Enhancement of Valley Polarization in Monolayer WSe2 Coupled with Microsphere‐Cavity‐Array

Monolayer transition metal dichalcogenides (ML‐TMDCs) possess degenerate levels with antiparallel spins in K and K′ valleys, providing the intrinsic valley polarization, which attracts great interest for potential applications on quantum information technology and on‐chip nanophotonics. Unfortunately, it is difficult to distinguish the degree of valley polarization (DoP) near room temperature due to the intensive phonon‐assisted intervalley scattering and the long‐range electron‐hole exchange interaction in ML‐TMDCs, limiting their practical applications. In this study, a novel design is proposed for great promotion of DoP in ML‐WSe2 with a microsphere cavity array, introducing Purcell effect and nanofocusing effect into the system. The radiative decay rate is dramatically enhanced owing to Purcell effect in microcavity in weak coupling regime, thus locking more polarized excitons in the corresponding valley under certain circularly‐polarized pumping. In addition, the nanofocusing effect contributes to increasing the number of charged excitons by suppressing the bright to dark exciton conversion. The present work achieves a great DoP of ML‐WSe2 with a simple configuration and promises broad applications from valleytronic devices to chiral optics in the future. A microsphere‐cavity‐array (MCA) structure is developed to improve the degree of valley polarization (DoP) and PLQY of ML‐WSe2. The MCA demonstrates superior capability to suppress dark exciton formation and intervalley scattering. The boosted localized density of charged excitons by nanofocusing and enhanced population decay by WGM‐induced Purcell effect in MCA/ML‐WSe2 achieves the PLQY of 12.4% with DoP > 0.20 near room temperature.