Role of dark exciton states in the relaxation dynamics of bright 1s excitons in monolayer WSe2

Abstract

Monolayer transition metal dichalcogenides (1L-TMDs) are excellent platforms for exciton physics. In tungsten-based 1L-TMDs, the existence of dark excitons at lower energy has important roles for bright exciton relaxation. However, the detailed relaxation dynamics from bright to dark excitons have not been revealed sufficiently. In this paper, we studied the rise dynamics of out-of-plane polarized photoluminescence (PL) from spin-forbidden dark excitons in monolayer WSe2. Under conditions of resonant excitation to the bright 1s excitons, PL from the spin-forbidden dark exciton has a finite rise time of a few tens of picoseconds, which suggests that intermediate states, probably hot indirect dark excitons, should play an important role in the relaxation pathway from the bright to the spin-forbidden dark excitons. The excitation density dependence indicates that exciton-exciton scattering should promote faster relaxation to the spin-forbidden dark excitons.