Plasmonic tuning of dark-exciton radiation dynamics and far-field emission directionality in monolayer WSe ₂

Manipulation of excitonic emission properties is important for numerous photonic applications. Of particular interest are developing easy-to-implement yet effective approaches for controlling the radiation dynamics and directionality of spin-forbidden dark excitons (X_D) in two-dimensional semiconductors. Here, we investigate the spectral, temporal, and directional characteristics of room-temperature X_D emission from a tungsten diselenide monolayer coupled to a dissipative plasmonic nanocavity. Under resonant plasmon-exciton coupling, the radiative decay rate of X_D is accelerated by nearly four orders of magnitude, and correspondingly, the X_D lifetime is shortened to a subnanosecond level, making it comparable to that of bright excitons. Fitting the measured lifetimes with a Purcell-formalism-based cavity quantum electrodynamics model allows estimating of the intrinsic room-temperature X_D lifetime to be about 24 ± 2.3 microseconds. Furthermore, the measured radiation patterns of the dark excitons show that subtle variations in the nanocavity orientation can effectively tailor the X_D emission directionality, important for quantum technologies and optoelectronics applications.