  Anti-Stokes photoluminescence (PL) is light emission at a higher photon
energy than the excitation, with applications in optical cooling, bioimaging,
lasing, and quantum optics. Here, we show how plasmonic nano-cavities activate
anti-Stokes PL in WSe$_2$ monolayers through resonant excitation of a dark
exciton. The tightly confined plasmonic fields excite the out-of-plane
transition dipole of the dark exciton, leading to light emission from the
bright exciton at higher energy. Through statistical measurements on hundreds
of plasmonic cavities, we show that coupling to the dark exciton is key to
achieving a near hundred-fold enhancement of the upconverted PL intensity. This
is further corroborated by experiments in which the laser excitation wavelength
is tuned across the dark exciton. Finally, we show that an asymmetric
nanoparticle shape and precise geometry are key for consistent activation of
the dark exciton and efficient PL upconversion. Our work introduces a new
excitation channel for anti-Stokes PL in WSe$_2$ and paves the way for
large-area substrates providing optical cooling, anti-Stokes lasing, and
radiative engineering of excitons.
