  Excitons, Coulomb-bound electron and hole pairs, are elementary
photo-excitations in semiconductors, that can couple directly to light through
radiative relaxation. In contrast to these bright excitons, dark excitons
X$\rm{_D}$ with anti-parallel electron spin polarization exist, with generally
forbidden radiative emission. Because of their associated long lifetimes, these
dark excitons are appealing candidates for quantum computing and
opto-electronic devices. However, optical read-out and control of X$\rm{_D}$
states have remained a major challenge due to their decoupling from light.
Here, we present a novel tip-enhanced nano-optical approach to precisely switch
and programmably modulate the X$\rm{_D}$ emission even at room temperature.
Using monolayer two-dimensional transition metal dichalcogenide (TMD)
WSe$\rm{_2}$ on a gold film as model system, we demonstrate ${\sim} 6 \times
10^5$-fold enhancement in dark exciton photoluminescence quantum yield. This is
achieved by the unique coupling of the nano-optical antenna-tip to the dark
exciton \textit{out-of-plane} optical dipole moment, with an extraordinary
Purcell factor of $\ge 2 \times 10^3$ of the tip-sample nano-cavity. Compared
to the necessity of cryogenic temperatures and high magnetic fields in
conventional approaches, our work provides a new way to harness excitonic
properties in low-dimensional semiconductors and new strategies for quantum
opto-electronic devices.
