Fig. 1: Dark exciton brightening and directional emission.
a Schematic of directional emission of dark excitons in the WSe2 monolayer with normally incident pumping light. The dark excitons PL signal can then be directionally emitted through the Friedrich-Wintgen bound states in the continuum (BIC) supported by the same PhC slab. b Split-band configuration of bright and dark exciton states. The spin-forbidden optical transition of the dark exciton (XD) is brightened by the converted \({{{{{{\bf{E}}}}}}}_{{{{{{\boldsymbol{\perp }}}}}}}\) with enhancement on the top surface of the PhC slab. CB, conduction band; VB, valence band. c A scanning electron microscope (SEM) image of the PhC slab made of silicon nitride (Si3N4). d and e BIC, and cavity modes obtain optical confinement in momentum (k) and real (x) space, respectively. f A sketch of the optical band structure of the PhC slab with three types of the BICs: ① and ④ are the on-Γ symmetry-protected BICs, ② is the off-Γ accidental BIC, and ③ is the Friedrich-Wintgen BIC due to the destructive interference of resonances belonging to different bands (red and blue).
