Optical polarizability and the observation of dark np states in WSe2.
In a recent work Poellmann et al. [Nat. Mat. 14, 889 (2015)] were able to probe, at room temperature, the dark excitonic energy level 2p in a transition metal dichalcogenide via the transition 1s-->2p between the two excitonic states. This is a remarkable experiment around which we developed in this paper a fully microscopic description of this type of dark excitonic transitions. We have used Fowler's and Karplus' methods to access the microscopic polarizability of a non-interacting exciton gas, where the ground state of the excitonic energy series is first populated by a pump laser, followed by a probe which allows excitation of the 1s-->2p transition. When compared with experimental data our theory accounts quantitatively for both the position of the resonance in the measured optical conductivity and the dispersion of the real part of the dielectric function.