  Monolayers of transition metal dichalcogenides (TMDs) have recently emerged
as a promising optoelectronic platform. To leverage their full potential,
however, it is important to understand and engineer the properties of the
different exciton species that exist in these monolayers, as well as to control
their transport through the material. A promising approach relies on
engineering strain landscapes in atomically thin semiconductors that excitons
navigate. In WSe$_2$ monolayers, for example, localized strain has been used to
control the emission wavelength of excitons, induce exciton funneling and
conversion, and even realize single-photon sources and quantum dots. Before
these phenomena can be fully leveraged for applications, including quantum
information processing, the details of excitons' interaction with the strain
landscape must be well understood. To address this, we have developed a
cryogenic technique capable of probing the dynamics of both bright and dark
excitons in nanoscale strain landscapes in TMDs. In our approach, a
nanosculpted tapered optical fiber is used to simultaneously generate strain
and probe the near-field optical response of WSe$_2$ monolayers at 5 K. When
the monolayer is pushed by the fiber, its lowest energy photoluminescence (PL)
peaks red shift by as much as 390 meV, (corresponding to 20% of the bandgap of
an unstrained WSe$_2$ monolayer). The red-shifting peaks are polarized
perpendicularly to the WSe$_2$ plane and have long rising times (10 ps) and
lifetimes (52 ps), indicating that they originate from nominally spin-forbidden
dark excitons. Taken together, these observations indicate that dark excitons
are funneled to the high-strain regions during their long lifetime and are the
principal participants in drift and diffusion at cryogenic temperatures. Our
work elucidates the important role that dark excitons play in locally strained
WSe$_2$.
