Emergence of spin-forbidden dark excitons in monolayer TMDs under a vortex beam

Because of the quantum confinement, non-local charge screening in 2D, and strong spin-orbit coupling, monolayer transition metal dichalcogenides (TMDs) exhibit various quantum phenomena that can be tamed with light beams for tantalizing technological applications. TMDs host tightly bound excitons, which dominate their optoelectronic response even at room temperatures. Light beams are often used to study these materials with the polarization - often termed the spin angular momentum of the light - providing the mechanism for exciting excitonic states. Light beams, however, can also carry an orbital angular momentum by creating helical structures of their phase front. We present a theoretical analysis of the interaction between the monolayer TMDs and the vortex beams with finite orbital angular momenta. Using symmetry arguments, we derive optical selection rules that govern the allowed transitions to various exciton series. In particular, we show that applying vortex beams on a monolayer TMD crystal brings about the formation of long-lived spin-forbidden dark excitons, which make compelling candidates for solid-state qubits for applications in quantum information technologies.