  Transition-metal dichalcogenides (TMDs) host tightly bound quasi-particles
called excitons. Based on spin and momentum selection rules, these excitons can
be either optically bright or dark. In tungsten-based TMDs, momentum-forbidden
dark exciton is the energy ground state and therefore it strongly affect the
emission properties. In this work, we brighten the momentum forbidden dark
exciton by placing WS$_2$ on top of nanotextured substrates which put the
WS$_2$ layer under tensile strain, modifying electronic bandstructure. This
enables phonon assisted scattering of exciton between momentum valleys, thereby
brightening momentum forbidden dark excitons. Our results will pave the way to
design ultrasensitive strain sensing devices based on TMDs.
