  Hexagonal boron nitride is an indirect band gap material with a strong
luminescence in the ultraviolet. This luminescence originates from bound
excitons recombination assisted by different phonon modes. The coupling between
excitons and phonons is so strong that the resulting light emission is as
efficient as the one of direct band gap materials. In this manuscript we
investigate how uniaxial strain modifies the electronic and optical properties
of this material, and in particular how it affects the exciton-phonon coupling.
Using a formulation of this coupling based on finite-difference displacements,
recently developed by some of us, we investigate how phonon-assisted
transitions change under strain. Our results open the way to the study of
phonon-assisted luminescence in strained materials from first principles. Our
findings are important both for experiments that directly probe \hbn under
strain or for those in which it is used as substrate for other 2D material with
a lattice mismatch.
