Brightening of dark excitons in monolayers of semiconducting transition metal dichalcogenides
We present low temperature magneto photoluminescence experiments which demonstrate the brightening of dark excitons by an in plane magnetic field $B$ applied to monolayers of different semiconducting transition metal dichalcogenides. For both WSe$_2$ and WS$_2$ monolayers, the dark exciton emission is observed at \sim$50 meV below the bright exciton peak and displays a characteristic doublet structure which intensity is growing with $B^2$ while no magnetic field induced emission peaks appear for MoSe$_2$ monolayer. Our experiments also show that the MoS$_2$ monolayer has a dark exciton ground state with a dark bright exciton splitting energy of \sim$100 meV.