  Strain presents a straightforward tool to tune electronic properties of
atomically thin nanomaterials that are highly sensitive to lattice
deformations. While the influence of strain on the electronic band structure
has been intensively studied, there are only few works on its impact on optical
properties of monolayer transition metal dichalcogenides (TMDs). Combining
microscopic theory based on Wannier and Bloch equations with nearestneighbor
tight-binding approximation, we present an analytical view on how uni- and
biaxial strain influences the optical fingerprint of TMDs including their
excitonic binding energy, oscillator strength, optical selection rules, and the
radiative broadening of excitonic resonances. We show that the impact of strain
can be reduced to changes in the lattice structure (geometric effect) and in
the orbital functions (overlap effect). In particular, we demonstrate that the
valley-selective optical selection rule is softened in the case of uniaxial
strain due to the introduced asymmetry in the lattice structure. Furthermore,
we reveal a considerable increase of the radiative dephasing due to
strain-induced changes in the optical matrix element and the excitonic wave
functions.
