Exploring new optical resonances in TMDs beyond the bright exciton

The rich band structures of transition metal dichalcogenides (TMDs) give rise to several transitions beyond standard bright excitons, including intervalley momentum-forbidden excitons (with electrons and holes located in two different valleys) and spin-forbidden excitons (with electrons and holes having opposite spin). Moreover, the malleability of TMDs allows for the manipulation of their atomic structure that opens exciting opportunities to tailor their optical response at the atomic scale. Here we report on the control of different exciton species in TMDs for optoelectronic applications. We show evidence that the formation and emission of momentum-forbidden excitons are related to compressive strain due to a phonon-assisted intervalley scattering process that can be used as an ultrasensitive optical strain sensing mechanism. We also report on the repulsion-driven propagation of dark spin-forbidden excitons that, due to their permanent dipole, can travel for several micrometres transporting spin-valley information. Finally, we show that when the semiconducting and the metallic phase coexist in a monolayer of TMDs, additional excitonic resonances appear in the optical spectrum due to the creation of new band nesting effects occurring at the phase boundaries.