Formation of dark excitons in monolayer transition metal dichalcogenides by a vortex beam: Optical selection rules

Monolayer transition metal dichalcogenides 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. In this work, we consider a Laguerre-Gaussian beam possessing an orbital angular momentum in addition to the spin angular momentum to create excitons in monolayer transition metal dichalcogenides. We derive optical selection rules that govern the allowed transitions to various exciton series using symmetry arguments. Our symmetry considerations show that we can create dark excitons using these high-order optical beams opening up new avenues for creating long-lived dark excitons with the potential of exploiting them in quantum information processing and storage.