Switching on Dark Excitons in WSe ₂ Monolayer at Room Temperature with Plasmonic Au Nanodisk‐on‐Mirror Cavities

Abstract Dark excitons in transition metal dichalcogenide (TMDC) monolayers (MLs) are promising candidates for Bose–Einstein condensation and quantum information processing owing to their long lifetime. Previous studies employing nanoparticle‐on‐mirror and tip‐on‐mirror cavities have successfully enabled room‐temperature dark exciton emissions through the Purcell effect. However, these approaches suffer from mode complexity, the geometric variability of the contact facet, and the extreme precision of the tip spacing, which limit further control over emission intensities. Herein, the switching‐on of dark excitons in WSe 2 ML at room temperature is demonstrated using plasmonic Au nanodisk‐on‐mirror cavities with the ML sandwiched in between. The hybrid structures combine simplicity, reproducibility, and flexible control over the dark exciton emission intensity. The dark exciton emissions are controlled by two orthogonal plasmon modes with strongly confined out‐of‐plane‐polarized electric field and can be switched on and off. The emission intensity ratio between dark excitons and bright excitons is enhanced up to 22 times by flexibly controlling the nanodisk diameter to match the plasmon energy to the dark exciton energy. The plasmonic nanodisk‐on‐mirror cavities feature enlarged field enhancement, narrowed mode linewidths, and precise geometry, providing a facile platform for switching on and controlling dark exciton emissions in TMDC MLs under readily accessible conditions.