  With the both spin and valley degrees of freedom, the low-lying excitonic
spectra of photo-excited transition-metal dichalcogenide monolayers (TMDC-MLs)
are featured by rich fine structures, comprising the intra-valley bright
exciton states as well as various intra- and inter-valley dark ones. The latter
states can be classified as those of the spin- and momentum-forbidden dark
excitons according to the violated optical selection rules. Because of the
optical invisibility, the two types of the dark states are in general hardly
observed and even distinguished in conventional spectroscopies although their
impacts on the optical and dynamical properties of TMDC-MLs have been well
noticed. In this Letter, we present a theoretical and computational
investigation of the exciton fine structures and the temperature-dependent
photo-luminescence spectra of strained tungsten diselenide monolayers
(WSe$_2$-MLs) where the intra-valley spin-forbidden dark exciton lies in the
lowest exciton states and other momentum-forbidden states are in the higher
energies that are tunable by external stress. The numerical computations are
carried out by solving the Bethe-Salpeter equation for an exciton in a
WSe$_2$-ML under the stress-control in the tight-binding scheme established
from the first principle computation in the density functional theory.
According to the numerical computation and supportive model analysis, we reveal
the distinctive signatures of the spin- and momentum-forbidden exciton states
of strained WSe$_2$-MLs in the temperature-dependent photo-luminescences and
present the guiding principle to infer the relative energetic locations of the
two types of DX's.
