driven by scattering and inter-valley exchange (Maialle-Silva-Sham mechanism), with measured valley coherence times of 98–520 fs, far shorter than the ~1 ps exciton radiative lifetime. This mismatch hinders coherent control and optical readout. Theoretical frameworks for exchange-induced pseudospin precession and motional narrowing have been established, and dielectric environment is known to alter exciton binding energy, bandgap renormalization, and exchange interaction. However, achieving strong steady-state valley coherence has remained challenging in monolayer semiconductors.

- Device stacks: Four heterostructures were fabricated to systematically vary dielectric screening and exciton lifetime: (1) hBN-MoS2-hBN (HMH), (2) FLG-hBN-MoS2-hBN-FLG (GHMHG), (3) MoS2-FLG-hBN (MGH), and (4) FLG-MoS2-FLG (GMG). Few-layer graphene (FLG) was used as a top and bottom encapsulant and as a fast exciton filter via interlayer transfer. - Optical measurements: Polarization-resolved photoluminescence (PL) at T = 5 K under near-resonant 633 nm linearly polarized excitation measured co- and cross-linear components to extract DOLP. Circularly polarized excitation/detection was used to quantify DOCP. Additional PL with 532 nm excitation probed A2s and A1s features and assessed dielectric screening across stacks and TMD species (MoS2, MoSe2, WS2). Polarization-dependent time-resolved PL (TRPL) was performed to estimate exciton lifetimes; instrument response function limited resolution was noted. - Modeling and calculations: The Bethe-Salpeter equation (two-band model) was solved numerically, with parameters fitted to match experimental A2s−A1s separations, to estimate A1s binding energies and continuum levels for different dielectric environments. Long-range exchange interaction J_LR(Q) within the light cone was computed to assess screening-induced suppression of exchange. The steady-state Maialle-Silva-Sham (MSS) pseudospin dynamics were solved to compute DOLP ⟨Sx⟩ versus scattering rate, incorporating exciton-impurity scattering at 5 K (excitons-phonon scattering neglected). The homogeneous Lorentzian linewidth from Voigt fits of PL peaks served as an experimental proxy for scattering. Comparisons of experiment and simulation evaluated the motional narrowing regime.

- Near-unity valley coherence: In the FLG-MoS2-FLG (GMG) stack, DOLP reached 96 (±6)% on average under near-resonant 633 nm excitation at 5 K, with several spots exhibiting ~100% DOLP. A peak DOLP of 97.6% was also observed in a FLG-WS2-FLG stack via TRPL. - Comparative DOLP across stacks: HMH: 44.5 (±10)%; GHMHG: 37 (±9)%; MGH: 77 (±5)%; GMG: 96 (±6)%. - Circular polarization: DOCP was 81.6 (±2)% in GMG versus 20.5 (±9)% in HMH, consistent with an in-plane exchange field that favors linear over circular polarization retention. - Screening signatures: A2s−A1s separations (from 532 nm excitation PL) decreased with increasing screening: HMH 144.5 meV; GHMHG 63.5 meV; GMG 43.3 meV. Corresponding BSE-derived A1s binding energies: HMH 379 meV; GHMHG 162.5 meV; GMG 122 meV. Calculations showed strong suppression of the long-range exchange interaction |J_LR(Q)| within the light cone with increased screening. - Exciton lifetime (TRPL): <5 ps (HMH), 6–8 ps (GHMHG), <5 ps (GMG). The longer lifetime in GHMHG reflects screening-induced enhancement; in GMG, fast interlayer transfer to graphene filters out long-lived excitons, keeping the effective lifetime short. - Motional narrowing: Simulations of ⟨Sx⟩ versus scattering reproduce a V-shaped dependence; experiments show DOLP increasing with homogeneous linewidth (Γ_hom), placing operation in the motional narrowing regime in both HMH and GMG. The impurity scattering dominates linewidth and is comparable across samples. - Combined mechanism: Achieving near-100% DOLP in GMG arises from (i) suppressed exchange by dielectric screening, (ii) short effective lifetime via graphene-mediated filtering, and (iii) operation in the motional narrowing regime.

The results address the central challenge of fast valley decoherence in monolayer TMDs by engineering both the dielectric environment and the exciton dynamics. Dielectric screening from FLG encapsulation reduces the electron-hole overlap and Coulomb interaction, which in turn lowers the long-range inter-valley exchange field responsible for pseudospin precession and decoherence. However, screening also tends to increase the exciton radiative lifetime, which would normally allow more time for decoherence. The GMG architecture overcomes this by enabling rapid interlayer transfer of excitons to graphene, effectively filtering out long-lived excitons from contributing to PL and ensuring that the observed emission reflects a short effective lifetime where coherence can be preserved. Furthermore, operation in the motional narrowing regime (high scattering rate compared to precession rate) cancels accumulated phase randomness and enhances coherence. The combined effects yield steady-state PL with near-unity DOLP, indicating excitons remain valley coherent throughout their lifetime. Comparative measurements across four stacks disentangle the roles of screening, lifetime, and scattering, showing that screening alone (GHMHG) is insufficient without lifetime control, while screening plus filtering (GMG) achieves full coherence consistent with MSS-based simulations and BSE-informed exchange suppression.

This work demonstrates nearly 100% valley-coherent excitons in monolayer MoS2 by combining strong dielectric screening via few-layer graphene encapsulation with rapid interlayer transfer (filtering) and operation in the motional narrowing regime. Polarization-resolved PL and TRPL, supported by Bethe-Salpeter and MSS calculations, establish suppressed inter-valley exchange, controlled exciton lifetimes, and a scattering regime favorable for coherence retention. The achievement of near-unity DOLP in steady-state PL, with exciton coherence persisting over the entire lifetime, enables optical readout of valley coherence in monolayer semiconductors and marks, to the authors’ knowledge, the first such observation. Future research directions are not explicitly detailed in the text.

- The observed valley coherence is reported as being limited by the exciton lifetime; direct measurement of intrinsic coherence beyond the lifetime is not available. - TRPL lifetimes in HMH and GMG are shorter than the instrument response function resolution limit, introducing uncertainty in exact values. - Exciton-phonon scattering was neglected in decoherence modeling at 5 K; contributions at higher temperatures were not addressed. - Results are primarily at cryogenic temperature (5 K) and under near-resonant excitation; broader temperature and excitation conditions are not covered in the provided text.